4/8
If the clips are too tight, stop and bend them outwards slightly, then try again. Don't force the tube into the clips
(you will crush it)!!!
The tube is polarized! Make sure the + mark or CBM20 writing (if it’s an SBM-20 tube) is on the left side of the
board, closest to the tube clip marked +.
21. Test it!
Note: If you purchased a plastic case with your kit (or it came as part of a bundle), you should complete the case
assembly instructions on page 5 before powering on your kit for the first time.
Warning: This kit is capable of generating high voltages (300-600V). Don't touch any part of the circuit
(particularly the bottom of the PCB) while it is on! The current is low, so while the potential for injury is
low, it may startle you and cause you to drop your kit (and break your Geiger tube!)
Clean any stray leads off of your workbench so that they won’t short against the PCB. Make sure switch S1 is off
(away from the ON position) and adjust VR1 all the way counter-clockwise.
Insert two AAA batteries in the battery holder and turn on S1. Listen for clicks and watch the LED. SLO W LY
rotate VR1 clockwise until you hear clicks. Once you start to hear clicks, rotate VR1 about 45 degrees more and
stop. The exact adjustment is not very critical.
If you got this far, congratulations! You have built a working Geiger counter!
If your kit isn’t working, do not dismay! Check the troubleshooting instructions on page 6.
Usage Instructions
The LED will flash and the speaker will click every time a beta particle or gamma ray hits the tube!
Note: The tube that comes with the kit (if you purchased one) is not sensitive to alpha particles.
If you get tired of hearing the clicks, you can push the button S2 to MUTE the sound.
The PULSE connector (J6) has the following pinout:
1. VCC (nominally 3V)
2. pulse output – a short (100us) active high pulse every time the Geiger tube fires
3. GND
More information, design files, source code: http://mightyohm.com/geiger
Data Logging
To connect your computer to the serial port (J7), you will need a USB-serial converter that operates at 3.3V TTL
levels. The serial header is set up to work with the common FTDI-232R-3V3 serial cable.
Data is sent over the serial port at 9600 baud, 8N1.
The data is reported in comma separated value (CSV) format:
CPS, #####, CPM, #####, uSv/hr, ###.##, SLOW|FAST|INST
There are three modes. Normally, the sample period is LONG_PERIOD (default 60 seconds). This is SLOW
averaging mode. If the last five measured counts exceed a preset threshold, the sample period switches to
SHORT_PERIOD seconds (default 5 seconds). This is FAST mode, and is more responsive but less accurate.
Finally, if CPS > 255, we report CPS*60 and switch to INST mode, since we can't store data in the (8-bit) sample
buffer.
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