ELECTRONICS KERALAM

Hobby Circuits


             30 L.E.D പ്രോജക്ടുകള്‍


LED കള്‍ ഉപയോഗിച്ചുള്ള കുറച്ചു സര്‍ക്യൂട്ടുകള്‍ ...
തുടക്കക്കാര്‍ക്ക് വളരെയധികം പ്രയോജനപ്രദമായിരിക്കും..വളരെ ലളിതമായ ഇംഗ്ലീഷില്‍ വിവരിച്ചിരിക്കുന്നു
The LED (Light Emitting Diode) is the modern-day equivalent to the light-globe.
It has changed from a dimly-glowing indicator to one that is too-bright to look at.
However it is entirely different to a "globe."
A globe is an electrical device consisting of a glowing wire while a LED is an electronic device.
A LED is more efficient, produces less heat and must be "driven" correctly to prevent it being damaged.
This eBook shows you how to connect a LED to a circuit plus a number of projects using LEDs.
It's simple to use a LED - once you know how.

CONNECTING A LED
A LED must be connected around the correct way in a circuit and it must have a resistor to limit the current.
The LED in the first diagram does not illuminate because a red LED requires 1.7v and the cell only supplies 1.5v. The LED in the second diagram is damaged because it requires 1.7v and the two cells supply 3v. A resistor is needed to limit the current to about 25mA and also the voltage to 1.7v, as shown in the third diagram.  The fourth diagram is the circuit for layout #3 showing the symbol for the LED, resistor and battery and how the three are connected. The LED in the fifth diagram does not work because it is around the wrong way. 

CHARACTERISTIC VOLTAGE DROP
When a LED is connected around the correct way in a circuit it develops a voltage across it called the CHARACTERISTIC VOLTAGE DROP.
A LED must be supplied with a voltage that is higher than its "CHARACTERISTIC VOLTAGE" via a resistor - called a VOLTAGE DROPPING RESISTOR  or CURRENT LIMITING RESISTOR - so the LED will operate correctly and provide at least 10,000 to 50,000 hours of illumination.
A LED works like this:  A LED and resistor are placed in series and connected to a voltage.
As the voltage rises from 0v, nothing happens until the voltage reaches about 1.7v. At this voltage a red LED just starts to glow. As the voltage increases, the voltage across the LED remains at 1.7v but the current through the LED increases and it gets brighter.
We now turn our attention to the current though the LED.  As the current increases to 5mA, 10mA, 15mA, 20mA the brightness will increase and at 25mA, it will be a maximum. Increasing the supply voltage will simply change the colour of the LED slightly but the crystal inside the LED will start to overheat and this will reduce the life considerably.
This is just a simple example as each LED has a different CHARACTERISTIC VOLTAGE DROP and a different maximum current.
In the diagram below we see a LED on a 3v supply, 9v supply and 12v supply. The current-limiting resistors are different and the first circuit takes 6mA, the second takes 15mA and the third takes 31mA. But the voltage across the red LED is the same in all cases. This is because the LED creates the
CHARACTERISTIC VOLTAGE DROP and this does not change.
It does not matter if the resistor is connected above or below the LED. The circuits are the SAME in operation:


HEAD VOLTAGE

Now we turn our attention to the resistor.
As the supply-voltage increases, the voltage across the LED will be constant at 1.7v (for a red LED) and the excess voltage will be dropped across the resistor. The supply can be any voltage from 2v to 12v or more.
In this case, the resistor will drop 0.3v to 10.3v.
This is called HEAD VOLTAGE - or HEAD-ROOM or OVERHEAD-VOLTAGE. And the resistor is called the CURRENT-LIMIT resistor.
The following diagram shows HEAD VOLTAGE:
The voltage dropped across this resistor, combined with the current, constitutes wasted energy and should be kept to a minimum, but a small HEAD VOLTAGE is not advisable (such as 0.5v). The head voltage should be a minimum of 1.5v - and this only applies if the supply is fixed.
The head voltage depends on the supply voltage. If the supply is fixed and guaranteed not to increase or fall, the head voltage can be small (1.5v minimum).
But most supplies are derived from batteries and the voltage will drop as the cells are used.
Here is an example of a problem:
Supply voltage:  12v
7  red LEDs in series = 11.9v
Dropper resistor = 0.1v
As soon as the supply drops to 11.8v, no LEDs will be illuminated.
Example 2:
Supply voltage 12v
5 green LEDs in series @ 2.1v = 10.5v
Dropper resistor = 1.5v
The battery voltage can drop to 10.5v
But let's look at the situation more closely.
Suppose the current @ 12v = 25mA.
As the voltage drops, the current will drop.
At 11.5v, the current will be 17mA
At 11v, the current will be 9mA
At 10.5v, the current will be zero
You can see the workable supply drop is only about 1v.
Many batteries drop 1v and still have over 80% of their energy remaining. That's why you need to design your circuit to have a large
HEAD VOLTAGE.
A large Head Voltage is also needed when a plug-pack (wall wart) is used. These devices consist of a transformer, set of diodes and an electrolytic. The voltage marked on the unit is the voltage it will deliver when fully loaded. It may be 200mA, 300mA or 500mA. When this current is delivered, the voltage will be 9v or 12v. But if the current is less than the rated current, the output voltage will be higher. It may be 1v, 2v or even 5v higher.
This is one of the characteristics of a cheap transformer. A cheap transformer has very poor regulation, so to deliver 12v @ 500mA, the transformer produces a higher voltage on no-load and the voltage drops as the current increases. 
You need to allow for this extra voltage when using a plug-pack so the LEDs do not take more than 20mA to 25mA.  

TESTING A LED 
If the cathode lead of a LED cannot be identified, place 3 cells in series with a 220R resistor and illuminate the LED.  4.5v allows all types of LEDs to be tested as white LEDs require up to 3.6v.  Do not use a multimeter as some only have one or two cells and this will not illuminate all types of LEDs. In addition, the negative lead of a multimeter is connected to the positive of the cells (inside the meter) for resistance measurements - so you will get an incorrect determination of the cathode lead.



CIRCUIT TO TEST ALL TYPES OF LEDs
IDENTIFYING A LED
 A LED does not have a "Positive" or "Negative" lead. It has a lead identified as the "Cathode" or Kathode" or "k". This is identified by a flat on the side of the LED and/or by the shortest lead.
This lead goes to the 0v rail of the circuit or near the 0v rail (if the LED is connected to other components).
Many LEDs have a "flat" on one side and this identifies the cathode. Some surface-mount LEDs have a dot or shape to identify the cathode lead and some have a cut-out on one end.
Here are some of the identification marks:

 

LEDs ARE CURRENT DRIVEN DEVICES
A LED is described as a CURRENT DRIVEN DEVICE.  This means the illumination is determined by the amount of current flowing through it.
This is the way to see what we mean: Place a LED and 100R resistor in series and connect it to a variable power supply.
As the voltage is increased from 0v, to 1v, the LED will not produce any illumination, As the voltage from the power-supply increases past 1v, the LED will start to produce illumination at about 1.6v to 1.7v (for a red LED). As the voltage is increased further, the illumination increases but the voltage across the LED does not increase. (It may increase 0.1v) but the brightness will increase enormously. That's why we say the LED is a CURRENT DRIVEN DEVICE.
The brightness of a LED can be altered by increasing or decreasing the current. The effect will not be linear and it is best to experiment to determine the best current-flow for the amount of illumination you want. High-bright LEDs and super-bright LEDs will illuminate at 1mA or less, so the quality of a LED has a lot to do with the brightness. The life of many LEDs is determined at 17mA. This seems to be the best value for many types of LEDs.

1mA to 5mA LEDs
Some LEDs will produce illumination at 1mA. These are "high Quality" or "High Brightness" LEDs and the only way to check this feature is to test them @1mA as shown below. 

THE 5v LED 
Some suppliers and some websites talk about a 5v white or blue LED. Some LEDs have a small internal resistor and can be placed on a 5v supply. This is very rare.
Some websites suggest placing a white LED on a 5v supply. These LEDs have a characteristic voltage-drop of 3.6v and should not be placed directly on a voltage above this value.
The only LED with an internal resistor is a FLASHING LED. These LEDs can be placed on a supply from 3.5v to 12v and flash at approx 2Hz. The LED is very weak on 3.5v but it flashing can be used to drive a powerful LED (see circuits section). It can also be used to produce a beep for a beeper FM transmitter.
NEVER assume a LED has an internal resistor. Always add a series resistor. Some high intensity LEDs are designed for 12v operation. These LEDs have a complete internal circuit to deliver the correct current to the LED. This type of device and circuitry is not covered in this eBook.

LEDs IN SERIES
LEDs can be placed in series providing some features are taken into account. The main item to include is a current-limiting resistor.
A LED and resistor is called a string. A string can have 1, 2, 3 or more LEDs.
Three things must be observed:
1. MAXIMUM CURRENT through each string = 25mA.
2. The CHARACTERISTIC VOLTAGE-DROP must be known so the correct number of LEDs are used in any string.
3. A DROPPER RESISTOR must be included for each string.
The following diagrams show examples of 1-string, 2-strings and 3-strings:


LEDs IN PARALLEL
LEDs CANNOT be placed in parallel - until you read this:
LEDs "generate" or "possess" or "create" a voltage across them called the
CHARACTERISTIC VOLTAGE-DROP  (when they are correctly placed in a circuit).
This voltage is generated by the type of crystal and is different for each colour as well as the "quality" of the LED (such as high-bright, ultra high-bright etc). This characteristic cannot be altered BUT it does change a very small amount from one LED to another in the same batch. And it does increase slightly as the current increases.
For instance, it will be different by as much as 0.2v for red LEDs and 0.4v for white LEDs from the same batch and will increase by as much as 0.5v when the current is increased from a minimum to maximum.
You can test 100 white LEDs @15mA and measure the CHARACTERISTIC VOLTAGE-DROP to see this range.
If you get 2 LEDs with identical
CHARACTERISTIC VOLTAGE-DROP, and place them in parallel, they will each take the same current. This means 30mA through the current-limiting resistor will be divided into 15mA for each LED.
However if one LED has a higher
CHARACTERISTIC VOLTAGE-DROP, it will take less current and the other LED will take considerably more. Thus you have no way to determine the "current-sharing"  in a string of parallel LEDs.  If you put 3 or more LEDs in parallel, one LED will start to take more current and will over-heat and you will get very-rapid LED failure.  As one LED fails, the others will take more current and the rest of the LEDs will start to self-destruct. The reason why they take more current is this: the current-limit resistor will have been designed so that say 60mA will flow when 3 LEDs are in parallel. When one LED fails, the remaining LEDs will take 30mA each.
Thus LEDs in PARALLEL should be avoided.
Diagram A below shows two green LEDs in parallel. This will work provided the Characteristic Voltage Drop across each LED is the same.
In diagram B the Characteristic Voltage Drop is slightly different for the second LED and the first green LED will glow brighter.
In diagram C the three LEDs have different Characteristic Voltage Drops and the red LED will glow very bright while the other two LEDs will not illuminate. All the current will pass through the red LED and it will be damaged.
The reason why the red LED will glow very bright is this: It has the lowest Characteristic Voltage Drop and it will create a 1.7v for the three LEDs. The green and orange LEDs will not illuminate at this voltage and thus all the current
from the dropper resistor will flow in the red LED and it will be destroyed.
 

THE RESISTOR
The value of the current limiting resistor can be worked out by Ohms Law.
Here are the 3 steps:
1. Add up the voltages of all the LEDs in a string.   e.g:  2.1v + 2.3v + 2.3v + 1.7v = 8.4v
2. Subtract the LED voltages from the supply voltage.  e.g:  12v - 8.4v = 3.6v
3. Divide the 3.6v (or your voltage) by the current through the string. 
for 25mA:   3.6/.025 =144 ohms
for 20mA:   3.6/.02  = 180 ohms
for 15mA:   3.6/.015 = 250 ohms
for 10mA:   3.6/.01   = 360 ohms
This is the value of the current-limiting resistor.

Here is a set of strings for a supply voltage of 3v to 12v and a single LED:

 

Here is a set of strings for a supply voltage of 5v to 12v and a white LED: Here is a set of strings for a supply voltage of 5v to 12v and two LEDs:


LED series/parallel array wizard
The LED series/parallel array wizard below, is a calculator that will help you design large arrays of single-colour LEDs.
This calculator has been designed by Rob Arnold and you will be taken to his site:
 http://led.linear1.org/led.wiz when you click: Design my array
The wizard determines the current limiting resistor value for each string of the array and the power consumed. All you need to know are the specs of your LED and how many you'd like to use. The calculator only allows one LED colour to be used. For mixed colours, you will have to use the 3 steps explained above. The result is not always correct. Read the discussion below: "THE DANGERS OF USING A "LED WIZARD" to understand the word "HEAD VOLTAGE."  The HEAD VOLTAGE should be as high as possible to allow for the differences in Characteristic Voltage and the variations in power supply voltage. 

Source voltage Help
diode forward voltage Help
diode forward current (mA) Help
number of LEDs in your array
View output as: ASCII schematic wiring diagram Help
help with resistor colour codes








Resistor Calculator

Use this JavaScript resistor calculator to work out the value of the current-limiting resistor:
Source voltage =
LED forward voltage drop =
LED current in milliamps =
Current-limiting resistance in Ohms =
Closest 5% Resistor =
Resistor wattage =
Actual current =
Power dissipated by LED      (watts) =
Power dissipated by resistor (watts) =
LED VOLTAGE AND CURRENT
LED characteristics are very broad and you have absolutely no idea of any value until you test the LED.
However here are some of the generally accepted characteristics:
 


THE DANGERS OF USING A "LED WIZARD"
You can find a LED WIZARD on the web that gives you a circuit to combine LEDs in series and/or parallel for all types of arrays.
Here is an example, provided by a reader. Can you see the major fault?


