GCSE Electronic Products Revision
Text ©Edward Clarvis 2010
Astable
Circuits
Astable Multivibrators

An astable multivibrator is a circuit with an output that turns on and off repetitively theyare also know as pulse generators. In the example shown the LED would flash on and off.
You need to remember how to wire up the circuit because you may be asked to complete a partly drawn circuit or explain what happens when component values are changed.
F =
1.44
Calculating the frequency

The speed that the output turns on and off is set by the values of

R1, R2 and C1

The rate that the output turns on and off is called the frequency and it can be calculated using the formula shown. You will be given the formula in your exam paper but you must know how to use it.
F =
1.44
F =
1.44
(1k + 2x10K) x 100uF
F =
1.44
F =
1.44
F =
1.44
F = 2.1 Hertz
1
2
3
4
In an exam you must show all of your working out. Even if you get the final answer wrong you will still get marks for your workings out.
Always include the UNITS.

Missing the “Hertz” off will lose you a mark!
Here is an example where...

R1 = 1K
R2 = 10K
C1 = 100uF