These circuits generate high voltages and can cause dangerous shocks! Do not build these devices unless you are experienced and qualified to work on high voltage devices.
Here is a 700 volt or less power supply for powering a small Geiger tube or other very low current device. The circuit is very efficient when no current is being consumed which is typically the case in a Geiger counter. Geiger tubes draw about 100 uA when they pulse but the pulses are very short and relatively far apart (hopefully). The current drain from the 9 volt battery is less than 1 mA with no load. The circuit will supply a bit more current when operated from 12 volts. The output voltage is set by the string of devices that includes the neon bulbs. Select a combination of neon lamps, varistors, zeners, etc to achieve the desired voltage. Good results may be had by selecting one or two ordinary small-signal silicon diodes with the desired breakdown voltage. The current in the diode will be quite low and no damage will result.
The circuit shows the power supply in a typical Geiger counter circuit but it may be desirable to use a larger resistor from the high voltage to the tube - see the manufacturer's recommendations. A 10 megohm is a safe value for most tubes.
Note: This generator cannot supply much current. An ordinary 10 megohm voltmeter will pull the output voltage quite low; a very high impedance voltmeter is needed to directly measure the output voltage. A 1000 megohm resistor in series with a 10 megohm voltmeter will give a 100 to 1 voltage reduction and will not overload the circuit. Large Geiger tubes may also draw too much current in moderate x-ray fields so a more robust generator may be needed. (See the 3V version below.)
Here is another version that will supply more current for a larger Geiger tube and runs on two cells (3 volts).
The transformer in the prototype is a small audio interstage transformer with opposite ends of the primary and secondary connected together to boost the output voltage. The lower impedance winding may be connected to the collector to minimize the collector voltage for a given output voltage. Other transformers will also work, including phone isolation transformers, as long as the impedance is relatively high on both windings.
The above circuit was used to construct a home-made Geiger counter employing a 10 inch Geiger tube (LND 78014). The pulses from the tube were converted into 3 volt, fixed-width ( a few hundred microseconds) pulses suitable for averaging using the following circuit:
The pulses may be counted with a digital circuit or may simply be averaged to apply to an analog meter:
The indicated switch is a two-pole, six-position switch. The resistor values in the meter circuit were selected to give the proper reading on one of the two pre-existing meter scales which differ by a factor of two. The older meter required 60 uA to reach full scale even though it was originally a 50 uA movement. In the actual prototype, an additional position and pole were included. The third pole switches in a 1 meg to ground at the input of the op-amp to reduce the signal by two for one more range. the same 19.6k resistor is used for both positions. In a new design, the op-amp and meter circuit would be modified for the chosen meter and desired scales. The op-amp must be one that operates on 3 volts or less and must have a very high input impedance.
The Geiger counter prototype was built into a homemade wooden box:
The tube is mounted with cable clamps over a hole in the bottom of the box covered by a perforated aluminum shield. The holes in the perforated aluminum directly below the tube were drilled to a larger diameter to block less radiation.
The voltage multiplier is mounted on the underside of a Plexiglas subchassis:
The other circuitry is mounted on two pieces of laminate on the top side of the subchassis:
The front panel holds the meter, switches, resistors, speaker and 4.7uF capacitor.
The wiring gets a little messy when the end of the project is in sight! It was discovered that the connection from the capacitor to the op-amp must be well insulated. Bend the op-amp positive input lead up into the air and directly solder the wire. Add an additional low leakage small value film capacitor (0.1 uF is fine) to ground at the op amp plus input since the 4.7uF is some distance away.
The front panel is also made from lexan painted on the outside with metallic hammertone paint.
The performance of this detector is quite good. The background radiation near 0.016 mR/hour give a 2/3 full-scale reading making slight changes in the background easy to see. The extra-long averaging time (44 megohm) gives a very steady reading, albeit slow to respond. This is a sensitive counter and is easily "pegged" with an ordinary test radiation source or even a Coleman lantern mantle.