Home > Circuit Bricks > Oscillator - astable
UPDATED:.20:50 21 October 2013


  • This astable is based upon a 555 timer IC.
  • An astable continually produces output pulses (switches on and off repeatedly).
  • The speed of this switching can be controlled by 2 (or 3) components.
  • Useful for making lights flash or producing a tone generator.
The frequency (or speed) at which the pulses occur is measured in Hertz (pulses per second) and can range from less than 1Hz to about 50,000Hz (50KHz). The output signal is called a square wave because of the shape it produces when connected to an oscilloscope.
Circuit Diagram

When connected to 0v, the timer will not operate. This should be connected to +v for normal use, directly or via a 1K resistor.


The output will repeatedly change from high to low at a set frequency. The output can only sink/source about 25mA, enough for an LED, buzzer or low power bulb.

You will need to add a driver to drive bigger devices such as lamps or large LED clusters.


The frequency can be calculated by the following formula:

Frequency = 1.44 / (R1 + 2R2) x C1

This can be difficult to use so for an easier method try the following:

Select a value of 1K for R1 as shown in the diagram.
Choose one of the following values for the capacitor, depending on the frequency you require:

Less than 1 Hz. = 10uF,
Up to 1 KHz.
= 100nF,
Over 1 KHz.
= 1nF

Using the graph below, select the line of C1 and read off the value of R2 for the required frequency.

Try using a preset or a potentiometer for R2. Then you can adjust the frequency manually.
The 555 timer comes in an 8pin DIL package.


When using ICs, it is always best to use an IC socket so the IC can be removed easily if needed.

Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers