The Astable Multivibrator

(a) Circuit diagram




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






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.