The characteristic voltage (the colour of the LED) is not important in this discussion. Obviously white LEDs will not work as they require 3.4v to 3.6v to operate.
The main fault is the dropper resistor.
Read our article on LEDs.
The most important component is the DROPPER RESISTOR.
It must allow for the difference between the maximum and minimum supply voltage and ALSO the maximum and minimum CHARACTERISTIC VOLTAGE of the string of LEDs.
When we say a red LED has a CHARACTERISTIC VOLTAGE of 1.7v, we need to measure the exact maximum and minimum value for the LEDs we are installing.
Some high-bright and super-high-bright LEDs have a Characteristic Voltage of 1.6v to 1.8v and this will make a big difference when you have 8 LEDs in series.
Secondly, the 12v supply may rise to 13.6v when the battery is being charged and fall to 10.8v at the end of its life.
Thirdly, you need to know the current required by the LEDs.
The normal value is 17mA for long life.
This can rise to 20mA but must not go higher than 25mA
You should also look at the minimum current. Many high-bright LEDs will perform perfectly on 5-10mA and become TOO BRIGHT on 20mA.
As you can see, it is much more complex than a WIZARD can handle.
That's why it produced the absurd result above.
The maximum characteristic voltage for 8 red LEDs is 8x1.8v = 14.4v
This means you can only put 6 LEDs in series. = 10.8v  
The LEDs will totally die when the battery reaches 10.8v
The value of the dropper resistor for 6 LEDs and a supply of 12v @20mA = 60 ohms. When the battery voltage rises to 13.6v during charging, the current will be: 46mA. This is too high.
The CURRENT LIMITING resistor is too low. 
We need to have a higher-value CURRENT LIMITING resistor and fewer LEDs.
Use 5 LEDs:
The characteristic voltage for 5 LEDs will be:  5 x 1.7v = 8.5v
Allow a current of 20mA when the supply is 12.6v   Dropper resistor = 200 ohms.
Current at 10.8v will be 11mA. And current at 13.6v will be 25mA
Now you can see why the value of the CURRENT LIMITING RESISTOR has to be so high.
SOLDERING LEDs
LEDs are the most heat-sensitive device of all the components.
When soldering surface-mount LEDs, you should hold the LED with tweezers and "tack" one end. Then wait for the LED to cool down and solder the other end very quickly. Then wait a few seconds and completely solder the first end. Check the glow of each LED with 3 cells in series and a 220R resistor. If you have overheated the LED, its output will be dim, or a slightly different colour, or it may not work at all. They are extremely sensitive to heat - mainly because the crystal is so close to the soldering iron.

HIGH-BRIGHT LEDs
LEDs have become more efficient over the past 25 years.
Originally a red LED emitted 17mcd @20mA. These LEDs now emit 1,000mcd to 20,000mcd @20mA.
This means you can lower the current and still produce illumination. Some LEDs operate on a current as low as 1mA 

LEDs as LIGHT DETECTORS
LEDs can also be used to detect light.
Green LEDs are the best, however all LEDs will detect light and produce a voltage equal to the CHARACTERISTIC VOLTAGE-DROP, providing they receive sufficient light. The current they produce is miniscule however high-bright and super-bright LEDs produce a higher output due to the fact that their crystal is more efficient at converting light into electricity.
The Solar Tracker project uses this characteristic to track the sun's movement across the sky.
BI-COLOUR, TRI-COLOUR, FLASHING LEDS and 7-colour LEDs
LEDs can also be obtained in a range of novelty effects as well as a red and green LED inside a clear or opaque lens. You can also get red, blue, white, green or any combination inside a LED with 2 leads.
Simply connect these LEDs to a 6v supply and 330R series dropper resistor to see the effects they produce.
Some LEDs have 3 leads and the third lead needs to be pulsed to change the pattern.
Some LEDs can be reversed to produce a different colour. These LEDs contain red and green and by reversing the voltage, one or the other colour will illuminate.
When the voltage is reversed rapidly, the LED produces orange.
Sometimes it is not convenient to reverse the voltage to produce orange.
In this case three leaded LEDs are available to produce red, green and orange.

FLASHING LEDs
Flashing LEDs contain a chip and inbuilt current-limiting resistor. They operate from 3.5v to 12v.


NOVELTY LEDs
Novelty LEDs can have 2 or three leads. They
contain a microcontroller chip, inbuilt current-limiting resistor and two or three colours.
The two leaded LEDs cycle through a range of colours, including flashing and fading.
The three leaded LEDs have up to 16 different patterns and the control lead must be taken from 0v to rail volts to activate the next pattern.

 
LEDs LEDs LEDs
There are hundreds of circuits that use a LED or drive a LED or flash a LED and nearly all the circuits in this eBook are different.
Some flash a LED on a 1.5v supply, some use very little current, some flash the LED very brightly and others use a flashing LED to create the flash-rate.
You will learn something from every circuit. Some are interesting and some are amazing. Some consist of components called a "building Block" and they can be added to other circuits to create a larger, more complex, circuit.
This is what this eBook is all about.
It teaches you how to build and design circuits that are fun to see working, yet practical.

You will learn a lot  . . . . even from these simple circuits. 
Colin Mitchell
TALKING ELECTRONICS.
talking@tpg.com.au

SI NOTATION
All the schematics in this eBook have components that are labelled using the System International (SI) notation system. The SI system is an easy way to show values without the need for a decimal point. Sometimes the decimal point is difficult to see and the SI system overcomes this problem and offers a clear advantage.
Resistor values are in ohms (R), and the multipliers are: k for kilo, M for Mega. Capacitance is measured in farads (F) and the sub-multiples are u for micro, n for nano, and p for pico.  Inductors are measured in Henrys (H) and the sub-multiples are mH for milliHenry and uH for microHenry.
A 10 ohm resistor would be written as 10R and a 0.001u capacitor as 1n.
The markings on components are written slightly differently to the way they are shown on a circuit diagram (such as 100p on a circuit and 101 on the capacitor or 10 on a capacitor and 10p on a diagram) and you will have to look on the internet under Basic Electronics to learn about these differences. 

 


For photos of nearly every electronic component, see this website: https://www.egr.msu.edu/eceshop/Parts_Inventory/totalinventory.php


How good is your power of observation?
Can you find the LED:


                 
POWERING A PROJECT
The safest way to power a project is with a battery. Each circuit requires a voltage from 3v to 12v. This can be supplied from a set of AA cells in a holder or you can also use a 9v battery for some projects.
If you want to power a circuit for a long period of time, you will need a "power supply."
The safest power supply is a Plug Pack (wall-wort, wall wart,
wall cube, power brick, plug-in adapter, adapter block, domestic mains adapter, power adapter, or AC adapter). Some plug packs have a switchable output voltage: 3v, 6v, 7.5v, 9v, 12v) DC with a current rating of 500mA. The black lead is negative and the other lead with a white stripe (or a grey lead with a black stripe) is the positive lead.
This is the safest way to power a project as the insulation (isolation) from the mains is provided inside the adapter and there is no possibility of getting a shock.
The rating "500mA" is the maximum the Plug Pack will deliver and if your circuit takes just 50mA, this is the current that will be supplied. Some pluck packs are rated at 300mA or 1A and some have a fixed output voltage. All these plug packs will be suitable.
Some Plug Packs are marked "12vAC."  This type of plug pack is not suitable for these circuits as it does not have a set of diodes and electrolytic to convert the AC to DC. All the circuits in this eBook require DC.

PROJECTS
     
FLASHING A LED
These 7 circuits flash a LED using a supply from 1.5v to 12v.
They all have a different value of efficiency and current consumption. You will find at least one to suit your requirements.
The simplest way to flash a LED is to buy a FLASHING LED as shown in figure A. It will work on 3v to 9v but it is not very bright - mainly because the LED is not high-efficiency.
A Flashing LED can be used to flash a super-bright red LED, as shown in figure B.
Figure C shows a flashing LED driving a buffer transistor to flash a white LED. The circuit needs 4.5v - 6v. 
Figure D produces a very bright flash for a very short period of time - for a red, green, orange or white LED.
Figure E uses 2 transistors to produce a brief flash - for a red, green, orange or white LED.
Figure F uses a single cell and a voltage multiplying arrangement to flash a red or green LED.
Figure G flashes a white LED on a 3v supply. 
     
CONSTANT CURRENT
These four circuits delivers a constant 12mA to any number of LEDs connected in series (to the terminals shown) in the following arrangements.
The circuits can be connected to 6v, 9v or 12v and the brightness of the LEDs does not alter.
You can connect:
1 or 2 LEDs to 6v,
1, 2 or 3 LEDs to 9v or
1, 2, 3 or 4 LEDs to 12v.    
The LEDs can be any colour.
The constant-current section can be considered as a MODULE and can be placed above or below the load:
     
WHITE LED on 1.5v SUPPLY
This circuit will illuminate a white LED using a single cell.
See LED Torch Circuits article for more details.
     to Index
2 WHITE LEDs on 1.5v SUPPLY
This circuit will illuminate two white LEDs using a single cell.
See LED Torch Circuits article for more details.
     to Index
WHITE LED FLASHER
This circuit will flash a white LEDs using a single cell.
See LED Torch Circuits article for more details.
     to Index
SHAKE TIC TAC LED TORCH
In the diagram, it looks like the coils sit on the “table” while the magnet has its edge on the table. This is just a diagram to show how the parts are connected. The coils actually sit flat against the slide (against the side of the magnet) as shown in the diagram:
The output voltage depends on how quickly the magnet passes from one end of the slide to the other. That's why a rapid shaking produces a higher voltage. You must get the end of the magnet to fully pass though the coil so the voltage will be a maximum. That’s why the slide extends past the coils at the top and bottom of the diagram.

The circuit consists of two 600-turn coils in series, driving a voltage doubler. Each coil produces a positive and negative pulse, each time the magnet passes from one end of the slide to the other.
The positive pulse charges the top electrolytic via the top diode and the negative pulse charges the lower
electrolytic, via the lower diode.
The voltage across each electrolytic is combined to produce a voltage for the white LED. When the combined voltage is greater than 3.2v, the LED illuminates. The electrolytics help to keep the LED illuminated while the magnet starts to make another pass.
                      
     to Index
 
LED DETECTS LIGHT
The LED in this circuit will detect light to turn on the oscillator. Ordinary red LEDs do not work. But green LEDs, yellow LEDs and high-bright white LEDs and high-bright red LEDs work very well.
The output voltage of the LED is up to 600mV when detecting  very bright illumination.
When light is detected by the LED, its resistance decreases and a very small current flows into the base of the first transistor. The transistor amplifies this current about 200 times  and the resistance between collector and emitter decreases. The 330k resistor on the collector is a current limiting resistor as the middle transistor only needs a very small current for the circuit to oscillate. If the current is too high, the circuit will "freeze."
The piezo diaphragm does not contain any active components and relies on the circuit to drive it to produce the tone.
     to Index
8 MILLION GAIN!
This circuit is so sensitive it will detect "mains hum."  Simply move it across any wall and it will detect where the mains cable is located. It has a gain of about 200 x 200 x 200 = 8,000,000 and will also detect static electricity and the presence of your hand without any direct contact. You will be amazed what it detects!  There is static electricity EVERYWHERE! The input of this circuit is classified as very high impedance.
 

Here is a photo of the circuit, produced by a constructor.
to Index 
 LEDs on 240v
I do not like any circuit connected directly to 240v mains. However Christmas tress lights (globes) have been connected directly to the mains for 30 years without any major problems.
Insulation must be provided and the lights (LEDs) must be away from prying fingers.
You need at least 50 LEDs in each string
to prevent them being damaged via a surge through the 1k resistor - if the circuit is turned on at the peak of the waveform. As you add more LEDs to each string, the current will drop a very small amount until eventually, when you have 90 LEDs in each string, the current will be zero.
For 50 LEDs in each string, the total characteristic voltage will be 180v so that the peak voltage will be 330v - 180v = 150v. Each LED will see less than 7mA peak during the half-cycle they are illuminated (because the voltage across the 0.22u is 150v and this voltage determines the current-flow). The 1k resistor will drop 7v - since the RMS current is 7mA  (7mA x 1,000 ohms = 7v). No rectifier diodes are needed. The LEDs are the "rectifiers."  Very clever. You must have LEDs in both directions to charge and discharge the capacitor. The resistor is provided to take a heavy surge current through one of the strings of LEDs if the circuit is switched on when the mains is at a peak. This can be as high as 330mA if only 1 LED is used, so the value of this resistor must be adjusted if a small number of LEDs are used. The LEDs above detect peak current. The LEDs are turned on and off 50 times per second and this may create "flickering" or "strobing."  To prevent this flicker, see the DC circuit below:

A 100n cap will deliver 7mA RMS or 10mA peak in full wave or 3.5mA RMS (10mA peak for half a cycle) in half-wave.  (when only 1 LED is in each string).

