Let’s say you want to blink an LED. You might grab an Arduino and run the Blink sketch, or you might lace up a few components to a 555. But you needn’t go so fancy! [The Design Graveyard] explains how this same effect can be achieved with a single transistor.
The circuit in question is rather odd at first blush. The BC547 NPN transistor is hooked up between an LED and a resistor leading to a 12V DC line, with a capacitor across the emitter and collector. Meanwhile, the base is connected to… nothing! It’s just free-floating in the universe of its own accord. You might expect this circuit to do nothing at all, but if you power it up, the LED will actually start to flash.
The mechanism at play is relatively simple. The capacitor charges to 12 volts via the resistor. At this point, the transistor, which is effectively just acting as a poor diode in this case, undergoes avalanche breakdown at about 8.5 to 9 volts, and starts conducting. This causes the capacitor to discharge via the LED, until the voltage gets low enough that the transistor stops conducting once again. Then, the capacitor begins to charge back up, and the cycle begins again.
It’s a weird way to flash an LED, and it’s not really the normal way to use a transistor—you’re very much running it out of spec. Regardless, it does work for a time! We’ve looked at similar circuits before too. Video after the break.
[Thanks to Vik Olliver for the tip!]
Its a relaxation oscillator.
i dnt know about relaxation, the article says the transistor is being traumatized (breakdown). :P
Sounds familiar
https://hackaday.com/2025/10/16/positive-results-with-negative-resistance/
My first was “Hire a TaskRabbit” but this was educational for me, so Thank You!
Shouldn’t this be entered in the component abuse challenge? Sounds abusive enough…
pff. Icould have done this with a
Oh, never mind…
It’s acting as a ziener diode. But it’s likey not going my to be good for it as a transitor for long.
LOL. Repeatedly putting a typical transistor junction into avalanche is a great way to put a hole in it … and not the kind that you want.