Purchasing and modifying a Geiger Counter

With all that is happening around the world, one piece of equipment that hopefully none of us will ever have to use is a Geiger Counter. There are many surplus counters on the market but most have drawbacks or deficiencies. You really do get what you pay for.

If you want one that does just about everything, the Gamma Scout is probably the top of the line at US$299.00. It measures Alpha, Beta, and Gamma. Displays readings on a four digit LCD display, in Becquerels, Sieverts, or accumulated pulses. It also logs the readings and is able to dump the log into your PC for recording purposes. It is also on all the time with a battery life of about 10 years. It comes with a leather carrying case and software for dumping the logs. 

The becquerel and sievert are SI (International) units. The sievert (Sv) replaces the rem. 1 sievert=100 rem. For a table of radiation conversions click here: Conversion Table.

The only drawback I found in the Gamma Scout is the lack of an audible signal indicating a tick (count). Since the unit has a 3 volt battery that lasts 10 years, I guess the audio portion would somewhat decrease the life of the battery.

I have gone into the unit and have found a way of adding an audible sounder that will make the unit a little more desirable. With the addition of a few parts, your Geiger counter will also "tick" like the real thing.

Of course modifying your detector will void any warranty you have with Gamma Scout.

Warning. High voltage is present around the geiger-müller tube and you will be soldering to the low voltage of a live circuit!

It is not recommended to disconnect the battery or you may shut down the processor chip or possibly loosing calibration data.

The simplest way to modify the unit is to tap into the pin that goes to the counter processor through a 100k ohm resistor and bring it out to the DB9 serial connector, Pin 9, which is not used in this unit. Pin 5 being ground. Your audio portion can be external as this provides a 3 volt to ground pulse for every count. This can also be used for your own external counter, log, or alarm system.

A more desirable way is to place the audio driver circuit and separate battery within the housing itself. You could provide a jumper from pin 9 to pin 5 to turn the audio on and take it out if storing the unit. The current draw from the audio portion is almost nil so you could use a separate lithium battery to run it. I would not advise tapping into the existing battery because you would decrease the life of the unit and one mistake in your modification and you may damage the unit beyond repair. Besides, 3 volts doesn't quite give you enough audio. I recommend two 3 volt lithium's in series to drive the speaker.

The following is a summary of how to modify the counter.

Here is the breadboard version. Note the DB9 at the bottom of the unit.


Rear cover removed. The GM tube is made in the U.S.


The 100k resistor tap point is just to the right of center in the picture.


This is the driver from the Geiger tube to the processor.


The above is the existing detector. The added parts are denoted with an asterisk.


The above circuit is shown as an external unit. To install within the housing, the power switch is a jumper between pin 5 and 9 of the DB9. If mounting internally, extreme care should be taken as to where you place the components. The speaker should be mounted on the housing back with holes drilled into it. The enclosure will ensure a deeper sounding "click" coming from the speaker.

The circuit draws so little current that I could not measure it. The only way to measure the current is to calculate using the width of the pulse and the current with the mosfet saturated, and the average number of ambient pulses over a period of time. The battery is two 3 volt lithium button cells with solder tabs, soldered in series.