The current-capability of a capacitor needs more explanation.  In the  diagram on the left we see a capacitor feeding a full-wave power supply. This is exactly the same as the LEDs on 240v circuit above. Imagine the LOAD resistor is removed. Two of the diodes will face down and two will face up. This is exactly the same as the LEDs facing up and facing down in the circuit above. The only difference is the mid-point is joined. Since the voltage on the mid-point of one string is the same as the voltage at the mid-point of the other string, the link can be removed and the circuit will operate the same.
This means each 100n of capacitance will deliver 7mA RMS (10mA peak on each half-cycle).
In the half-wave supply, the capacitor delivers 3.5mA RMS (10mA peak on each half-cycle, but one half-cycle is lost in the diode)  for each 100n to the load, and during the other half-cycle the 10mA peak is lost in the diode that discharges the capacitor. 
You can use any LEDs and try to keep the total voltage-drop in each string equal. Each string is actually working on DC. It's not constant DC but varying DC. In fact is it zero current for 1/2 cycle then nothing until the voltage rises above the total characteristic voltage of all the LEDs, then a gradual increase in current over the remainder of the cycle, then a gradual decrease to zero over the falling portion of the cycle, then nothing for 1/2 cycle. Because the LEDs turn on and off, you may observe some flickering and that's why the two strings should be placed together.

SINGLE LED on 240v
A single LED can be illuminated by using a 100n or 220n capacitor with a rating of 400v. These capacitors are called "X2" and are designed to be connected to  the mains.
The LED will be 240v above earth if the active and neutral are swapped and this represents a shock of over 340v if anything is exposed. The power diode in the first diagram is designed to discharge the 0.22u during one half of the cycle so that the capacitor will charge during the other half-cycle and deliver energy to the LED. The 1k resistor limits the peak in-rush current when the circuit is first turned on and the mains happens to be at a peak.   

Two LEDs can be driven from the same circuit as one LED will be illuminated during the first half cycle and the other LED will be driven during the second half of the cycle.  

 
LEDs can also be connected to the mains via a power diode and current-limiting resistor. But the wattage lost (dropped) in the resistor is about 2.5 watts and a 3 watt resistor will be needed to illuminate a 70mW white LED.
This is an enormous waste of energy and a capacitor-fed supply shown above is the best solution.
When 50 to 80 white LEDs are connected in series, a resistor can be used. For 50 white LEDs, use a 4k7 2watt resistor to provide 10mA average current.
For 100 white LEDs, use a 2k2 1watt resistor to provide 10mA average current.
The circuit will not work with more than 95 LEDs as the characteristic voltage-drop across the combination will be more than the peak of the supply (340v).
 
DC CONNECTION
To prevent "flickering' or "strobing," the LEDs must be driven with DC. This requires a BRIDGE.
The 0.22u will deliver 15mA when one LED is connected to the output. As additional LEDs are connected, the current gradually reduces to zero with 100 LEDs.  
40 LEDs will be provided with:
345 - 145 = 200v  = 200/345 x 15 = 8.6mA
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MAINS NIGHT LIGHT
The circuit illuminates a column of 10 white LEDs. The 10u prevents flicker and the 100R also reduces flicker.

This circuit is classified as a CONSTANT CURRENT GENERATOR or CONSTANT CURRENT CIRCUIT.
This means any component placed on the output of the circuit will pass 7mA if the capacitor is 100n on a 240v supply or 4.7 x 7mA  = 33mA if the capacitor is 470n.
This also applies to a short-circuit on the output.
If no load is connected, the output voltage will be 230v x 1.4 = 320v and if the voltage across the load is 100v, the output will be reduced to about 20mA. If the output voltage is 200v, the current will be 10mA and if the output voltage is 300v, the current will be 0mA. In our case the output voltage will be about 35v and the current will be 30mA.
This means you cannot add LEDs endlessly. A time will come when they will simply not illuminate.
LED DIMMER
This circuit will adjust the brightness of one or more LEDs from 5% to 95%.
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DRIVING A BI-COLOUR LED
Some 3-leaded LEDs produce red and green. This circuit alternately flashes a red/green bi-coloured LED:
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BI-POLAR LED DRIVER
Some 2-leaded LEDs produce red and green. These are called Bi-polar LEDs. This circuit alternately flashes a red/green bi-polar LED:
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RGB LED DRIVER
This is a simple driver circuit that drives the 3 LEDs in an RGB LED to produce a number of interesting colours. Even though the component values are identical in the three oscillators, the slight difference in tolerances will create a random display of colours and it will take a while for the pattern to repeat.
The colours change abruptly from one colour to another as the circuit does not use Pulse Width Modulation to produce a gradual fading from one colour to another.
This LED is called COMMON ANODE. This has been done so it can be connected to transistors or other devices that "SINK." 
The second circuit a common cathode LED.
Note the different pinout.
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RGB LED FLASHER
This LED flashes at a fast rate then a slow rate. It only requires a current-limiting resistor of 100R for 4.5v to 6v supply or 470R for 7v to 12v supply.


 

There are two different types of RGB LEDs. The RGB LED Driver circuit above uses an RGB LED with 4 leads and has 3 coloured chips inside and NOTHING ELSE.
The LED described in the video has 2 leads and requires a dropper resistor so that about 20mA flows. The LED also contains a microcontroller producing PWM signals. If you cannot get the 2-leaded LED, you can use a 4-leaded LED plus the circuit below. It is an analogue version of the circuit inside the self-flashing LED, for the slow-rate:

As with everything Chinese, the self-flashing LED is too gimmicky.
It is better to produce your own colour-change via the circuit above. You can alter the rate by changing the value of the components and/or remove one or more of the 100u's. The circuit for a common cathode RGB LED is shown in the RGB LED Driver above.
KNIGHT RIDER
In the Knight Rider circuit, the 555 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 555 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line.
The 10 outputs Q0 to Q9 become active, one at a time, on the rising edge of the waveform from the 555. Each output can deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (330R in the circuit above).
The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display.
Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.

 




 













Here is a simple Knight Rider circuit using resistors to drive the LEDs. This circuit consumes 22mA while only delivering 7mA to each LED. The outputs are "fighting" each other via the 100R resistors (except outputs Q0 and Q5).

                 
TRAFFIC LIGHTS
Here's a clever circuit using two 555's to produce a set of traffic lights for a model layout.
The animation shows the lighting sequence and this follows the Australian-standard. The red LED has an equal on-off period and when it is off, the first 555 delivers power to the second 555. This illuminates the Green LED and then the second 555 changes state to turn off the Green LED and turn on the Orange LED for a short period of time before the first 555 changes state to turn off the second 555 and turn on the red LED. A supply voltage of 9v to 12v is needed because the second 555 receives a supply of about 2v less than rail. This circuit also shows how to connect LEDs high and low to a 555 and also turn off the 555 by controlling the supply to pin 8.  Connecting the LEDs high and low to pin 3 will not work and since pin 7 is in phase with pin 3, it can be used to advantage in this design. 

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4 WAY TRAFFIC LIGHTS
This circuit produces traffic lights for a "4-way" intersection. The seemingly complex  wiring to illuminate the lights is shown to be very simple.
DRIVING MANY LEDS
The 555 is capable of sinking and sourcing up to 200mA, but it gets very hot when doing this on a 12v supply.
The following circuit shows the maximum number of white LEDs that can be realistically driven from a 555 and we have limited the total current to about 130mA as each LED is designed to pass about 17mA to 22mA maximum. A white LED drops a characteristic 3.2v to 3.6v and this means only 3 LEDs can be placed in series.
 
3x3x3 CUBE
This circuit drives a 3x3x3 cube consisting of 27 white LEDs. The 4020 IC is a 14 stage binary counter and we have used 9 outputs. Each output drives 3 white LEDs in series and we have omitted a dropper resistor as the chip can only deliver a maximum of 15mA per output. The 4020 produces 512 different patterns before the sequence repeats and you have to build the project to see the effects it produces on the 3D cube.
 
UP/DOWN FADING LED
These two circuits make a LED fade on and off. The first circuit charges a 100u and the transistor amplifies the current entering the 100u and delivers 100 times this value to the LED via the collector-emitter pins. The circuit needs 9v for operation since pin 2 of the 555 detects 2/3Vcc before changing the state of the output so we only have a maximum of 5.5v via a 220R resistor to illuminate the LED. The second circuit requires a very high value electrolytic to produce the same effect.
 
 
UP/DOWN FADING LED-2
The circuit fades the LED ON and OFF at an equal rate. The 470k charging and 47k discharging resistors have been chosen to create equal on and off times.

 
BIKE TURNING SIGNAL
This circuit
can be used to indicate left and right turn on a motor-bike. Two identical circuits will be needed, one for left and one for right.                                    
 
POLICE LIGHTS
These three circuits flash the left LEDs 3 times then the right LEDs 3 times, then repeats. The only difference is the choice of chips.


 
LED DICE with Slow Down
This circuit produces a random number from 1 to 6 on LEDs that are similar to the pips on the side of a dice. When the two TOUCH WIRES are touched with a finger, the LEDs flash very quickly and when the finger is removed, they gradually slow down and come to a stop. LED Dice with Slow Down kit is available from Talking Electronics.


 
ROULETTE
This circuit creates a rotating LED that starts very fast when a finger touches the TOUCH WIRES. When the finger is removed, the rotation slows down and finally stops. 
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DICE TE555-4


This circuit uses the latest 
TE555-4 DICE chip from Talking Electronics. This 8-pin chip is available for $2.50 and drives a 7-Segment display. The circuit can be assembled on proto-type board.
 
LED FX TE555-5


This circuit uses the latest 
TE555-5 LED FX chip from Talking Electronics. This 8-pin chip is available for $2.50 and drives 3 LEDs. The circuit can be assembled on matrix board.
The circuit produces 12 different sequences including flashing, chasing, police lights and flicker.
It also has a feature where you can create your own sequence and it will show each time the chip is turned on. 



SOLAR GARDEN LIGHT
This is the circuit in a $2.00 Solar Garden Light.
The circuit illuminates a white LED from a 1.2v rechargeable cell.
SOLAR TRACKER
This circuit is a SOLAR TRACKER. It uses green LEDs to detect the sun and an H-Bridge to drive the motor. A green LED produces nearly 1v but only a fraction of a milliamp when sunlight is detected by the crystal inside the LED and this creates an imbalance in the circuit to drive the motor either clockwise or anticlockwise. The circuit will deliver about 300mA to the motor.   


BATTERY MONITOR  MkI
A very simple battery monitor can be made with a dual-colour LED and a few surrounding components. The LED produces orange when the red and green LEDs are illuminated.
The following circuit turns on the red LED below 10.5v
The orange LED illuminates between 10.5v and 11.6v.
The green LED illuminates above 11.6v
BATTERY MONITOR  MkII
This battery monitor circuit uses 3 separate LEDs.
The red LED turns on from 6v to below 11v.
It turns off above 11v and
The orange LED illuminates between 11v and 13v.
It turns off above 13v and
The green LED illuminates above 13v
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LOW FUEL INDICATOR
This circuit
has been designed from a request by a reader. He wanted a low fuel indicator for his motorbike. The LED illuminates when the fuel gauge is 90 ohms. The tank is empty at 135 ohms and full at zero ohms. To adapt the circuit for an 80 ohm fuel sender, simply reduce the 330R to 150R. (The first thing you have to do is measure the resistance of the sender when the tank is amply.)

LED ZEPPELIN
 
The game consists of six LEDs and an indicator LED that flashes at a rate of about 2 cycles per second. A push button is the "Operations Control" and by carefully pushing the button in synchronisation with the flashing LED, the row of LEDs will gradually light up.
But the slightest mistake will immediately extinguish one, two or three LEDs. The aim of the game is to illuminate the 6 LEDs with the least number of pushes.
We have sold thousands of these kits. It's a great challenge.

THE DOMINO EFFECT      

Here's a project with an interesting name. The original design was bought over 40yearsa ago, before the introduction of the electret microphone. They used a crystal earpiece.
We have substituted it with a piezo diaphragm and used a quad op-amp to produce two building blocks. The first is a high-gain amplifier to take the few millivolts output of the piezo and amplify it sufficiently to drive the input of a counter chip. This requires a waveform of at least 6v for a 9v supply and we need a gain of about 600.
The other building block is simply a buffer that takes the high-amplitude waveform and delivers the negative excursions to a reservoir capacitor (100u electrolytic). The charge on this capacitor turns on a BC557 transistor and this effectively takes the power pin of the counter-chip to the positive rail via the collector lead.
The chip has internal current limiting and some of the outputs are taken to sets of three LEDs. 
The chip is actually a counter or divider and the frequency picked up by the piezo is divided by 128 and delivered to one output and divided by over 8,000 by the highest-division output to three more LEDs The other lines have lower divisions.
This creates a very impressive effect as the LEDs are connected to produce a balanced display that changes according to the beat of the music.
The voltage on the three amplifiers is determined by the 3M3 and 1M voltage-divider on the first op-amp. It produces about 2v. This makes the output go HIGH and it takes pin 2 with it until this pin see a few millivolts above pin3. At this point the output stops rising.
Any waveform (voltage) produced by the piezo that is lower than the voltage on pin 3 will make the output go HIGH and this is how we get a large waveform.
This signal is passed to the second op-amp and because the voltage on pin 6 is delayed slightly by the 100n capacitor, is also produces a gain.
When no signal is picked up by the piezo, pin 7 is approx 2v and pin 10 is about 4.5v.  Because pin 9 is lower than pin 10, the output pin 8 is about 7.7v (1.3v below the supply rail) as this is as high as the output will go - it does not go full rail-to-rail.
The LED connected to the output removes 1.7v, plus 0.6v between base and emitter and this means the transistor is not turned on. 
Any colour LEDs can be used and a mixture will give a different effect.
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10 LED CHASER
Here's an interesting circuit that creates a clock pulse for a 4017 from a flashing LED. The flashing LED takes almost no current between flashes and thus the clock line is low via the 1k to 22k resistor. When the LED flashes, the voltage on the clock line is about 2v -3v below the rail voltage (depending on the value of the resistor) and this is sufficient for the chip to see a HIGH.

