LED Flasher Circuit Using 555 Timer IC

This is a simple LED flasher project that uses a common 555 timer IC for its operation. It is configured as an astable mode which means that its output is a square wave oscillator. Two LEDs are connected to its output in such a way that when one LED is ON, the other LED will turn OFF. It uses only 10 simple parts that are easily available at any electronic shops.
Capacitor C2 charges exponentially through resistors R1, R2 and the resistance of the trimpot. When C2 has charged to about 2/3 VCC it stops charging and it discharges to about 1/3 VCC through R2 and the trimpot resistance via pin 7. This is the standard operation of a 555 timer. When a Vcc of 5 V to 15 V DC is applied to the circuit, the LED will start to flash. The frequency of the flashing can be changed by varying the resistance of the potentiometer or trimpot.

Parts List
The parts list of the simple LED project is as shown below.

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The 555 Timer IC

The 555 Timer is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine".

The 555 gets its name from the three 5-k Ohm resistors used in typical early implementations. It is still in wide use, thanks to its ease of use, low price and good stability. As of 2003[update], 1 billion units are manufactured every year.

The 555 timer is one of the most popular and versatile integrated circuits ever produced. It includes 23 transistors, 2 diodes and 16 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).

The 555 has three operating modes:

* Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, Frequency Divider,Capacitance Measurement, Pulse Width Modulation (PWM) etc

* Astable - Free Running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

* Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

The connection of the pins is as follows:

Nr. Name Purpose

1 GND Ground, low level (0V)
2 TR A short pulse high → low on the trigger starts the timer
3 Q During a timing interval, the output stays at +VCC
4 R A timing interval can be interrupted by applying a reset pulse to low (0V)
5 CV Control voltage allows access to the internal voltage divider (2/3 VCC)
6 THR The threshold at which the interval ends (it ends if U.thr → 2/3 VCC)
7 DIS Connected to a capacitor whose discharge time will influence the timing interval
8 V+, VCC The positive supply voltage which must be between 3 and 15 V

In the astable mode, the high time from each pulse is given by

high = 0.693.(R1 + R2).C

and the low time from each pulse is given by

low = 0.693.R2.C

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.


These specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade (military, medical, etc).

* Supply voltage (VCC) 4.5 to 15 V
* Supply current (VCC = +5 V) 3 to 6 mA
* Supply current (VCC = +15 V) 10 to 15 mA
* Output current (maximum) 200 mA
* Power dissipation 600 mW
* Operating temperature 0 to 70 °C Read More!

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