Geiger Tube Simulator

Geiger Tube Simulator

This little gadget will simulate a 500 volt or less Geiger tube when driven by a signal generator of sufficient amplitude, typically 5 volts p-p or greater. The purpose is to generate a steady, precise number of counts per minute so that the Geiger counter's meter may be calibrated.

The circuit rectifies the signal generator's output to provide power for the circuit so no battery or power supply is needed. The generator's frequency is divided by 100 so that low counts per minute may be simulated with ordinary bench generators that typically only go down to 10 Hz (600 CPM).
The two input diodes may be just about any schottky type or even older germanium diodes. The generator should be able to supply over 5 volts peak-to-peak. Higher amplitude is fine and will stretch the pulse width slightly. I vary the amplitude to modulate the pulse width when experimenting. The pulse width is also set by the 2.2 nF; increasing the value will increase the pulse width. The width is about 200 uS with the values shown in a typical Geiger counter circuit operating at 400 volts. The 2N3440 was chosen for high breakdown voltage with a 22k resistor connected from base to emitter. The prototype's transistor breaks down at about 600 volts, making this device useful for simulating tubes operating at up to 500 volts. The breakdown voltage was tested at a low current. Since the divider is wired to divide by 100, the counts per minute is equal to the frequency of the generator multiplied by 0.6. Setting the generator to 100 Hz will give 60 counts per minute. To simulate a higher voltage tube, a higher breakdown transistor will be necessary. Make sure to test the transistor's breakdown voltage with the 22k from base to emitter. The breakdown will be higher than with the base open but lower than with the base connected to the emitter directly.

Construction technique isn't critical. The prototype is built into a plastic box with a BNC for the signal generator connection and miniature binding posts for the simulated Geiger tube output. Please don't notice the typo on the label! ('CMP' instead of 'CPM') Note that this circuit is for Geiger counters that ground one end of the Geiger tube (most do). It might work with other types if the Geiger counter case is floating relative to ground