I built this glow-lamp stabilized Geiger counter for a friend of mine. Uses any 400-450V Geiger tube, plus with the stabilizer disconnected, it can provide a maximum of 600V. When the battery cannot supply enough voltage to produce ~400v, the neon lamps blink and a motor-like noise is heard on the speaker.
The pulses from the GM tube itself are sent to a small 8-ohm speaker, since a 1.5V battery cannot drive a single LED, although combining the output with a transistor and an external supply, it is possible to do. It would be way cooler to make a LED-display counter circuit, though.
The operating frequency is ~4kHz and when the tube's disconnected, it makes a real harsh sound. This works also as a test if the supply's working correctly. (It shuts up when you reconnect the tube back again.)
So folks, what do you think?
