# How does an astable multivibrator LED blinking circuit work?

I am beginner to electronics. I come to know about oscillator using RC circuits (Capacitor charge and discharge and time constant tells the behavior of circuit).

Then I saw following circuit that blinks 2 LEDs timely in sequence. Will somebody explain its working? I know that capacitor will charge and during the charging the LED will off and when they discharge they will on the LED. But why transistor are there?

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I did some formatting and spelling edits to the post but I am unable to improve on the second paragraph. –  jpc Mar 30 '11 at 13:39
I have made this but both leds light at the same time.It doesn´t blink.Can someone help me? –  user9385 Apr 22 '12 at 13:53

This circuit is called an astable multivibrator, and the reason this circuit works is a little difficult to put into words.

Take a look at this circuit simulation that shows visually what is happening. You can slow the speed right down and look carefully at how the current is flowing.

While one of the capacitors is charging, current flows to the base of the alternate transistor, making the emitter-collector path conduct, making one of the LEDs light. When the capacitor is charged, it stops conducting and switches off the transistor, and then the other capacitor begins to charge switching on the other transistor, at the same time the first capacitor discharges, then the cycle repeats.

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Why is the one voltage of the transistors not getting as high as the other? IS that a fault with the simulation as surely they should get to the same with the same resistors and cap sizes. –  Dean Mar 30 '11 at 18:29
@Dean: Actually, one of the resistors is 10 ohms less than the other... this is to start the circuit oscillation in the unrealistic world of the simulator. –  BG100 Mar 30 '11 at 19:04