Maplin published a design for a Geiger counter a year after the Chernobyl explosion in the September 1987 issue of the Maplin Magazine. I bought the kit for the remote head and constructed this. I never built their meter, because I used two CD4017 decade counters to make a faux dekatron display, feeding the output of the chain into a mechanical counter bought from H.L. Smith in Edgware Road in the early 1980s. Edgware road used to be a haven of weird and wonderful surplus electronics shops.
I’m not really sure why Maplin didn’t go the counter way, it’s not like seven-segment LED displays didn’t exist then. The dekatron display is better than a digital count for the first couple of digits, because the spin of the display gives a feeling of count rate, in a way that changing digital display numbers doesn’t.
The kit was shockingly expensive at the time – the Maplin Magazine shows the kit was £79.95 in 1987. That’s about £221 nowadays. Perhaps markups were much better in Maplin’s geeky heydays, whereas now they are competing against Banggood and Dealextreme. The original schematic shows an oddball AG1407 GM tube, it looks like I couldn’t afford the kit when it came out and built it in the early 1990s, when they had substituted the LND 712 tube, which is still made, LND 712 datasheet here. Datasheet seems to indicate 6E-5 R/hr for a count rate of 1CPS for Co60 source, so a conversion 0.0036R/hr for 1CPM
The dekatron display was inspired by the Geiger counter at school – I think it looked very much like this Panax unit. It similarly used two decade counters to calm down the count rate to be slow enough for the mechanical counter.
There’s a PDF schematic of this dekatron Geiger counter over at UK Vintage Radio. I’m not absolutely sure I would design the EHT volts in as a front panel control stuck in front of a bunch of teenage kids with the natural temptation to go Spinal Tap on it which wouldn’t be good for the GM tube, and I’m not sure ‘elf’n’safety would allow the BNC connector with 500V on the centre pin now. There was a wider variety of interesting radioactive things about then – luminous watch dials and old aircraft meters tended to give a good response on this.
Geiger counters and radiation monitoring
A single geiger counter in a single location has no context, but if several in a similar location give a rise in background then something is up. It all sort of begs a Cloud application, which wasn’t possible in 1987. The trouble with the Cloud is that people always try and screw you for money and hold your data to ransom. After an initial dalliance with the Cloud I reverted to using rrd on a local Raspberry Pi. RRD is good for tracking environmental sensors, it supports averaging and data reduction for long term storage. It is based on a circular database that wraps round after a user-determined time, but by averaging frequent updates into secondary stores you can track variables with a coarser resolution over years. The circular database means the storage doesn’t keep on getting bigger filling up your hard drive; the downside is you have to determine temporal resolution and data longevity right from the start. The averaging and data reduction is shown in these plots, over an hourly, weekly and yearly basis. Apart from the spike when I took the head apart while running for the pictures in this post, nothing untoward happened over any scale up to a whole year.
Radmon is run by an individual, rather than a company. I modded the counter to add a Ciseco RF device that sends the count every minute via 863MHz SRD data, and a Raspberry Pi receives this and populates a RRD database. On the Pi I can use RRDtool and this shell script to extract the last count from the rrd database.
CPM=$(/usr/bin/rrdtool lastupdate /home/pi/rrd/db/geiger.rrd | sed ':a;N;$!ba;s/.: ([0-9])/\1/g')
Crontab uploads the count regularly. I’ll make an exception for cloud that isn’t run by a corporation, particularly as I tap off the data after the local RRD chart generation, so I don’t lose local functionality or resolution.
Most people on Radmon seem to use the Soviet SBM-20 tube that is to be had on Ebay for about £20. Unlike the Maplin tube the SBM-20 doesn’t respond to alpha particles as it has no mica window. However, I don’t think alpha is that common in a civilian environment.
The Maplin tube gives a lower count, probably about half the value of nearby stations. It is a fair bit shorter than the long SBM-20 so the active volume is lower, perhaps by about half. This stacks up with the Co60 conversion factor of .0036 derived from the data sheet – the corresponding value seems to be .0057 for the SBM-20 tube in common usage. It’s shown clearly in this video
From memory the only civilian use of alpha radiation is in some kinds of smoke detectors. It’s an isotope of Americium I think and I don’t think that the alpha gets out of the plastic case.
I have a panax, scaler type 102st, from redhill in Surrey. I couldn’t bare to see it disposed of. Question- do they contain any dangerous radioactive isotopes? As it’s in my living room for it’s ornamental value, along with some other pieces of old equipment. Please feel free to contact me, all the best, Melvin.
No, don’t worry about it if it looks like the one in the photo. No meters with luminescent paint. A radioactive Geiger counter display would be a bad thing to design, somewhat masking the signal you are looking for 😉
In service the 500V on a BNC socket, albeit via 2M7 wouldn’t be considered electrically safe in front of the general public these days. And as is the general rule with any vintage gear don’t just plug it in if it has been unused for decades, as at best the electrolytics will need to re-form and at worst they will blow anyway, and rubber mains cable from back in the day can harden and the insulation crack away.
Sadly the schematic only shows the signal conditioning and GM tube EHT generator, not how the dekatrons are wired, I’d imagine you need at least 100V to get the neon glow on a dekatron.
But no radiation hazard, assuming it hasn’t been irradiated at the AWRE or something. I’d imagine they use something better than a school science device for testing bombs…