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
Best comment ever!
Sounds familiar
https://hackaday.com/2025/10/16/positive-results-with-negative-resistance/
+1
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.
Needless to say that all transistor manufacturers tell you to avoid this.
Nevertheless it works. I’ve used this trick back then when semiconductors were expensive, so you
tried to use as few as possible. I once made a blinking LED hose with several of these circuits, and it worked for years.
It was what we did when we made fake “Alarm active” blinkers in cars, IIRC.
Ah, remembered where I’ve seen this before, it is a neon oscillator circuit where BC547 is used instead of neon bulb.
It’s hard to believe that some people actually would grab a RPI/Arduino just to blink a LED. Sad when you think about it.
Why?
Blink: The circuit above.
Blink!: 555 circuit
BLINK!!!: Arduino
HULK BLINK!!!: Raspberry Pi
I use a car to blink LEDs.
God mode engaged.
Congratulations, I spilled some coffee reading this!
Yes a RPI/Arduino would seem to be overkill – BUT – if you want a specific blink rate using a 555 and getting the correct values for a specific blink rate/pattern can take a significant amount of time even using the online 555 calculators for us that don’t do this on a regular basis. There are related processors that are smaller and cheaper – Adafruit sells the Trinket M0 that is small and uses the Arduino IDE for programming, a valid solution in my view if you value your time at any more than maybe $ 1.00/hr or so. The Trinket M0 is about $ 9– there are a few older processors that cost a bit less but are still not to bad to program.
Yeah wasting the power is a little bit sad but even sadder is to think that we can’t think of anything else we want to do. Live our whole lives searching for meaning and all along it was just the desire of the universe to blink an LED expressing itself through our souls.
Hey, the unijunction transistor is essentially made for this!
If the requirement is to only use one transistor, why not configure it as a LC or phaseshift sinewave oscillator, and attach the LED around the collector?
Interesting that the the avalanche mechanism can be used in reliable ~10V “zeners” (actually avalance diodes). But when reverse biasing the emitter-base region of BJT’s it can degrade gain. This is due to
generation of recombination centers at the silicon/oxide interface, which increases base current and lowers transistor gain.
You can simulate it in LT-spice if you use a component from a third party library: http://bordodynov.ltwiki.org/