Abusing Transistors to Flash LEDs
This unusual LED flasher circuit exploits a little-known property of transistors to blink lights without a microcontroller or 555 timer.
Blinking an LED is the prototypical first electronics experiment for many beginner hobbyists. This most commonly involves a microcontroller, such as those found on an Arduino development board, or the more adventurous maker might use a 555 timer chip. But YouTuber Zafer Yildiz has demonstrated a very different way to accomplish this old rite of passage. Yildiz has built an LED flasher circuit that involves no complex integrated circuits.
At first glance, the circuit appears to be missing something. It consists of just 470 µF electrolytic capacitors, 1K resistors, BC547 NPN bipolar junction transistors, and, of course, LEDs. The strangest part of all is that the base pin of each transistor is left disconnected. Huh? How does this circuit result in a regular flashing pattern?
It relies on a little-known property of this type of transistor called an avalanche breakdown. When 12V is applied to the circuit, it quickly charges the capacitor. The capacitor is connected across the collector and emitter of the transistor. When the capacitor’s voltage level reaches around 9V, an avalanche breakdown occurs in the transistor, which allows current to flow. The capacitor then begins to discharge, illuminating the LED. Once the capacitor’s voltage falls below the level of an avalanche breakdown, the transistor switches the LED off again and the process starts over.
Not only is this an interesting effect, but it is also a very simple build. Whether you are looking for your first electronics project, or a cool new desk toy, recreating this project would be a fun way to spend a couple of hours.