(circuit designed on 9-10-2010)
   
Emergency PHONE-LINE LIGHT
Here's a project that uses the phone line to illuminate a set of white LEDs.
The circuit delivers a current of 4.5mA as any current above 10mA will be detected by the exchange as the hand-set off the hook. 
Be warned: This type of circuit is not allowed as it uses the energy from the phone line (called "leeching") and may prevent the phone from working.
 
EQUAL BRIGHTNESS
A 2-leaded dual colour LED can be connected to the outputs of a microcontroller and the brightness can be equalized by using the circuits shown.
 
FLICKERING LED
A Flickering LED is available from eBay and some electronics shops.
It can be connected to a supply from 2v to 6v and needs an external resistor when the supply is above 3v. The LED has an internal circuit to create the flickering effect and limit the current. We suggest adding a 150R resistor when the supply is above 3v and up to 6v. Above 6v, the current-limit resistor should be increased to 220R for 9v and 330R for 12v.
You can connect the flickering LED to an ordinary LED and both will flicker. Here are some arrangements:

The Pulse-Width Modulation to activate the flickering can be observed on an oscilloscope by connecting the probe across the LED. It is a very complex waveform. It is approx 1v in amplitude and approx 15 x 1kHz pulses to create each portion of the on-time, something like this:
The pulses vary in width to create a brighter illumination.

 
CONSTANT-CURRENT 7805 DRIVES  1 WATT LED
The circuit can be reduced to 2 components:


The 7805 can be converted into a content-current device by connecting a resistor as shown above.
We will take the operation of the circuit in slow-motion to see how it works.
As the 12v rises from 0v, the 7805 starts to work and when the input voltage is 4v, the output is 1v as a minimum of 3v is lost across the 7805. The voltage rises further and when the output is 5v, current flows through the 15R resistor and illuminates the LED. The LED starts to illuminate at 3.4v and the voltage across the 15R at the moment is 1.6v and the output current will be 100mA. The input voltage keeps rising and now the output voltage is 7v. The current through the LED increases and now the voltage across the LED is 3.5v. The voltage across the 15R is 3.5v and the current is 230mA.
The input voltage keeps rising and the output voltage is now 8.6v The current through the LED increases and the voltage across the LED is now 3.6v. The voltage across the 15R is 5v and the current is 330mA. The input voltage keeps rising but a detector inside the 7805 detects the output voltage is exactly 5v above the common and the output voltage does not rise any more. The input voltage can rise above 13v, 14v  . . . . 25v or more but the output voltage will not rise.
If the output voltage rises, more current will be delivered to the LED and the voltage across the 15R will increase. The 7805 will not allow this to happen.
The LED will have 3.6v across it. The 15R will have 5v across it and the output will be 8.6v. The input voltage will have to be at least 12.6v for the 7805 to operate.
                                         to 
1-WATT LED - very good design

Circuit takes 70mA on LOW brightness and 120mA on HIGH Brightness
This circuit has been specially designed for a 6v rechargeable battery or 5 x 1.2v NiCad cells. Do not use any other voltage.
It has many features:
The pulse-operation to the two 1-watt LEDs delivers a high current for a short period of time and this improves the brightness.
The circuit can drive two 1-watt LEDs with extremely good brightness and this makes it more efficient than any other design.
The circuit is a two-transistor high-frequency oscillator and it works like this:
The BD139 is turned ON via the base, through the white LED and two signal diodes and it amplifies this current to appear though the collector-emitter circuit. This current flows though the 1-watt LED to turn it ON and also through the 30-turn winding of the inductor. At the same time the current through the 10R creates a voltage-drop and when this voltage rises to 0.65v, the BC547 transistor starts to turn ON. This robs the base of the BD139 of "turn-on voltage" and the current through the inductor ceases to be expanding flux, but stationary flux.
The 1n capacitor was initially pushing against the voltage-rise on the base of the BC547 but it now has a reverse-effect of allowing the BC547 to turn ON.
This turns off the BD139 a little more and the current through the inductor reduces.
This creates a collapsing flux that produces a voltage across the coil in the opposite direction. This voltage passes via the 1n to turn the BC547 ON and the BD139 is fully turned OFF.
The inductor effectively becomes a miniature battery with negative on the lower LED and positive at the anode of the Ultra Fast diode. The voltage produced by the inductor flows through the UF diode and both 1-watt LEDs to give them a spike of high current. The circuit operates at approx 500kHz and this will depend on the inductance of the inductor.
The circuit has about 85% efficiency due to the absence of a current-limiting resistor, and shuts off at 4v, thus preventing deep-discharge of the rechargeable cells or 6v battery.
The clever part of the circuit is the white LED and two diodes. These form a zener reference to turn the circuit off at 4v. The 10k resistor helps too.
The circuit takes 70mA on low brightness and 120mA on HIGH brightness via the brightness-switch.
The LEDs actually get 200mA pulses of current and this produces the high brightness.

The Inductor
The coil or inductor is not critical. You can use a broken antenna rod from an AM radio (or a flat antenna slab) or an inductor from a computer power supply. Look for an inductor with a few turns of thick wire (at least 30) and you won't have to re-wind it.


The cost of surplus is from 10 cents to 50 cents, but you are sure to find something from a computer power supply.
Pick an inductor that is about 6mm to 10mm diameter and 10mm to 15mm high. Larger inductor will not do any damage. They simply have more ferrite material to store the energy and will not be saturated. It is the circuit that delivers the energy to the inductor and then the inductor releases it to the LEDs via the high speed diode. 

IMPROVEMENT
By using the following idea, the current reduces to 90mA and 70mA and the illumination over a workbench is much better than a single high-power LED. It is much brighter and much nicer to work under.
Connect fifteen 5mm LEDs in parallel (I used 20,000mcd LEDs) by soldering them to a double-sided strip of PC board, 10mm wide and 300mm long. Space them at about 20mm. I know you shouldn't connect LEDs in parallel, but the concept works very well in this case. If some of the LEDs have a characteristic high voltage and do not illuminate very brightly, simply replace them and use them later for another strip.
You can replace one or both the 1-watt LEDs with a LED Strip, as shown below:

No current-limit resistor.  .  . why isn't the LED damaged?
Here's why the LED isn't damaged:
When the BD139 transistor turns ON, current flows through the LEDs and the inductor. This current gradually increases due to the gradual turning-on of the transistor and it is also increasing through the inductor. The inductor also has an effect of slowing-down the "in-rush" of current due to the expanding flux cutting the turns of the coil, so there is a "double-effect" on avoiding a high initial current.  That's why there is little chance of damaging the LEDs.
When it reaches 65mA, it produces a voltage of .065 x 10 = 650mV across the 10R resistor, but the 1n is pushing against this increase and it may have to rise to 150mA to turn on the BC547. LEDs can withstand 4 times the normal current for very short periods of time and that's what happens in this case. The BD139 is then turned off by the voltage produced by the inductor due to the collapsing magnetic flux and a spike of high current is passed to the LEDs via the high speed diode. During each cycle, the LEDs receive two pulses of high current and this produces a very high brightness with the least amount of energy from the supply. All the components run "cold" and even the 1-watt LEDs are hardly warm.

Charging and Discharging
This project is designed to use all your old NiCad cells and mobile phone batteries.
It doesn't matter if you mix up sizes and type as the circuit takes a low current and shuts off when the voltage is approx 4v for a 6v pack.
If you mix up 600mA-Hr cells with 1650mA-Hr, 2,000mA-Hr and 2,400mA-Hr, the lowest capacity cell will determine the operating time.
The capacity of a cells is called "C."
Normally, a cell is charged at the 14 hour-rate.
The charging current is 10% of the capacity. For a 600mA-Hr cell, this is 60mA. In 10 hours it will be fully charged, but charging is not 100% efficient and so we allow another 2 to 4 hours.
For a 2,400mA-Hr cell, it is 240mA. If you charge them faster than 14-hr rate, they will get HOT and if they get very hot, they may leak or even explode. But this project is designed to be charged via a solar panel using 100mA to 200mA cells, so nothing will be damaged.
Ideally a battery is discharged at C/10 rate. This means the battery will last 10 hours and for a 600mA-Hr cell, this is 60mA. If you discharge it at the "C-rate," it will theoretically last 1 hour and the current will be 600mA. But at 600mA, the cells may only last 45 minutes. If you discharge is at C/5 rate, it will last 5 hours.
Our project takes 120mA so no cell will be too-stressed. A 600mA-Hr cell will last about  4-5 hours, while the other cells will last up to 24 hours.   Try to keep the capacity of each cell in a "battery-pack" equal.

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FL
The
 


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FLASHING RAILROAD LIGHTS
This circuit flashes two red LEDs for a model railway crossing.
 
                 
If 3rd band is gold, Divide by 10
If 3rd band is silver, Divide by 100
(to get 0.22ohms etc)

Not copyright        You can copy and use anything for your own personal use. 


                                         555സര്‍ക്യൂട്ടുകള്‍ 
THE 555 PINS
Here is the identification for each pin:

When drawing a circuit diagram, always draw the 555 as a building block, as shown below with the pins in the following locations. This will help you instantly recognise the function of each pin:



Pin 1 GROUND.  Connects to the 0v rail.
Pin 2 TRIGGER. Detects 1/3 of rail voltage to make output HIGH. Pin 2 has control over pin 6. If pin 2 is LOW, and pin 6 LOW,  output goes and stays HIGH. If pin 6 HIGH, and pin 2 goes LOW, output goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and will trigger with about 1uA.
Pin 3 OUTPUT. (Pins 3 and 7 are "in phase.") Goes HIGH (about 2v less than rail) and LOW (about 0.5v less than 0v) and will deliver up to 200mA.
Pin 4 RESET. Internally connected HIGH via 100k. Must be taken below 0.8v to reset the chip.
Pin 5 CONTROL. A voltage applied to this pin will vary the timing of the RC network (quite considerably). 
Pin 6 THRESHOLD.  Detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. This pin has a very high impedance (about 10M) and will trigger with about 1uA.
Pin 7 DISCHARGE. Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must be HIGH. If pin 2 is HIGH, pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW.  (Pins 7 and 3 are "in phase.") Pin 7 is equal to pin 3 but pin 7 does not go high - it goes OPEN.  But it goes LOW and will sink about 200mA. You can connect pin 7 to pin 3 to get a slightly better SINK capability from the chip.
Pin 8 SUPPLY. Connects to the positive rail.


555 in a circuit - note the circle on the chip to identify pin 1
This is sometimes called a "push-out-pin" (hole) and sometimes
it has no importance. But in this case it represents pin 1. 
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THE SIMPLEST 555 OSCILLATOR
The simplest 555 oscillator takes output pin 3 to capacitor C1 via  resistor R1.
When the circuit is turned on, C1 is uncharged and output pin 3 is HIGH. C1 charges via R1 and when Pin 6 detects 2/3 rail voltage, output pin 3 goes LOW. R1 now discharges capacitor C1 and when pin 2 detects 1/3 rail voltage, output pin 3 goes HIGH to repeat the cycle.
The amount of time when the output is HIGH is called the MARK and the time when the output is LOW is called the SPACE.
In the diagram, the mark is the same length as the space and this is called 1:1 or 50%:50%.
If a resistor and capacitor (or electrolytic) is placed on the output, the result is very similar to a sinewave.

C1 to POSITIVE RAIL
C1 can be connected to the positive rail. This is not normal practice, however it does work.
The output frequency changes when the capacitor is changed from the negative rail to the positive rail. Theoretically the frequency should not change, but it does, and that's why you have to check everything.  The frequency of operation in this arrangement is different to connecting the components via pin7 because pin3 does not go to full rail voltage or 0v. This means all the output frequencies are lower than those in the "555 Frequency Calculator."  
The table shows the frequency for the capacitor connected to the 0v rail and 12v rail:




C1 to 0v rail
C1 to 12v rail
1k 1n 505kHz 1k 1n 255kHz
1k 10n 115kHz 1k 10n 130kHz
1k 100n 23kHz 1k 100n 16kHz
10k 1n 112kHz 10k 1n 128kHz
10k 10n 27kHz 10k 10n 16kHz
10k 100n 3700Hz 10k 100n 1600Hz

 

CHANGING THE MARK-SPACE RATIO
This ratio can be altered by adding a diode and resistor as shown in the following diagrams. In the first diagram, the 555 comes ON ("fires-up") with pin 3 low and pin 2 immediately detects this low and makes pin 3 HIGH. The 10n is quickly charged via the diode and 4k7 and this is why the MARK is "short." When the capacitor is 2/3Vcc, pin 6 detects a HIGH and the output of the 555 goes LOW. The 10n is discharged via the 33k and this creates the long-duration SPACE (LOW). The second diagram creates a long-duration HIGH:

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HOW TO REMEMBER THE PINS:
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THE FASTEST 555 OSCILLATOR
The highest frequency for a 555 can be obtained by connecting the output to pins 2 and 6. This arrangement takes about 5mA and produces an output as shown. The max frequency will depend on the supply voltage, the manufacturer, and the actual type of 555 chip.


