Galactic Electronics Projects

Build this Geiger counter from commonly available parts (except the Geiger-Mueller tube).

This is a simple Geiger counter that uses a small 500 Volt Geiger-Mueller tube and is powered by a 9V battery. The counter produces audible clicks through a small speaker that represent ionizing radiation passing through the Geiger-Mueller tube. The high voltage power supply is basically the same high voltage power supply featured in another project listed in Galactic Electronic's web site. Battery life is about 14 hours continuous operation with an alkaline battery. This Geiger counter has a low battery indicator. The "Low Bat" LED glows when the battery power is not enough to sustain the 500 Volts required by the Geiger-Mueller tube.

New Geiger-Mueller tubes are not inexpensive when purchased in small quantities but can be found on the surplus market at a lower price. I was fortunate enough to have a friend that used to work in the radiation detection business and he gave me a small 500 Volt Geiger-Mueller tube. The other parts in this Geiger counter project are commonly available components.

Geiger-Mueller tubes have their limitations in sensitivity of weak radiation fields and are also saturated in very strong radiation fields (strong enough to be lethal). When the Geiger counter presented in this project is operated under normal background radiation conditions, an occasional click is heard, every 5 to 10 seconds or so. There are not many household items these days that produce enough radiation to generate a noticeable increase in click rate above background radiation. I tested my Geiger counter with an old compass with glow-in-the-dark markings on the dial. This compass is over 50 years old and was manufactured when Radium was used in glow-in-the-dark paints. This compass produced a fairly high click rate in my Geiger counter (several counts per second). Other sources of radiation may be antique green glassware 80+ years old, which used Uranium to give the glass a green color. Also, brick used to line the fire box in fireplaces in older homes may also exhibit some radioactivity.

Here's how it works -

The oscillator uses a transistor, capacitor and a transformer. The oscillator design I used is based on blocking oscillators I have seen used in Xenon flash-tube power supplies in cameras. The transformer is an audio output transformer run in reverse (output being used as input). I used a common audio output transformer purchased from Radio Shack in USA (part # 273-1380). The transformer output in this circuit is about 80 Volts. These transformers are not designed to deal with high voltages but they do - and they are inexpensive. The output of the transformer is connected to a four stage voltage multiplier. The rectifiers (1N4935) are high voltage, fast recovery diodes. The 1N4004 will work in the voltage multiplier but fast recovery diodes yield a higher voltage output. The capacitors are 200V, polyester tubular type.

The high voltage power supply does produce a quiet whining sound in the speaker despite the 330uf filter capacitor across the battery.

The output of the transformer is then connected to the voltage multiplier. The center tap of the transformer is grounded through a 47K resistor. This resistor stabilizes the circuit and prevents it from "running away" (despite the regulator), which eventually destroys the oscillator transistor. I don't understand why this works but I put the 47K resistor in the circuit intuitively thinking it might give a ground reference for the oscillator and help stabilize it - and it does.

The output of the voltage multiplier is then filtered by a 1500 Volt, .01 uf capacitor. The feedback for the regulator is connect to a LM358 op-amp through a 22M resistor. An adjustable voltage divider is made with the 50K pot for high voltage adjustment. The LM358 is operating as a comparator modulating the base of the oscillator transistor. I chose the LM358 because it operates from a single polarity power supply. I was concerned about the 358's low current sensitivity, which can be an issue with a 22M resistor feeding the input from the high voltage. However, it seems to work well. I used a LM385Z, 1.2V voltage reference in the regulator circuit. The circuit does not need a precision reference but I have a few of them on-hand so I used it. A plain Zener diode reference would probably work just as well.

Be careful to adjust the high voltage to 500 Volts before connecting the Geiger-Mueller tube. Over-voltage on the GM tube may damage it by causing ionization and conduction like a neon light bulb inside the tube. After connecting the GM tube, carefully adjust the voltage upwards until the GM tube starts detecting background radiation.

The high voltage is connected to the Geiger-Mueller tube through a 10M resistor. Then there is a voltage divider connected to the case of the GM tube. This voltage divider is composed of 10K and 100K resistors with a 220 pf bypass capacitor. The bypass capacitor value should be supplied with the specifications for your GM tube. In my case, where I am not sure exactly which GM tube I have (I think it is a Ludlum LND 713), I had to find the value experimentally. The output of this voltage divider, which lowers the high voltage pulse from the GM tube, is feed to an NPN transistor inverter which in turn feeds a one-shot multivibrator made from a 555. The one-shot produces the audible click.

The reason for using a one-shot is that the voltage pulses coming from the GM tube are only 1 microsecond to a few milliseconds in length (depending on the GM tube). This is not long enough to produce an audible sound in most small, low cost speakers. The one-shot stretches out the pulse long enough to hear it. I used a small, aluminum coned speaker I salvaged from an old modem. It may be desirable to use a CMOS 555 because of the low power consumption but I don't recommend it because the CMOS device can't pull enough current to produce of click of sufficient volume in the speaker.

The other half of the LM358 is used as a simple comparator to monitor the battery level. The output of a voltage divider (100K and 24k resistors) cross the battery is compared to the 1.2 Volt reference. When the battery voltage falls below approximately 6.2 Volts, the LED glows to indicate a low batter supply.

The schematic -

Construction notes -

In this project I took the time to manufacture a printed circuit board using the Ferric Chloride etching technique. I purchased high quality, single sided fiberglass PC board stock from a local surplus outlet (Electronic Surplus, Inc. in Cleveland, OH). I produced the circuit traces (circuit mask) using dry transfer traces I purchased from Radio Shack. I chose fiberglass PC board stock because of it's high resistance which I think is desirable considering the high voltages involved.

The circuit board is pictured below. It is shown upside-down so that the battery does not obscure the circuit board. The voltage multiplier can be seen in the upper left. The LM358 and 555 are seen in the lower right. The Geiger-Mueller tube can be seen in the lower left of the circuit board. I realize now that the GM tube should have been mounted at the bottom of the circuit board (as shown) perpendicular to it's current position. This way the GM tube will be most sensitive to radiation in front of the user, instead of to the left side as the GM tube is currently mounted.

The plastic enclosure used was purchased from Radio Shack. I drilled concentric rings of holes for the speaker outlet and four holes for mounting the circuit board. There is a power switch which is not shown in the schematic.

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(c) Jon Qualey, October 2006

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