Blinking Led Circuit Using Two Transistors

Actually, this is the circuit of the astable multivibrator. We can make the blinking LED with the help of the astable multivibrator circuit.

An astable multivibrator is also known as the free-running oscillator. Because of capacitor coupling, the transistor does not off permanently.

It has two quasi-states and it makes the periodic transition between these two states.

Polarity is perfect for NPN transistor. For PNP transistor polarity must be reversed.


Let us consider before time, t=0 transistor Q2 is ON and carrying Ib current. Also, Q1 is below cut-off and in the off-stage.

C1 charges through R2 and “c” rises exponentially towards Vcc.

At t=0 “c” reaches the cut-off voltage. This makes Q1 ON stage and Q1 starts conducting. Due to this “Vc1” rises exponentially towards Vpp. Because “a” is capacitive coupling to “d”, the rise in “Vc1” makes Q2 in off stage. Due to this, C2 starts discharging.

When “Vc2 ” goes below cut-off due to discharging Q1 becomes OFF. This makes Q2 ON.

The time period is approximately equal to



$$R_1=R_2=R \text{ and Also }C_1=C_2=C$$

then time period is equal to

$$T= 1.38RC$$

The time period is independent of supply voltage Vcc.


When Q1 conducts Q2 becomes off and vice versa. Only one transistor conducts at a time.

By connecting LED in the emitter of the transistor we can make the blinking LED due to the fact that when Q1 Conduct Q2 don’t and vice versa.

When Q1 conducts Red led glow. Also, when Q2 conducts Green led glow.