The circuit presented below consists of a high voltage supply, and means of converting tube avalanches to audible ticks. A small, regulated flyback supplies the high voltage needed. Each Geiger Mueller tube will have it's own "plateau" which is the working voltage range. Voltages higher up the plateau increase radiation sensitivity, voltages lower down do the opposite. I chose 420V since it lays within the plateau of most tubes.

By changing the zener diode values one can adjust the output votlage to whatever is needed by the tube in question. For the main transformer a disposable camera transformer is required, these vary little from camera to camera and all will have the necessary windings. I assume they have a common pin out, but if not experiment until you get the circuit oscillating. The approximate inductance of the windings can be determined by the winding size in the schematic (I know, very crude), so if you have an L/C meter handy finding the pin out shouldn't be too hard. The ingenious flyback supply isn't my design, I just ripped it off from here. Once the tube is energized and a particle hits it, it will short out. This causes a negative potential at the diode side of the 100pF capacitor which turns on the PNP Darlington transistor combo (the two BD140s). Once on they flash the ultra-bright LED and chirp the buzzer. The voltage at diode UF4007 is approx 5-6V. A real darlington transistor could substitute the two BD140s, I only used them because I had some available.