Simple Transistor Bistable Circuit

This is an astable multivibrator circuit that blinks filament light bulbs, which I call the blinking past. Everyone makes this classic time-honoured bistable circuit when starting out on the royal road to electronics, and when I first made it, I was ten years old. I used the BC108 transistor in a TO-18 metal can, and back then, LEDs were not readily available so I used torch light bulbs instead, which made the circuit much nicer. Therefore, for this application, I am using a pair of commonly available MES bulbs rated at 6 V and 60 mA. in the UK, a dad usually makes this circuit with his children during Christmas, because the blinking lights are very festive!

When I first built the circuit, it was on the S-Dec breadboard, which was a finely crafted British made piece of electronic equipment, but today, you will just have to settle for the cheap Chinese breadboards you can get for a buck or two.

Transistor

Many people will remember the old BC107, BC108, and BC109 transistors in the TO-18 metal can, however their direct replacements became the BC547, BC548, and BC549 in TO-92 package. Although you can use almost any general-purpose transistor in this application, it is so much nicer if you can use the original and classic BC108.

If you have never used the BC108, the best way to distinguish between the base, collector, and emitter terminals, is to view it from underneath. The emitter lead is the one closest to the metal tab, and then comes base, followed by the collector lead.

If you decide to use the BC548, then it has a different package and pinout, however this diagram should help.

Torch Light Bulbs

You will require 11 mm E10 MES Miniature Lamp Light Bulbs with a rating of 6 V / 60 mA. Preferably lower voltage and current is better, however the E10 is very common and cheap. Also remember to get the bulb holders.

Resistors and Capacitors

For the resistor and capacitor, you should start with 200 µF and 22 kΩ, and then try other values to see how it changes the blinking rate. You should notice that increasing their value increases the period. For the power supply, nice 9 V PP3 battery is the best for this application, however voltages close to this will also work fine.

Cost / eBay

You should be able to make this for less than five pounds including the battery, and all of these parts, including old stock BC108, are usually available on eBay.