Two-transistor flasher (astable multivibrator)
Open in the lab ▸Make my own copy
Two BC547s, two capacitors and a few resistors make a circuit that cannot sit still. When Q1 switches on, its collector drops, and C1 passes that drop to Q2’s base, pulling it below zero and holding Q2 off until C1 recharges through R3. Then Q2 switches on and does the same to Q1. Each LED hangs from a collector, so it lights while its transistor is off: they take turns. The time each side stays off is set by its base resistor and capacitor (R2·C2 and R3·C1). R3 is a little larger than R2, so the two halves are not exactly equal; make one capacitor smaller and that half gets shorter. Scope a base to see the dip below zero that holds the other side off.
What is on the bench
23 parts, 30 wires.
| Resistor | 6 |
|---|---|
| NPN transistor | 2 |
| Electrolytic capacitor | 2 |
| LED | 2 |
| 9 V battery | 1 |
Put it on your page
The project runs live inside the frame: readers can press buttons, turn knobs and read the meters.
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Link
https://zencircuitlab.com/p/zclmultivib
Published by its author under the lab's community terms: it may be opened, copied and changed by anyone. Simulation results are for learning; check a real build against the parts' datasheets.