Simple Geiger counter
The circuit diagram above shows a simple Geiger counter. I designed it actually for a robot which will seek for the highest radiation exposure in a room, but many other applications are possible (for example Hardware random number generator). Beside the GM tube only standard components are used. The PCB mount transformer T1 is a 230V/6V version, operated in the opposite way as usual. The HV can be adjusted with the potentiometer R5 between approx. 300 and 1000V, therefore the circuit covers many different GM tube types. Check out the data sheet of the GM tube for the operating voltage and adjust the voltage before connecting the GM tube! Measuring point is between the cathode of diode D5 and 0V. Take care that your multimeter has a measurement range of min. 1000 V DC! Please also note, that the protective resistor R10 must be as near as possible on the anode of the GM tube. The capacitors C4 to C8 must be suitable for 630V AC. Keep also the creepage distances in mind.
Commonly available radioactive sources for experiments, tests and calibration can be found here.