View the output on a CRO. Our 555 "Test Chip" produced a frequency of 300kHz at 5v and also at 12v.  (CMOS versions will operate at a higher frequency.) Note the very short LOW TIME.
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INSIDE THE 555

Note: Pin 7 is "in phase" with output Pin 3 (both are low at the same time).
Pin 7 "shorts" to 0v via a transistor. It is pulled HIGH via R1.
Maximum supply voltage 16v - 18v
Current consumption approx 10mA
Output Current sink @5v = 5 - 50mA     @15v = 50mA
Output Current source @5v = 100mA     @15v = 200mA
Maximum operating frequency 300kHz - 500kHz

Faults with Chip:
Consumes about 10mA when sitting in circuit
Output voltage can be up to 2.5v less than rail voltage
Output can be  0.5v to 1.5v above ground
Sources up to 200mA
Some chips sink only 50mA,  some will sink 200mA

A NE555 was tested at 1kHz, 12.75v rail and 39R load.
The Results:
Output voltage 0.5v low, 11.5v high at output current of 180mA
The "test chip" performance was excellent.
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HOW TO USE THE 555
There are many ways to use the 555. They can be used in hundreds of different circuits to do all sorts of clever things. They can also be used as three different types of oscillators:
(a) Astable Multivibrator  - constantly oscillates
For frequencies above 1 cycle per second, it is called an oscillator (multivibrator or square wave oscillator).
For frequencies below 1 cycle per second it is called a TIMER or DELAY. 

(b) Monostable  - changes state only once per trigger pulse - also called a ONE SHOT
(c) Voltage Controlled Oscillator - called a VCO.
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THE ASTABLE (or FREE RUNNING)
MULTIVIBRATOR

The capacitor C charges via R1 and R2 and when the voltage on the capacitor reaches 2/3 of the supply, pin 6 detects this and pin 7 connects to 0v. The capacitor discharges through R2 until its voltage is 1/3 of the supply and pin 2 detects this and turns off pin 7 to repeat the cycle.
The top resistor is included to prevent pin 7 being damaged as it shorts to 0v when pin 6 detects 2/3 rail voltage.
Its resistance is small compared to R2 and does not come into the timing of the oscillator.

The  following graph applies to the Astable circuit:

Using the graph:
Suppose R1 = 1k, R2 = 10k and C = 0.1u (100n).
Using the formula on the graph, the total resistance  = 1 + 10 + 10 = 21k
The scales on the graph are logarithmic so that 21k is approximately near the "1" on the 10k. Draw a line parallel to the lines on the graph and where it crosses the 0.1u line, is the answer. The result is approx 900Hz.

Suppose R1 = 10k, R2 = 100k and C = 1u
Using the formula on the graph, the total resistance  = 10 + 100 + 100 = 210k
The scales on the graph are logarithmic so that 210k is approximately near the first "0" on the 100k. Draw a line parallel to the lines on the graph and where it crosses the 1u line, is the answer. The result is approx 9Hz.

The frequency of an astable circuit can also be worked out from the following formula:

 frequency =            1.4          
(R1 + 2R2) × C
555 astable frequencies
C R1 = 1k
R2 = 6k8
R1 = 10k
R2 = 68k
R1 = 100k
R2 = 680k
0.001µ 100kHz 10kHz 1kHz
0.01µ 10kHz 1kHz 100Hz
0.1µ 1kHz 100Hz 10Hz
100Hz 10Hz 1Hz
10µ 10Hz 1Hz 0.1Hz
0.001µ = 1n
0.01µ   = 10n
0.1µ     = 100n
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HIGH FREQUENCY OSCILLATORS
360kHz is the absolute maximum as the 555 starts to malfunction with irregular bursts of pulses above this frequency. To improve the performance of the oscillator, a 270R and 1n can be added as shown in the second circuit:
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LOW FREQUENCY OSCILLATORS - called TIMERS
If the capacitor is replaced with an electrolytic, the frequency of oscillation will reduce. When the frequency is less than 1Hz, the oscillator circuit is called a timer or "delay circuit." The 555 will produce delays as long as 30 minutes but with long delays, the timing is not accurate.
 
555 Delay Times:
C R1 = 100k
R2 = 100k
R1 = 470k
R2 = 470k
R1 = 1M
R2 = 1M
10µ 2.2sec 10sec 22sec
100µ 22sec 100sec 220sec
470µ 100sec 500sec 1000sec
The following circuits show a 1-5 minute timer and 10 minute timer:
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CMOS 555
A low power version of the 555 is available from many manufacturers and basically it is a CMOS version of the TTL 555 device.
The CMOS 555 has the same pinouts as the TTL version and can be fitted into the same 8 pin socket but if the circuit needs more current than can be supplied by the CMOS version, it will not produce the same results.
It is the low current capability of the CMOS version that will be the major reason why you cannot directly replace the TTL version with the CMOS version.
It will operate from 1v (only some manufacturers) to 15v and will work up to 3MHz in astable mode.
Current consumption @5v is about 250uA (1/4mA)
But the major thing to remember is the output current capability.
At 2v, the chip will only deliver 0.25mA and sink only 1mA.
At 5v, the chip will deliver 2mA and sink only 8mA

At 12v the chip will
deliver 10mA and sink 50mA
At 15v the chip will deliver 100mA and sink 100mA

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SQUARE WAVE OSCILLATOR KIT:

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Bi-stable or "Latch" or "2-state" 555 The bi-stable 555 has two steady states. SET turns ON the LED and RESET turns the LED off. The 555 comes on in reset mode as Pin2 does not see a LOW to SET the 555.
See also: Divide By Two
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Monostable or "One Shot" or Pulse Extender When the circuit is turned on, the output is LOW and a brief negative pulse on pin 2 will make the output go HIGH for a period of time determined by the value of R and C. If pin 2 is low for longer than this period, the output will remain HIGH while pin 2 is LOW and immediately go LOW when pin 2 goes HIGH.

CIRCUIT OPERATION
When the circuit is turned on, the capacitor is uncharged. Pin 6 sees a LOW and pin 2 sees a HIGH.
Remember: Pin 2 must be LOW to make the output HIGH.
Pin 6 must be HIGH to make the output LOW.
Neither pin is "controlling the chip" at start-up and the chip is designed to output a LOW with these start-up conditions.
In other words, the chip starts in RESET mode. Pin 7 is LOW and the capacitor does not charge.
When pin 2 see a LOW pulse, the chip goes to SET mode and the output goes HIGH. Pin 7 goes OPEN and capacitor C charges via R. When pin 6 sees 2/3 rail voltage, the chip goes to RESET mode with pin 3 and 7 LOW. The capacitor instantly discharges via pin 7 and the circuit waits for a negative pulse on pin 2.
 
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THE 555 AS A VOLTAGE CONTROLLED OSCILLATOR (VCO)
By adjusting the voltage on pin 5, (the CONTROL pin) the frequency of the oscillator can be adjusted quite considerably. See Police Siren for an application.
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THE 555 AS A RAMP GENERATOR
When a capacitor is charged via a constant current, the waveform across it is a ramp.
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FREQUENCY DIVIDER
A 555 can be used to divide a frequency by almost any division.
It works this way:
A 555 is set-up to produce the required output frequency.
Pin 2 is then taken to the input frequency and this turns the 555 into a Monostable Multivibrator.
The circuit will detect a LOW on pin 2  to start the timing cycle and pin 3 will go HIGH. The 555 will not respond to any more pulses on pin 2 until pin 6 detects a HIGH via the charging of the capacitor. The value of C and the 1M pot need to be adjusted to produce the desired results.

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DIVIDE BY 2
A 555 can be used to divide-by-2
When pins 2 and 6 are connected, they detect 1/3 and 2/3 of rail voltage.   When the detected voltage is below 1/3, the output goes HIGH and when the voltage is above 2/3, the output goes LOW.
The push switch detects the output voltage and after a short period of time the electrolytic will charge or discharge and it will be HIGH or LOW.
If the switch is pressed for a short period of time, the output will change. If the switch is kept pressed, the output will oscillate at a low frequency.

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"No-No's"
Here are some mistakes to avoid:
1. Pin 7 gets connected to the 0v rail via a transistor inside the chip during part of the operation of the 555. If the pot is turned to very low resistance in the following circuit, a high current will flow through the pot and it will be damaged:


2. The impedance of the 100u electrolytic will allow a very high current to flow and the chip will get very hot.   Use 10u maximum when using 8R speaker. (The temp of the chip will depend on the frequency of the circuit.)


3. The reset pin (pin 4) is internally tied HIGH via approx 100k but it should not be left floating as stray pulses may reset the chip.


4. Do not draw 555 circuits as shown in the following diagram. Keep to a standard layout so the circuit is easy to follow.

5. Here's an example from the web. It takes a lot of time to work out what the circuit is doing:

The aim it to lay-out a circuit so that it shows instantly what is happening. That's why everything must be in recognised locations.

Here is the corrected circuit: From this diagram it is obvious the circuit is an oscillator (and not a one-shot etc).


6. Don't use high value electrolytics and high resistances to produce long delays. The 555 is very unreliable with timing values above 5-10 minutes. The reason is simple. The charging current for the electrolytic is between 1 - 3 microamp in the following diagram (when the electro is beginning to charge) and drops to less than 1 microamp when the electro is nearly charged.
If the leakage of the electro is 1 microamp, it will never fully charge and the 555 will never "time-out."


7. Do not connect a PNP to the output of a 555 as shown in the following diagram. Pin 3 does not rise high enough to turn the transistor OFF and the current taken by the circuit will be excessive. Use an NPN driver.

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555's
Here is a list of 555's from different manufacturers plus the range of low voltage, low current 555's. The normal 555 is called a TTL or Transistor-Transistor-Logic chip and it consumes about 10mA when "sitting and doing nothing." It will work from 4v to 18v.
A low current version is available from the list below, (called a CMOS version) and consumes about 10uA to 100uA. Some of these chips work from 1.5v to 15v (ZSCT1555 = 9v max) but they can sink and source only about 100mA (less than 30mA at 2v).
The 555 is the cheapest and the others cost about double.
The normal 555 oscillates up to 300kHz. A CMOS version can oscillate to 3MHz.
You need to know the limitations as well as the advantages of these chips before substituting them for the normal 555:

Manufacturer Model Remark
Custom Silicon Solutions CSS555/CSS555C CMOS from 1.2V, IDD < 5uA
ECG Philips ECG955M
Exar XR-555
Fairchild Semiconductor NE555/KA555
Harris HA555
IK Semicon ILC555 CMOS from 2V
Intersil SE555/NE555/ICM7555
Lithic Systems LC555
Maxim ICM7555 CMOS from 2V
Motorola MC1455/MC1555
National Semiconductor LM1455/LM555/LM555C
National Semiconductor LMC555 CMOS from 1.5V
NTE Sylvania NTE955M
Raytheon RM555/RC555
RCA CA555/CA555C
STMicroelectronics NE555N/ K3T647
Talking Electronics TE555-1, -2, -3, -4
Texas Instruments SN52555/SN72555; TLC555 CMOS from 2V
Zetex ZSCT1555 down to 0.9V      (9v max)
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REPLACING A 556 WITH TWO 555's
Here is a handy reference to replace a 556 dual timer with two 555's:


The table shows the pin numbering for each timer:
 



555 556 - Timer 1 556 - Timer 2
Ground (–) 1 7
7
Trigger 2 6 8
Output 3 5 9
Reset 4 4 10
Control 5 3 11
Threshold 6 2 12
Discharge 7 1 13
 Vcc (+) 8 14 14
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SUBSTITUTING A 555 - part 1
Although a 555 is a wonderful chip, it may not be the best choice for some applications.
You may require an oscillator frequency higher than 1MHz, or a very low quiescent current. You may also need 4 or more 555's to get the timing and delays you require.
Here are some circuits to help you substitute a 555.

The 74c14 IC contains 6 Schmitt Trigger gates and each gate can be used to replace a 555 in SOME circuits. The voltage for a 74c14 is 3v to 15v. Maximum output current per gate is 15mA. Max frequency of operation: 2MHz - 5MHz. Quiescent current is 1uA if all inputs are 0v or rail voltage. 





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SUBSTITUTING A 555 - part 2
If you need a special function or special effect, it may take 2, 3 or more 555's to do the job. The 74c14 has 6 gates and can create 6 "building blocks."
Here are some circuits to show its versatility:

2 MINUTE TIMER
The relay is energized for a short time, 2 minutes after the push-button is pressed. The push-button produces a brief LOW on pin 1, no matter how long it is pushed and this produces a pulse of constant length via the three components between pin 2 and 3.
This pulse is long enough to fully discharge the 100u timing electrolytic on pin 5.
The 100k and electrolytic between pins 6 and 9 are designed to produce a brief pulse to energize the relay.




OUTPUT AFTER 2 MINUTES
Here is another very similar circuit. Use either the active HIGH or Active LOW switch and if the Active LOW switch is used, do not connect the parts or gate between pins 1 and 2 to the rest of the circuit.


PULSER
The 74c14 can be used for lots of different circuits. In the following design, the output produces 3mS pulses every second. The circuit is adjustable to a wide range of requirements.

TRIGGER TIMER
The next design interfaces a "Normally Open" and "Normally Closed" switch to a delay circuit.
The feedback diode from the output prevents the inputs re-triggering the timer (during the delay period) so that a device such as a motor, globe or voice chip can be activated for a set period of time.
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BUILDING THE CIRCUITS
The fastest way to put a circuit together is on BREADBOARD. The cheapest and best bread-board has power-rails and sets of "tie-points" or "holes" as shown in this photo:

Connect the components with hook-up wire (called jumpers) by stripping the ends to expose the wire at both ends. Or you can use 0.5mm tinned copper wire (make sure the jumpers do not touch each other).
Do not cut the leads of the components as you may want long leads on another project.

