BeginnerLighting
Two-Transistor Alternating LED Flasher
Build a discrete astable multivibrator that alternates two LEDs without a timer IC.
Parts from the database
- 2N3904 × 2Cross-coupled switching transistors
Other supplies
- breadboardCommon build supply
- regulated 5 V supplyCommon build supply
- two LEDsCommon build supply
- two 1 kΩ collector resistorsCommon build supply
- two 47 kΩ base resistorsCommon build supply
- two 10 µF electrolyticsCommon build supply
- two 330 Ω LED resistorsCommon build supply
- 100 nF bypass capacitorCommon build supply
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Connections
- Check exact 2N3904 pinouts first; connect both emitters to ground.
- Connect each collector to +5 V through 1 kΩ; from each collector also connect an LED and 330 Ω in series to ground so it lights when the collector is low.
- Connect each base to +5 V through 47 kΩ. Cross-couple each collector to the opposite base through a 10 µF capacitor; positive lead faces collector.
- Keep both timing branches mirrored and bypass the supply.
Build steps
- Verify one transistor switch at a time.
- Add the cross-coupling capacitors with power off.
- Power at 5 V and observe alternating flashes.
- Change both timing capacitors equally to alter the rate.
Software and firmware setup
This is a hardware-only circuit; no firmware is required.
Test your build
The LEDs alternate; actual rate depends on capacitor leakage, resistor tolerance, and transistor gain.
Troubleshooting
- Both on: verify pinout and crossed capacitor connections.
- Only one side flashes: inspect the second base resistor and capacitor polarity.
- Erratic rate: use fresh electrolytics and shorten base-node wires.
Safety and limitations
Use a current-limited 5 V supply; this low-current circuit is not for lamps or large loads.
Manufacturer and platform documentation
Community Notes for This Project
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