Old-School Light Chaser

This is based on a very old circuit, first published in the 1960s with germanium transistors and using small incandescent light bulbs.  At the time it was published, the only LEDs available were large red things (think HAL9000 from 2001, a Space Odyssey) that cost about £50 each.  A string of 48 of those would have cost you the price of a nice car.  A new one.  I rediscovered this in a document I found on the internet one day, which you can see here - it's the second circuit in the list.  Below is the schematic.

There's a good description of how it works in the link.  It's based on a circuit called an 'astable multivibrator', which is basically a 2-stage common emitter amplifier, capacitor coupled between the stages and with the output of the second stage fed back (also through a capacitor) to the input of the first stage.  The circuit here simply adds extra stages and some emitter to base diodes which prevent the transistors' base-emitter junctions from going into reverse breakdown.

I thought it would be neat to adapt this to use LEDs and build a long string of them, running on, say, 12V because I had several 12V wall-wart type power supplies doing nothing at the time.  I opted to use red, green & blue LEDs, 16 of each making a total of 48.  Using BC337s for the transistors (they were the cheapest I could get hold of, at about 2p each from CPC if you bought a bag of 100) and running on 12V, I found that there was enough leakage through the capacitors to keep the transistors slightly conducting, so added a bleed-off resistor in parallel with the diodes to fix that.

I've also added a switch to control the sequence - bypassing the transistor of the first stage will switch that light on and prevent the second stage (and therefore subsequent stages) from triggering.  By closing and re-opening the switch you can send extra 'pulses' down the line.

Here's a video of the light chaser just before the final assembly & covering with heat-shrink.

Since making the first one, I've tweaked the circuit a little, so now it's as shown below.  Capacitors are 10 microfarad electrolytics.  More construction details to follow in the next post.