Neatness is not important. The important thing is to build as many circuits as possible as each one will help you understand how the 555 works and how the external circuitry modifies the signal to produce the resulting effect. There is a point-to-learn in every circuit.
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POWERING A PROJECT
The safest way to power a project is with a battery. Use 4 x AA cells in a holder or a 9v battery if you only want to use the project for a short period of time.
If you want to use a 555 project for a long period of time you will need a "power supply."
The safest power supply is a Plug Pack (wall-wort, wall wart,
wall cube, power brick, plug-in adapter, adapter block, domestic mains adapter, power adapter, or AC adapter). The adapter shown in the diagram has a switchable output voltage: 3v, 6v, 7.5v, 9v, 12v) DC and is rated at 500mA. The black lead is negative and the other lead with a white stripe (or a grey lead with a black stripe) is the positive lead.
This is the safest way to power a project as the insulation (isolation) from the mains is provided inside the adapter and there is no possibility of getting a shock.
The rating "500mA" is the maximum the Plug Pack will deliver and if your circuit takes just 50mA, this is the current that will be supplied. Some pluck packs are rated at 300mA or 1A and some have a fixed output voltage. All these plug packs will be suitable.
Some Plug Packs are marked "12vAC."  This type of plug pack is not suitable for these circuits as it does not have a set of diodes and electrolytic to convert the AC to DC. All the circuits in this eBook require DC.

PROJECTS
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TOY ORGAN
 This circuit produces a tone according to the button being pressed. Only 1 button can be pressed at a time, that's why it is called a monophonic organ. You can change the 1k resistors to produce a more-accurate scale.
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TICKING BOMB
This circuit sound just like a ticking bomb.
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METAL DETECTOR
This circuit detects metal and also magnets. When a magnet is brought close to the 10mH choke, the output frequency changes.
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UNEVEN CLICKS
This circuit produces two clicks then a short space before two more clicks etc. Changing the voltage on pin, 5 via the diode, adjusts the timing of the chip.
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FLASHING RAILROAD LIGHTS
This circuit flashes two red LEDs for a model railway crossing.
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SCREAMER
This circuit will produce an ear-piercing scream, depending on the amount of light being detected by the Light Dependent Resistor.
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LASER RAY
This circuit produces a weird "Laser Ray" sound and flashes a white LED at approx 5Hz:
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LED DIMMER
This circuit will adjust the brightness of one or more LEDs from 5% to 95%.
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MOTOR PWM
See also: PWM Controller
The speed of a motor can be adjusted by this circuit, from 5% to 95%.
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PWM
See also: PWM Controller
The output of these circuits can be adjusted from 5% to 95%.
 
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VOLTAGE DOUBLER
A voltage higher than the supply can be created by a "Charge-Pump" circuit created with a 555, diodes and capacitors as shown in the following circuit. The output will deliver about 50mA
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NEGATIVE VOLTAGE
A negative supply can be produced by a "Charge-Pump" circuit created with a 555, diodes and capacitors as shown in the following circuit. The output will deliver about 50mA.
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555 AMPLIFIER
The 555 can be used as an amplifier. It operates very similar to pulse-width modulation. The component values cause the 555 to oscillate at approx 66kHz and the speaker does not respond to this high frequency.  Instead it responds to the average CD value of the modulated output and demonstrates the concept of pulse-width modulation. The chip gets very hot and is only for brief demonstrations.
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LIGHT DETECTOR
This circuit detects light falling on the Photo-cell (Light Dependent Resistor) to turn on the 555 and create a tone that is delivered to the speaker. Pin 4 must be held below 0.7v to turn the 555 off. Any voltage above 0.7v will activate the circuit. The adjustable sensitivity control is needed to set the level at which the circuit is activated.  When the sensitivity pot is turned so that it has the lowest resistance (as shown in red), a large amount of light must be detected by the LDR for its resistance to be low. This produces a voltage-divider made up of the LDR and 4k7 resistor. As the resistance of the LDR decreases, the voltage across the 4k7 increases and the circuit is activated.
When the sensitivity control is taken to the 0v rail, its resistance increases and this effectively adds resistance to the 4k7. The lower-part of the voltage-divider now has a larger resistance and this is in series with the LDR. Less light is needed on the LDR for it to raise the voltage on pin 4 to turn the 555 on. 
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DARK DETECTOR
When the level of light on the photo-cell decreases, the 555 is activated. Photo-cells (Photo-resistors) have a wide range of specifications. Some cells go down to 100R in full sunlight while others only go down to 1k. Some have a HIGH resistance of between 1M and others are 10M in total darkness. For this circuit, the LOW resistance (the resistance in sunlight) is the critical value.
More accurately, the value for a particular level of illumination, is the critical factor. The sensitivity pot adjusts the level at which the circuit turns on and allows almost any type of photo-cell to be used.
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FLIP FLOP and MEMORY CELL
When output pin 3 is HIGH, the 220n charges through the 220k to 6v. When pin 3 is LOW, the 220n discharges through the 220k to 0v. Pressing the switch upsets the 3v created by the two 10k voltage dividers, triggering the flip flop inside the 555 and changing the state of the output from HIGH to LOW or vice-versa. The output of the 555 drives a transistor to turn a globe on and off.
The second circuit is a Memory cell and is the basis of the memory in a computer. The SET button turns on the globe and the RESET button turns the globe off.
It works like this: When the circuit is turned on, pin 6 does not see a high and pin 2 does not see a low, so the 555 starts in reset mode.
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CAR TACHOMETERA 555 is configured as a monostable or one shot in this project. The period of the 555 is determined by the 47k and the capacitor from pin 6 to ground (100n). Time "T" = 1.1 RC  or  1.1 X 50,000 X 0.1 X10 -6  =  0.0055 or 5.5 mS (milli-seconds).
The 555 receives trigger pulses from the distributor points. These are limited by the 1k and 5v zener diode. These are AC coupled to the trigger input through the 100n coupling capacitor. The 50mA meter receives pulses of current through the 200k pot to show a reading.

Integration of the current pulses produces a visible indication of the cars engine speed on the 0-1mA meter.
Supply is taken from the cars 12v system and for the 555 it is reduced to a regulated 9v by the 15 ohm resistor in conjunction with the 9v zener diode. Note: the 10u electrolytic must be placed physically as close as possible to supply pin 8.
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FREQUENCY METER This circuit will detect audio frequencies and display them on a meter (actually called a "movement"). Connect the circuit to the output of an amplifier. It is best to detect one frequency at a time. Integration of the audio frequency produces a visible indication on the 0-1mA meter.
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SERVO TESTER
This circuit can be used to manually turn a servo clockwise and anti-clockwise.  By pushing the forward or reverse button for a short period of time you can control the rotation of the servo. It will also test a servo.
Here is a photo of a kit from Cana Kit for  $10.00 plus postage (it is a slightly different circuit) and a motor and gearbox, commonly called a "servo." The output shaft has a disk or wheel containing holes. A linkage or push-rod is fitted to a hole and when the disk rotates, the shaft is pushed and pulled. The shaft only rotates about 180° to actuate flaps or ailerons etc.

A pot can be used to control the position of the servo by using the following circuit. It produces a positive pulse between about 0.9 milliseconds and 2.1 milliseconds. The off period between pulses is about 40 milliseconds. This can be shortened by reducing the value of the 3M3 resistor.
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USELESS MACHINE
Here is a fun project using a servo motor and a circuit similar to the SERVO TESTER project above. It is available on the INSTRUCTABLES website.  Before you do anything, watch the video:
http://www.instructables.com/id/The-Most-Useless-Machine



The Instructables website contains all the construction details.  The circuit diagram shows the toggle switch is clicked towards the lid of the box and this starts the servo motor. The servo has an arm that comes out of the box and clicks the switch to the opposite position. This reverses the servo and the arm retreats into the box and hits the limit switch that turns the circuit off.
You may have to adjust the value of the 15k and 27k resistors and you will also see other videos on the Instructables website to help you with construction.
 As the website says: "It's the most useless invention, but everyone wants one."
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TRANSISTOR TESTER

The 555 operates at 2Hz. Output pin 3 drives the circuit with a positive then zero voltage. The other end of the circuit is connected to a voltage divider with the mid-point at approx 4.5v. This allows the red and green LEDs to alternately flash when no transistor is connected to the tester.
If a good transistor is connected, it will produce a short across the LED pair when the voltage is in one direction and only one LED will flash. If the transistor is open, both LED’s will flash and if the transistor is shorted, neither LED will flash.
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SWITCH DEBOUNCE
The output goes HIGH for 100mS when the switch is pressed.
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INCREASING OUTPUT
CURRENT
The
555 will deliver 200mA to a load but the chip gets extremely hot (12v supply). The answer is to use a buffer transistor.
For 200mA, use a BC547 or equivalent.
For 500mA use a BC337 or equivalent
For 1A, use a TIP31 or equivalent.
For 3A - 5A use a BD679 or equivalent with heatsink
For 5A to 10A use TIP3055 with heatsink 
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IMPROVING THE SINKING OF A 555
The output of a 555 goes low to deliver current to a load connected as shown in the circuit below. But when the chip is sinking 200mA, pin 3 has about 1.9v on it. This means the chip does not provide full rail voltage to the load.
This can be improved by connecting pin 7 to pin 3. Pin 7 has a transistor that connects it to 0v rail at the same time when pin 3 is LOW. They can both be connected together to improve sinking capability. In this case the low will be 800mV for 200mA instead of 1900mV, an improvement of 1100mV. This will add 1v1 to the load and also make the chip run cooler.
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CONSTANT CURRENT
The
555 will deliver 200mA to a load but this might be too much. You can add a dropper resistor (current limiting resistor) but the current will reduce as the supply voltage drops.
To provide a constant output current to a device such as an IR LED, the following circuit can be used. The current will be constant for any supply voltage but the best range will be 7v to 12v. 
The current is determined according to the value of R. You can use this table:  
R
Current
5R6 100mA
10R 60mA
22R 30mA
47R 15mA
100R 6mA
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INCREASING OUTPUT PUSH-PULL CURRENT
Some 555's do not swing rail-to-rail when 200mA is being delivered and the chip gets very hot when trying to deliver 200mA.
The solution is to add a push-pull output. The following arrangement has been chosen as it swings almost rail-to-rail but two faults need to be addressed.
Both transistors turn on during the brief interval when pin 3 is travelling from high to low or low to high.
This means the two transistors will put a "short" across the power rail.
The addition of the 4R4 will allow a high current to flow but the transistors will not be damaged. In addition, green LEDs on the base of each transistor reduces the time when both transistors are ON.
The animation shows how the transistors are turned on and off and deliver a high current to the load. The animation shows how NPN and PNP transistors follow an input signal in a push -pull arrangement using positive and negative supply rails. This is not the same as our circuit however the basic effect applies. The output is inverse of pin3 but pin3 only needs to deliver 10-50 milliamp and the transistors can deliver 1 amp or more to the load. This allows the 555 to be kept cool.


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DRIVING A BI-COLOUR LED
Some 3-leaded LEDs produce red and green. This circuit alternately flashes a red/green bi-coloured LED:
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BI-POLAR LED DRIVER
Some 2-leaded LEDs produce red and green. These are called Bi-polar LEDs. This circuit alternately flashes a red/green bi-polar LED:
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ZENER DIODE TESTER
This circuit will test zener diodes up to 56v. See Talking Electronics website, left index, 200 Transistor Circuits (circuits 1-100) and go to Zener Diode (making) to see how to make a zener diode and how to create a zener voltage from a combination of zeners.
Place the zener across the terminals in the circuit below and read the value across it with a multimeter set to 50v range.
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WAILING SIREN
By pressing the button, the wailing sound increases. Releasing the button decreases the wailing.
The circuit automatically turns off after about 30 seconds.
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CONTINUITY TESTER
This circuit will detect low resistances and high resistances to produce a tone from the speaker.
It will detect up to 200k and the circuit automatically turns off when the probes are not used.
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MORSE KEYER
This circuit will help you master the art of keying Morse Code:


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STUN GUN - Voltage Multiplier
This circuit produces a very high voltage and care must be used to prevent getting a nasty shock.  The transformer can produce over 1,000v and the 8-stage multiplier can produce up to 20,000v
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12v to 240v INVERTER
This circuit will produce 240v at 50Hz. The wattage will depend on the driver transistors and transformer. 
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170v SUPPLY FOR NIXIE TUBES
This circuit produces approx 170v for Nixie tubes and other neon tubes.  It is a switch-mode boost circuit.
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ACTIVE LOW TRIGGER
This circuit sits LOW and the output goes HIGH when the push-button is pressed. When the normally-closed push button is pressed, it opens and the uncharged 1u will be pulled to nearly 0v rail via the 10k and this will take pin 2 LOW to make output pin 3 HIGH for the duration determined by the 22u and 100k. If the push-switch stays open, the 1u will charge via the 100k and eventually the output of the 555 will go low.
But normally the switch must be pressed for a short period of time so that the timing components (100k and 22u) make the output go HIGH for a short period of time. This circuit is called an ACTIVE LOW TRIGGER
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ACTIVE HIGH TRIGGER
This circuit produces a HIGH output via a HIGH trigger:
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MACHINE GUN
This circuit produces a sound very similar to a machine gun:
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LATCH
This circuit is a LATCH and remains ACTIVE when the push-button has been pressed for an INSTANT and released.
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TOGGLE 555
This circuit will toggle the output each time the switch is pressed. The action cannot be repeated until the 10u charges or discharges via the 100k.
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KNIGHT RIDER
In the Knight Rider circuit, the 555 is wired as an oscillator. It can be adjusted to give the desired speed for the display. The output of the 555 is directly connected to the input of a Johnson Counter (CD 4017). The input of the counter is called the CLOCK line.
The 10 outputs Q0 to Q9 become active, one at a time, on the rising edge of the waveform from the 555. Each output can deliver about 20mA but a LED should not be connected to the output without a current-limiting resistor (330R in the circuit above).
The first 6 outputs of the chip are connected directly to the 6 LEDs and these "move" across the display. The next 4 outputs move the effect in the opposite direction and the cycle repeats. The animation above shows how the effect appears on the display.
Using six 3mm LEDs, the display can be placed in the front of a model car to give a very realistic effect. The same outputs can be taken to driver transistors to produce a larger version of the display.

