This is a device which I have constructed back in 2010, when I had no calibrated radiation metering devices such as the IT-65 at hand, and there was a need to distinguish between normal and abnormal intensity levels. The reason why I have chosen to do this from scratch was simple: back then, I had quite a few GM tubes lying around doing nothing and I’d been unhappy with a conversion applied to the IBG-58-T detector previously. At that time, I have also started my first X-ray experiments, so having a simple but sensitive “indicator” was, of course, a must.
It is pretty simple in terms of operation. There’s an NE555 oscillator powered from two old Li-Ion cells with a small 6 volt mains transformer, giving out approximately 1500 volts peak when unloaded. The current is rectified, filtered and sent to two neon glow lamps in series against ground. These have a special mixture, giving them an ignition voltage of approx. 180 volts each – you can find these in old fluorescent bulb starters. In operation, they stabilize the voltage output to approx. 370-440 volts, so they esentially give optical information about correct voltage on the output and also a low battery “warning” if they don’t light up. Of course, these glow lamps can be substituted with Zeners.
The Geiger Müller (GM) tube here is a Soviet STS-5 (SBM-20) and is connected to the output through a few megaohm resistor. In series with it, an optocouple is used for galvanic isolation – to get rid of a low-frequency hum, caused by the internal inverter. Since I did not want to make my own ferrite driver transformer, I’ve used a mains one (as described above) which has to be driven at low frequencies to get the best outputs.
The optocouple output feeds a small 45 ohm speaker and a LED diode through a Darlington pair, for pulse indication. Finally, a 3.5mm jack is provided to connect external headphones or, optionally, to a digital counting circuit.
One of the drawbacks with not designing a proper transformer and using the “glow lamp stabilization method” is the bigger current draw – 15 to 35 mA @ 7.2V. But I had no other use for the seven year old Li-Ion cells, so it did not really matter at the time. Additionally, they wouldn’t hold a reasonable charge for powering up anything more sophisticated than this.
With uraninite (pitchblende) again