Geiger Counter

Overview

For a while I had been planning to build a Geiger counter, I've always enjoyed putting electronics projects together and this got to teach me a few neat things about charge pumps, etc. The project was to build a board that would support the use of a SBM-20 Geiger-Muller tube.

The SBM-20 responds well to Hard Beta and Gamma radiation, it's placed in a weather proofed box with an extremely thin plastic window. Experimentation with a known radioactive source shows little to no reduction in the count with the window present. The following graph is a live representation of the output from that counter.

Recent Problems

The Geiger counter seemed to experience periods of time when it failed to detect any particles, these did not coincide with temperature or times of day.

It appears upon closer inspection that the tube became corroded with super glue fumes (from when I made a windows in an IP55 box). After cleaning these up with a fibre glass pencil it began to show some signs of life. Further it appears that mounting it on polystyrene also caused issues, removing it from this caused normal counts to return.

The device is now Bluetooth enabled which was previously stunted due to a faulty module, a newer cheaper module has been located and installed successfully. Finally two cable glands have been ordered to allow the device to be sealed and placed in any environment required.

Additionally it appears that on initial construction I used a Bipolar 555 rather then a CMOS 555, which explains why I had to remove a 3.9K Ohm resistor. I've also ordered a replacement 555 to bring the circuit back to it's original design.

Location

The detector is in Bowerham, Lancaster in the United Kingdom. Lancaster is approximately 5 miles from the reactors at Heysham Nuclear Power Station. Although I have no fear of Nuclear Power, it's just worth noting.

Hardware

The hardware design was based off these two guys work, thanks very much to both BroHogan and Jim Remington!

I swapped the Arduino for a PIC18LF25K22, removed the external pull up resistor and enabled the PIC's internal weak pull up. The first photograph is the finished build.

Mount for the Geiger Muller Tube is a piece of Veroboard and two 1/4" fuse clips.

Firmware

Communication with the Geiger Counter is via a RS232 port, the initial design included a SpartFun Bluetooth transceiver. The 0.1" pin mounts are present on the board however the model in stock appears to have stopped working and unfortunately the particular model with the pin out is now unavailable. The serial protocol was designed to be extremely simple.

The output from the HV side directly drives the Int0 line on the PIC, the ISR maintains a count of ticks for 60 seconds, and minute cumulative's for 15 minutes. The above graph is generated from a RRD file which populated from querying a SNMP server which in turn uses Perl to override a custom OID.

Because the company I own has a Private Enterprise Number I was able to allocate a unique OID:
iso(1) identified-organization(3) dod(6) internet(1) private(4) enterprise(1) alastria(32567) people(2) woodp(1) personal(1) projects(1) geigercounter(1)