 
















Here is a simple Knight Rider circuit using resistors to drive the LEDs. This circuit consumes 22mA while only delivering 7mA to each LED. The outputs are "fighting" each other via the 100R resistors (except outputs Q0 and Q5).

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FLASHING INDICATORS
This is a request from Daniel, one of our subscribers.
He needed to flash "turn indicators" using a 555 and a single 20 amp relay. Here is our suggestion. The timing resistor needs to be selected for the appropriate flash-rate. 


Flashing the "TURN INDICATORS"
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TOUCH SWITCH and TOUCH ON-OFF
The Touch Switch circuit will detect stray voltages produced by mains voltages and electrostatic build-up in a room. In the first circuit, pin 2 must see a LOW for the circuit to activate.  If sufficient static voltage is detected by the plate, the chip will change state. If not, you will need to touch the plate and the 0v rail. In the second circuit, two touch plates are provided and the resistance of your finger changes the voltage on pin 2 or 6 to toggle the 555.
 



The circuit can be made 100 times more sensitive by adding a transistor to the front-end as shown in the diagram below:
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SIREN 100dB
This is a very loud siren and if two or more piezo's are located in a room, the burglar does not know where the sound is coming from.
A robber will not stay anywhere with an ear-piercing sound as he cannot hear if someone is approaching.
It's the best deterrent you can get. The "F" contact on the piezo is "feedback" and is not needed in this circuit.
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POLICE SIREN
The Police Siren circuit uses two 555's to produce an up-down wailing sound. The first 555 is wired as a low-frequency oscillator to control the VOLTAGE CONTROL pin 5 of the second 555. The voltage shift on pin 5 causes the frequency of the second oscillator to rise and fall. 
 


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HEE HAW SIREN
Build the circuit and listen. Change the resistors and capacitors to get all sorts of different results.
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RAIN ALARM
This circuit consumes no current until moisture is detected on the rain plate.
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PWM CONTROLLER
See also: PWM

This controller will deliver up to 30 amps and control the motor from 5% to 95%.
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SOLAR TRACKER
Some ideas are simply not suited for a 555. This is one. A solar tracker should consume little or no current when waiting for a the sun to change position. A 555 takes 10mA+ and suitable circuits using other chips will take less than 1mA. That's why we have not designed a 555 circuit.





 
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HULDA CLARK ZAPPER
This is the circuit for Dr. Hulda Clark's Zapper, designed in 2003. The frequency is approximately 30kHz positive offset square wave. It has a red LED light that lights up when the unit is on. Perfect for regular zapping, extended zapping and other Hulda Clark related experiments.
This device is used to
cure, treat and prevent any disease. It will cure anything. Simply hold the two probes (one in each hand)  for 5-10 minutes then rest for 20 minutes, then repeat two more times.  Do this each day and you will be cured. Here is the .pdf of her book: A Cure For All Diseases.   Website: http://clarktestimonials.com/ Hundreds of people have been cured of everything from herpes to AIDS. 
On the other side of the coin is the claim that Dr
Hulda Clark is a complete quack.  Here is a website called: Quackwatch. The second diagram shows the two copper tubes and the circuit in a plastic box. I am still at a loss to see how any energy can transfer from this quack machine, through the skin (50k skin resistance and 9v supply) and zap a bug in your intestine. It's a bit like saying I will kill all the mice in a haystack by stabbing the stack with a needle.

 
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TILT SWITCH
The output is LOW at start-up due to the capacitor on pin 4. When the mercury switch closes, the output goes HIGH and remains HIGH until the reset button is pressed.  This circuit is called a LATCH. See Latch circuit and Memory Cell above.
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MOSQUITO REPELLER
This circuit produces a tone above the human audible range and this is supposed to keep the mosquitoes away. You need a piezo diaphragm that will respond to 15kHz and these are very difficult to find.
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DRIVING A RELAY
The 555 will activate a relay. When pins 2 and 6 are connected as an input, the chip requires only about 1uA to activate the output. This is equivalent to a gain of about 200,000,000 (200 million) and represents about 4 stages of amplification via transistors.
In the first circuit, the output will be opposite to the input. The relay can be connected "high" or "low" as show in the second diagram. One point to note: The input must be higher than 2/3V for the output to be low and below 1/3V for the output to be high. This is called HYSTERESIS and prevents any noise on the input creating "relay chatter."

NEGATIVE LOGIC
An interesting point to remember.
In the first diagram above, the relay is connected so that it is active when the output is low. This is called NEGATIVE or NEGATIVE LOGIC. It has the same reasoning as
-5 - (-5) = 0.
Or in English:  "I am not NOT going."
When the input is low in the first diagram, the output is HIGH and the relay is OFF. The circuitry creates two reversals and makes it easy to see that when the input is LOW, the relay is OFF.
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SCHMITT TRIGGER (Hysteresis)
also called AN INVERTER
also called A SCHMITT INVERTER
This circuit is the same as Driving A Relay circuit above. It is the same circuit with a different name.
We have also animated the circuit to show how the output goes high or low according to the input level. The animation shows a wide gap between the input levels when the time when the output goes HIGH or LOW and this gap is called the HYSTERESIS GAP.
This circuit is called a SCHMITT TRIGGER and it is used in many building-blocks (using a different chip - such as 74c14) to prevent false triggering.
It prevents false triggering because as the input rises, the output does not change until the input voltage is fairly high. If the input voltage falls, the output does not change until the input falls about 30%. This means small fluctuations (noise) on the input do not have any effect on the output. The output is the INVERSE of the input - in other words the 555 is a SCHMITT INVERTER. The second diagram shows a Schmitt Trigger building block.
 



SCHMITT TRIGGER
BUILDING BLOCK
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MUSIC BOX
This circuit produces 10 different tones and by selecting suitable values to change the voltage on pin 5, the result can be quite pleasing. Note: the two unused outputs of the 4017 produce a tone equal to that produced by the 555 when pin 5 has no external control voltage.
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REACTION TIMER GAME
This is a game for two players.
Player 1 presses the START button. This resets the 4026 counter chip and starts the 555 oscillator.
The 555 produces 10 pulses per second and these are counted by the 4026 chip and displayed on the 7-Segment display.
The second player is required to press the STOP button. This freezes the display by activating the Clock Inhibit line of the 4026 (pin 2).
Two time-delay circuits are included. The first activates the 555 by charging a 10u electrolytic and at the same time delivering a (high) pulse to the 4026 chip to reset it. The second timer freezes the count on the display (by raising the voltage on pin 2) so it can be read.
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TRAFFIC LIGHTS
Here's a clever circuit using two 555's to produce a set of traffic lights for a model layout.
The animation shows the lighting sequence and this follows the Australian-standard. The red LED has an equal on-off period and when it is off, the first 555 delivers power to the second 555. This illuminates the Green LED and then the second 555 changes state to turn off the Green LED and turn on the Orange LED for a short period of time before the first 555 changes state to turn off the second 555 and turn on the red LED. A supply voltage of 9v to 12v is needed because the second 555 receives a supply of about 2v less than rail. This circuit also shows how to connect LEDs high and low to a 555 and also turn off the 555 by controlling the supply to pin 8.  Connecting the LEDs high and low to pin 3 will not work and since pin 7 is in phase with pin 3, it can be used to advantage in this design. 

Here is a further description of how the circuit works:
Both 555's are wired as oscillators in astable mode and will oscillate ALL THE TIME when they are turned ON. But the second 555 is not turned on all the time!
The first 555 turns on and the 100u is not charged. This makes output pin 3 HIGH and the red LED is not illuminated.  However the output feeds the second 555 and it turns on.
Output pin 3 of the second 555  turns on the green LED and the second 100u charges to 2/3 rail voltage and causes the 555 to change states. The green LED goes off and the orange LED turns on.
The second 100u starts to discharge, but the first 100u is charging via a 100k and after the orange LED has been on for a short period of time, the first 555 changes state and pin 3 goes LOW.
This turns on the red LED and turns off the second 555.
The first 100u starts to discharge via the 100k and eventually it changes state to start the cycle again.
The secret of the timing is the long cycle-time of the first 555 due to the 100k and the short cycle due to the 47k on the second 555.
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4 WAY TRAFFIC LIGHTS
This circuit produces traffic lights for a "4-way" intersection. The seemingly complex  wiring to illuminate the lights is shown to be very simple.
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DRIVING MANY LEDS
The 555 is capable of sinking and sourcing up to 200mA, but it gets very hot when doing this on a 12v supply.
The following circuit shows the maximum number of white LEDs that can be realistically driven from a 555 and we have limited the total current to about 130mA as each LED is designed to pass about 17mA to 22mA maximum. A white LED drops a characteristic 3.2v to 3.6v and this means only 3 LEDs can be placed in series.
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TV REMOTE CONTROL JAMMER
This circuit confuses the infra-red receiver in a TV. It produces a constant signal that interferes with the signal from a remote control and prevents the TV detecting a channel-change or any other command. This allows you to watch your own program without anyone changing the channel !!    The circuit is adjusted to produce a 38kHz signal. The IR diode is called an Infra-red transmitting Diode or IR emitter diode to distinguish it from a receiving diode, called an IR receiver or IR receiving diode. (A Photo diode is a receiving diode). There are so many IR emitters that we cannot put a generic number on the circuit to represent the type of diode. Some types include: CY85G, LD271, CQY37N(45¢), INF3850, INF3880, INF3940 (30¢). The current through the IR LED is limited to 100mA by the inclusion of the two 1N4148 diodes, as these form a constant-current arrangement when combined with the transistor and 5R6 resistor.
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3x3x3 CUBE
This circuit drives a 3x3x3 cube consisting of 27 white LEDs. The 4020 IC is a 14 stage binary counter and we have used 9 outputs. Each output drives 3 white LEDs in series and we have omitted a dropper resistor as the chip can only deliver a maximum of 15mA per output. The 4020 produces 512 different patterns before the sequence repeats and you have to build the project to see the effects it produces on the 3D cube.
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UP/DOWN FADING LED
These two circuits make a LED fade on and off. The first circuit charges a 100u and the transistor amplifies the current entering the 100u and delivers 100 times this value to the LED via the collector-emitter pins. The circuit needs 9v for operation since pin 2 of the 555 detects 2/3Vcc before changing the state of the output so we only have a maximum of 5.5v via a 220R resistor to illuminate the LED. The second circuit requires a very high value electrolytic to produce the same effect.
 

If you just want fade-ON and fade-OFF, this circuit is all you need:
You can also drive "rope lights."
These can be surface-mount LEDs or totally-sealed LEDs and generally have two wires connected to one end for the 12v supply.
Three LEDs are generally connected in series inside the "rope" with a dropper resistor and some "ropes" can be cut after each set of three LEDs as shown in the diagram below:

Each set of three LEDs draws about 20mA so a rope of 24 LEDs takes about 160mA. Adjust the first two 100k resistors and 100u to set the fade-IN and fade-OUT feature.
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H-BRIDGE
This circuit drives a motor clockwise / anticlockwise via a 10k to 100k pot.
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H-BRIDGE WITH PWM
This circuit drives a motor clockwise / anticlockwise via a pot and reduces the speed to zero when the pot is in mid-position. The current is limited to 200mA and the voltage across the motor is less than 6v, but the circuit shows the principle of Pulse Width Modulation (providing powerful bursts of current to the motor to create a high or low RPM under load) and both forward / reverse RPM via the H-bridge arrangement.
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H-BRIDGE PUSH-PULL
DOG-BARK STOPPER
The two circuits above are also H-Bridge Push-Pull outputs, however the current is limited to 200mA or less. In this design the current can be 3 amps or more, depending on the supply voltage, the resistance of the load and the type of driver transistors. About 2v5 is lost between "c and e" due to the output of the 555 and the base-emitter voltage of the driver transistors. This circuit drives an ultrasonic transducer (speaker) at 20kHz to 40kHz to subdue dog barking.
If the unit is turned on by remote control every time the dog barks, the animal will soon learn to cease barking.

Look on eBay for Piezo Tweeter for about $3.00 plus $7.00 postage. The maximum frequency response will be about 30kHz.
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BIKE TURNING SIGNAL
This circuit
can be used to indicate left and right turn on a motor-bike. Two identical circuits will be needed, one for left and one for right.                                    
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555 ON 24v
If you need to operate a 555 on 24v, you will need to reduce the voltage to less than 18v. The following circuits reduce the voltage to 12v:

30mA:
If the 555 circuit takes less than 30mA (the 555 takes 10mA) you can use a 400mW zener diode to drop the 24v supply to 12v for the 555. In other words, 12v is dropped across the zener.


