Aware RM-60 Geiger Counter
by J. Beale July 2007
The Aware Electronics RM-60 geiger counter (photos above) is a small plastic box that connects to a computer serial port via a 4-wire cable and DB-25 adaptor. The counter is powered by the voltage on the serial port pins.  Here is a plot of background radiation levels I recorded in 1997 with this counter, and a pdf plot of diurnal variation in 2007.  Here is are some other geiger circuits. There is an online random-number generator using the Aware RM-80.

total parts count select parts of note
5    transistors
8    diodes
11    resistors
14    capacitors
0    ICs
3    2N3904A NPN
2    KN2907 PNP
1    1000 uf 35 V
1    .033 uf square
1    100 pf, 1 kV
1    470 pf, 1 kV
 1    tube, LND type 712
2    10 Mohm resistors
1    4.7 Mohm resistor
2    1 Mohm resistors
1    HV transformer
1    RJ-11 jack
Black Cat Systems also sells computer-interface geiger counters. Their GM-10 model appears to have similar specs to the Aware RM-60 pictured above.  They have a page on how the serial interface works and a circuit diagram.
Radalert Geiger Counter


The Radalert geiger counter shows counts per minute (cpm) or total counts on a LCD display, and can sound an alarm if the radiation level exceeds a preset value. It runs from an internal 9V battery but also has an external power input, and a data output for logging.  The photos above show a kit that I assembled in 1992. This kit is no longer sold, but the Radalert 100 appears to be similar, and in 2007 it is still sold by International Medcom.
Radiation Sources

"Vaseline Glass" bead

Most things in a typical house are not radioactive, so it may be difficult to tell if the geiger counter is actually measuring something, or just clicking at random. One possibility is a gas lamp mantle (contains thorium), although many mantles sold now use non-radiative alternatives. There is a type of yellowish-green glass called vaseline glass which has small amounts of uranium oxide as a coloring agent, and consequently is somewhat radioactive. I was able to locate some of this material at a local bead store by shining a UV flashlight on the trays of beads. The uranium ions give this type of glass a strong fluorescent response in UV.  Putting the bead (shown above) directly against the alpha window on the Radalert counter gives about 70-80 cpm, where the local background is around 12 cpm.  This is a very weak radioactive source, and is not a significant health hazard.  The count falls off to the background level when the bead is moved more than 1 cm away from the counter window.

Bead on geiger counter (lit by UV light)
back to projects page