I got this4 circuit from Hands-On Electronics, 1988. The American magazine no longer exists, but the circuit can be easily assembled. The transformer has 110 V primary and 6 + 6 V secondary with 200 to 500 mA currents. The critical component is the GM valve (Geiger Muller) and U1 is an LM386. The circuit must be powered by a source or large batteries, given its high consumption. The voltage doubler capacitors are polyester for 250 V or more.
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