Up to 500mA:
The next circuit will allow up to 500mA. The transistor will need to be placed on a large heatsink. It is an emitter-follower-regulator transistor and can be used with a 400mW zener. The output voltage is 0.6v lower than the zener voltage.



Up to 500mA with "Amplifier Zener"
A 400mW zener can be converted to a "Power Zener" by combining with a transistor as shown in the following circuit: 12.6v will be dropped across the rails. In other words, if the top rail is 24v, the bottom rail will be 11.4v.

Up to 1A:
Using the next circuit will allow the 555 to take 200mA and the load to take 800mA. The 7812 will need to be placed on a large heatsink. The 7812 is called a 3-terminal VOLTAGE REGULATOR.

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POLICE LIGHTS
These three circuits flash the left LEDs 3 times then the right LEDs 3 times, then repeats. The only difference is the choice of chips.



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LED DICE with Slow Down
This circuit produces a random number from 1 to 6 on LEDs that are similar to the pips on the side of a dice. When the two TOUCH WIRES are touched with a finger, the LEDs flash very quickly and when the finger is removed, they gradually slow down and come to a stop. LED Dice with Slow Down kit is available from Talking Electronics.


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ROULETTE
This circuit creates a rotating LED that starts very fast when a finger touches the TOUCH WIRES. When the finger is removed, the rotation slows down and finally stops. 
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MODEL RAILWAY TIME
Here is a circuit that will convert any clock mechanism into Model Railway Time.
For those who enjoy model railways, the ultimate is to have a fast clock to match the scale of the layout. This circuit will appear to "make time fly" by turning the seconds hand once every 6 seconds. The timing can be adjusted by changing the 47k. The electronics in the clock is disconnected from the coil and the circuit drives the coil directly. The circuit takes a lot more current than the original clock (1,000 times more) but this is one way to do the job without a sophisticated chip. 

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REVERSING A MOTOR-4 (see 1, 2, 3 in 200 Transistor Circuits)
In this example the power is applied via the start switch and the train moves to the away limit switch and stops. The 555 creates a delay of 1 minute and the train moves to the home limit and stops. Turn the power on-off to restart the action.
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AUTOMATIC CURTAIN CLOSER
Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk

This circuit uses a mixture of transistors, an IC and a relay and is used to automatically open and close a pair of curtains. Using switch S3 also allows manual control, allowing curtains to be left only partially open or closed. The circuit controls a motor that is attached to a simple pulley mechanism, to move the curtains.

Automatic Operation
The circuit can be broken into three main parts; a bi-stable latch, a timer and a reversing circuit. Toggle switch S3 determines manual or automatic mode. The circuit as shown above is drawn in the automatic position and operation is as follows. The bi-stable is built around Q1 and Q2 and associated circuitry and controls relay A/2. S1 is used to open the curtains and S2 to close the curtains. At power on, a brief positive pulse is applied to the base of Q2 via C2. Q2 will be on, and activate relay A/2.
The network of C3 and R4 form a low current holding circuit for the relay. Relay A/2 is a 12V relay with a 500 ohm coil. It requires slightly less current to keep it energized than it does to operate it. Once the relay has operated, the current through the coil is reduced by R4, saving power consumption. When Q2 is off, C3 will be discharged, but when Q2 becomes active (either at switch-on or by pressing S1) capacitor C3 will charge very quickly via the relay coil. The initial charging current is sufficient to energize the relay and current flow through R4 sufficient to keep it energized.
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STEPPER MOTOR CONTROLLER

This circuit controls the speed of a stepper motor via the 100k pot. The direction of rotation is determined by the double-pole double-throw switch.
The stepper motor used in this circuit came from an old scanner. It had 5 wires: red-black-yellow-brown-orange. The LEDs illuminate via the back-emf of the coils and prevent the spikes entering the transistors. The LEDs will flicker to show the pulses being received by the stepper motor.
The 27k stop-resistor limits the upper-frequency of the 555 and prevents the circuit producing pulses that are too fast for the stepper motor.
If the colour coding is different on your stepper motor and it fails to work, you only need to reverse two connections thus:
A   B   C   D
A   B   D   C   (reverse the two end connections) and if this fails,
A   D   B   C   (reverse the two middle connections)


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STEPPER MOTOR CONTROLLER TE555-1



This circuit uses the latest 
TE555-1 STEPPER MOTOR SPEED CONTROLLER and controls the speed of a stepper motor via the 100k pot. The direction of rotation is determined by the FORWARD and REVERSE switches and the motor does not take any current when a switch is not pressed.
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ANIMATED DISPLAY CONTROLLER TE555-2


This circuit uses the latest 
TE555-2 ANIMATED DISPLAY CONTROLLER chip from Talking Electronics. This 8-pin chip is available for $2.50 and produces 7 different animations on a 10 LED display. The animations are selected by the position of a 100k pot and when the animation is showing, the pot can be adjusted to increase the speed of the animation.
"Position 10" on the pot cycles through the 7 animations.

A kit of components (matrix board, PC board for LEDs, surface-mount resistors, capacitors, transistor, diode, switch, cells, battery holder, pot and 20 yellow LEDs with TE555-2 chip is available for $15.00 plus $5.00 postage. Click the link above and you will be sent an email with the costs. This is an ideal project you get you into surface-mount technology and you can add it to a model layout or build it into a Lego brick for a junior member.

Wiring the two ten-LED displays



The project has two 10-LED displays. One on the front and one on the back

Two of the 7 animations
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FOUR ALARMS SOUNDS TE555-3



The output is set to produce an alarm for 3 minutes then stops.



The chip set-up as an alarm
A1 A0 Alarm Output
0 0 Continuous 2.4kHz tone
0 1  Chirps
1 0 Siren
1 1 Space Gun
Control lines A0 and A1 are tied HIGH or LOW and when "enable" line is taken HIGH, the tone is emitted from pins 2 and 7 (in toggle mode). Pin 2 is LOW when the chip is at rest. To get a very loud output, pin 2 drives a Darlington transistor and piezo tweeter with a 10mH choke across the piezo to produce a waveform of nearly 100v. The circuit consumes 0.1mA when at rest.
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DICE TE555-4


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HEADLIGHT SELECTOR
This circuit was designed by a reader who needed to select between low and high beam by pressing a switch, then change back by pressing the switch again. The circuit always starts on low beam, regardless of the state it was turned off.
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12v DC to 12v DC BATTERY CHARGER
You cannot charge a 12v battery from a 12v battery. The battery being charged creates a "floating charge" or "floating voltage" that is higher than the charging voltage and the charging stops.
The following circuit produces a voltage higher than 12v via a CHARGE PUMP arrangement in which the energy in an electrolytic is fed to a battery to charge it.
The circuit produces about 900mA "charge current" and the diodes and transistors must be fitted with heat sinks. The LEDs are designed to prevent the two output transistors turning ON at the same time. The lower output transistor does not start to turn on until the voltage is above 5v and the top transistor does not turn on until the voltage drops 4v from the positive rail. This means both transistors will be turned on ONLY when the voltage passes a mid-point-gap of 4v. In our circuit, this time is very short and and the transition is so fast that no current (short-circuit current) flows via the two output transistors (as per our test). 
The electrolytic charges to about 10v via the lower transistor and top diode. The top BD679 then pulls the negative of the 2200u electrolytic towards the 12v6 rail and the positive is higher than 12v6 by a theoretical 10v, (about 9v in our case) however we need the ENERGY IN THE ELECTROLYTIC and in our circuit it is capable of delivering a current flow of about 900mA. This energy is passed to the battery via the lower diode.  Most batteries should not be charged faster than the "14-hour-rate." This basically means a flat battery will be charged in 14 hours. To do this, divide the AHr capacity by 14 to get the charge-rate. For example, a 17AHr battery should be charged at 1.2A or less. For lower-capacity batteries, the 2200u can be reduced to 1,000u. Charging is about 80% efficient. In other words, delivering 120% of the AHr capacity of a battery is needed to fully charge it. 
 
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1-10 MINUTE AUTO TURN OFF
This circuit  provides an automatic turn-off feature after a time that can be set from 1 minute to 10 minutes by the 470k pot.
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WATER LEVEL DETECTOR
This circuit can be used to automatically keep the header tank filled. It uses a double-pole relay. This is the transistor version of the circuit below.

Here is the circuit using a 555:
These are the facts you have to remember. In our circuit, Pins 2 and 6 detect a voltage when water is not touching the probes, due to the 100k resistors. When water touches the probes, neither pin "detects a voltage."
Don't worry about pin 2 detecting 1/3 of rail voltage and pin 6 detecting 2/3 of rail voltage. In our circuit the pins either detect a voltage or do not detect a voltage. Pin 2 detects a LOW and pin 6 detects a HIGH. Pin 2 does nothing when it detects a HIGH and pin 6 does nothing when it detects a LOW.
When the water is LOW, as shown in fig 1, both pins 2 and 6 are HIGH and the output of the 555 is LOW. 
As the water rises, as shown in fig 2, Pin 6 goes low but nothing happens to pin 3 except the chip "has been prepared via the internal flip-flop" to change when pin 2 goes LOW. When the water reaches pin 2, as shown in fig 3, this pin "fails to see a HIGH," the output of the chip goes HIGH and the pump turns off. 
As the water level goes down, as shown in fig 4, pin 2 sees a HIGH but this does not change the 555 as pin 2 only has an effect when it goes LOW.
When the water level goes down further, as shown in fig 5, pin 6 sees a HIGH and because pin 2 is not seeing a LOW, the chip will change states. The output goes LOW. 
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WARNING LIGHTS FLASHER
These two circuits flash two 5 watt to 21watt car lamps. The first circuit uses BD679 Darlington transistors and the second circuit uses a 555.

This 12v Warning Beacon is suitable for a car or truck break-down on the side of the road. The key to the operation of the circuit is the high gain of the Darlington transistors. The circuit must be kept "tight" (thick wires) to be sure it will oscillate.
A complete kits of parts and PC board costs $5.00 plus postage from: Talking Electronics. Email
HERE for details.

The 555 circuit uses two diodes to connect the chip to the negative rail and this allows the components to be fitted between the "high side" of the lamps and the "power switch."
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MISSING AUDIO DETECTOR
This circuit detects when audio is not received for about 4 seconds and turns on an alarm. 
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5 SECS DELAY THEN RELAY ON FOR 4 SECONDS
This circuit waits 5 seconds before turning on the relay for 4 seconds.
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DELAY BEFORE TURNING ON
This circuit does not turn on for XX seconds after power is applied. Adjustable from 1 second to 2 minutes.
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LOW CURRENT TIMER (delay)
The low-current version of the 555 is called 7555 (ICM7555CN),  and is a CMOS direct-equivalent version of the TTL 555. It costs more but can be purchased on eBay for $12 (for 10 items incl postage). The normal standing current for a 555 is about 10mA. The standing current for a 7555 is about 0.3mA. This circuit will produce a delay of about 5mins. Change the 1M and/or 100u for different delays.
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CROSSING LIGHTS
A magnet on the train activates the TRIGGER reed switch to turn on the amber LED for a time determined by the value of the first 10u and 47k.
When the first 555 IC turns off, the 100n is uncharged because both ends are at rail voltage and it pulses pin 2 of the middle 555 LOW. This activates the 555 and pin 3 goes HIGH. This pin supplies rail voltage to the third 555 and the two red LEDs are alternately flashed. When the train passes the CANCEL reed switch, pin 4 of the middle 555 is taken LOW and the red LEDs stop flashing.

 

 The circuit can also be constructed with a 40106 HEX Schmitt trigger IC (74C14). The 555 circuit consumes about 30mA when sitting and waiting. The 40106 circuit consumes less than 1mA.

 

FAULTS
Here are some circuits with faults. They come from
projects on the web:

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HEADLIGHT FLASHER
This circuit flashes headlights via a relay but the relay is only getting 9v4 due to the voltage-loss of the 555 and 0.6v of the transistor: The transistor should be common-emitter configuration.
In addition, the pot will be damaged if turned to zero ohms. A 1k should be placed in series with the pot (at pin7 end).

Here is a simpler circuit. It will need testing and adjusting to suit the relay you will be using:
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MISSING PULSE DETECTOR
This circuit is described on the web as a missing pulse detector. If the 1M pot is turned to zero ohms, it will be damaged when the transistor inside the 555 at pin 7 connects to 0v rail.
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MERCURY SWITCH DETECTOR
This circuit is a LATCH CIRCUIT and it detects when the mercury switch is tilted. But it is consuming 10mA while it is sitting around waiting for the mercury switch to make contact.
By replacing the 555 with two transistors, the circuit will consume zero current when waiting for the switch to close. Sometimes a 555 is not the ideal choice.







                       
THE FUTURE
This eBook has shown the enormous number of circuits
that can be produced with a 555.
However there is something we should point out.
The 555 has limitations and disadvantages.
It is not a chip you readily add to battery operated devices
as its current consumption is quite high at 10mA. (There is a whole
range of  low-current equivalents.)
Secondly, the 555 is not a chip you add to a complex circuit as
there are many other chips that can perform the task of a
555 and you will have additional gates within the chip for other
sections of the circuit. The 74c14 is an example. It has 6 Schmitt trigger
gates and each gate can be wired as an oscillator or delay and the chip
takes less than 1mA.
Before designing a circuit around a 555, you should look at  100 IC Circuits. It has many "building Blocks" to help you design
your own circuits.
 
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If 3rd band is gold, Divide by 10
If 3rd band is silver, Divide by 100
(to get 0.22ohms etc)



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