Astable Multivibrator

This is the simplest circuit to make two LED blink back and forth. The technical term for this circuit is “astable multivibrator”.

It’s really easy to put together, with only 10 parts.

[Part List]

* 2 x LEDs – any color. Red, yellow, and green are safe.
* 2 x NPN transistors – just about any small transistor. I used PN2222A, but you can use 2N3904, 2N4401, etc.
* 2 x Capacitor – around 10 – 22 uF. 6V or higher. Electrochemical capacitors are the easiest to find.
* 2 x 220 ohm resisters – anything from 200 to 1k ohm of any type should work.
* 2 x 47k ohm resisters – anything from 22k to 100k ohm should work. Value of these changes the timing of blinking (larger the value, slower the blinking).

You can easily obtain the parts from your local Redioshack.