This is a simple homemade miniature detector (indicator) of radioactivity (ionising radiation), which uses a Geiger-Muller sensor (GM) tube.
This is a detector (indicator), not meter, because it is not equipped with measuring device
but only the optical and acoustic indication. The device could be used as a basis for the analog meter or a digital Geiger counter ass well.
My Geiger - Muller (GM) tube is the STS-5 type (Produced in the former USSR), sensitive to beta and gamma radioation. The Geiger tube used requires a voltage of 400V. This is obtained in switched converted with T1 and T2. T1 acts as an oscillator, which drives the miniature transformer Tr1. Tr1 is the transformer for CCFL tubes. For better efficiency I replaced the air-gapped core with a core without gap. Primary is rewound to 2 x 15 turns 0.3 mm dia. The original secondary stays unchanged, it is about 1000 - 1200 turns. T2 acts as a feedback. When the voltage on C5 reaches 400V, current begins to flow through zeners ZD1 and ZD2 and will reduce the duty cycle of oscillator. Total ZD voltage is 400V. You can also use a different number of zeners, transils or varistors as long as the sum of voltage stays 400V. The aim when designing the circuit was very low consumption. I used the stabilisation with feedback, not only parallel regulation. Tr1 would itself be able to deliver 400V, but the use of multipliers reduces power consumption (because of losses in the transformer core, capacitive losses, etc.). GM tube works in a simpler - ungrounded connection. Pulses are amplified by transistors T3 and T4 and subsequently indicated using a small speaker about 8 to 32 ohm and superbright red LED diode. LED is powered by the doubler circuit, because its voltage drop is about 1.6 - 2 V and input voltage is not enough by itself to light it up. I used a small 16 ohm speaker from old LCD monitor. Geiger tube slot can be used the same as for the larger tube fuses. Detector indicates about 25 pulses per minute at normal background 10uR / h (equivalent to approximately 0.1 uSv / h).
The device I was able to build with dimensions of just 110 x 50 x 15mm. Length is determined by the length of the tube. The device is powered by AA or AAA 1.5 V battery or 1.2 V NiCd or NiMH accumulator. Consumption is only about 4 mA. AA NiMH batteries reach today (2011) 3200mAh capacity, and thus allow for one charge up to 800h of operation :). For comparison, the old RBGT-62 radiometer draws 75 mA from 3V and thus has a 37.5 x more power consumption.
You might also be interested in: Digital radiometer - dosimeter.