Astable Multivibrator — 2-Transistor LED Flasher (No 555)

Circuit Diagram

How to Build It

1. Place the two BC547 transistors on the breadboard, spaced apart.
2. Connect each transistor's collector to an LED (with its current-limiting resistor) and then to the positive rail (9V).
3. Connect each transistor's emitter to the negative rail (GND).
4. Connect an electrolytic capacitor from the first transistor's collector to the second transistor's base.
5. Connect a second capacitor from the second transistor's collector to the first transistor's base — this is the cross-coupling.
6. Connect a bias resistor from the positive rail to each transistor's base.
7. Power the circuit — the two LEDs should blink alternately, no digital chip involved.

Bill of Materials