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The Astable Multivibrator

(a) Circuit diagram

(b) Output voltages
(Suggested values for the resistors: R1 = 6.8kΩ, R2 = 6.8kΩ, R3 = 2.2kΩ, R4 = 2.2kΩ)

We will assume that transistor T1 is ON initially and transistor T2 is OFF.

When T1 is on O1 is at 0.2 V and B1 is at 0.7 V
T2 is off and O2 is at 6 V and B2 is at 5.3 V
C1 charges up through R1 and C2 charges up through R4
As soon as B2 reaches 0.7 V T2 switches on.
Note that C1 charges slowly through the large resistor R1 while C2 charges more quickly through the small resistor R4.

When T2 switches on O2 drops to nearly 0 V, B1 drops by 6 V to 5.3 V and T1 goes off. Both sides of C2 will drop by 6 V and it will take some time for C2 to adjust its charge, the potential difference across it cannot change instantly.

The cycle now repeats for the transistor.

The charge discharge rate and hence the switching action of the circuit depends on C and R.

© Keith Gibbs 2013