Cheng Sun

Transistor-only LED flasher


The LED flasher is the “hello world” of electronics. The circuit typically uses an RC circuit consisting of a capacitor charging and discharging repeatedly through a resistor for timing.

Here’s my slightly unusual take on it. The entire circuit consists solely of 6 transistors. There are no other components at all. How does it keep time?

LED flasher circuit schematic

The core of the circuit is a simple integrator – a current source charging a capacitor. A voltage-controlled switch discharges the capacitor when the voltage reaches a threshold.

To make this circuit compact and transistor-only, I had to use several different kinds of transistors, and abuse some of the lesser-known properties of each. Here are the tricks involved:

Front: LED flasher (front)
Back1: LED flasher (back)
Layout: LED flasher PCB layout
  1. I designed this circuit quite some time ago: the back of the PCB has 2018-07-01 printed on it. It took a while for me to get round to writing this up.