Brief Single 2N3904 NPN Bipolar Junction Transistor BJT LED Flasher Circuit

NPN BJT Collector and Emitter wired backwards will make for an unpredictable circuit as voltages rise. Doing so purposely though, with a LED and protective resistor on the collector side, floating base, and parallel capacitor, leads to an interesting circuit.

Single 2N3904 NPN bipolar junction transistor BJT LED flasher circuit schematic diagram by electronzap
Single 2N3904 NPN bipolar junction transistor BJT LED flasher circuit schematic diagram by electronzap
  • The transistor won’t conduct at first. The capacitor voltage will rise as it charges through the resistor connected to the positive supply.
  • Capacitor voltage will rise until the transistor breaks down, and current flows through it and the series load.
    1. The capacitor discharges through NPN BJT and load.
    2. The current will keep flowing through the transistor and load, until the capacitor drops to a low enough voltage. This is a bit below the breakdown voltage that started transistor conduction.
  • The lowered capacitor voltage wont breakdown the transistor, and the voltage will rise until it does again.


Nice assortments of semiconductors that I use in my videos. Includes 2N3904 NPN BJTs and 2N3906 PNP BJTs. Amazon affiliate link ad.

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