LearnTronics
Dual-LED Transistor Flasher
Recipe #26 — dual-led transistor flasher
Two transistors and two capacitors can form a free-running oscillator. This classic astable multivibrator makes the RC timing action visible by flashing two LEDs alternately.
How to trace this drawing:
Start at either collector dot. C1 carries the change at Q1's collector to
Q2's base. C2 does the opposite: it carries the change at
Q2's collector to Q1's base. The two transistor symbols are
identical NPN transistors drawn in the same orientation.
There is no stable resting state. When Q1 turns on, its collector change is coupled through C1 and pushes Q2 toward off. C2 later lets Q2 recover and the circuit flips. Then the same process happens in the opposite direction.
Why this is called astable
Astable means neither state is permanently stable. The circuit continuously alternates between its two states, so LED1 and LED2 flash back and forth.
Timing is still RC
Each cross-coupling capacitor charges through a base resistor. In a roughly symmetrical circuit, each half-cycle is on the order of 0.69 RBC. Real transistor voltages make that only an approximation.
This is an oscillator made from ordinary transistors
No 555 timer is needed. Positive feedback provides the switching and RC charging provides the delay. The next lessons package related ideas into the very convenient 555 timer IC.