Tag: geiger counter

CAJOE ebay geiger counter kit

I bought this noname kit, from somewhere in China. I may want two of these in the end, to compare radiation near one site compared with the general background, so getting cost down1 is worth having, and I can’t moan about the price, delivered for < £25 including tube.  But I can moan about the total absence of documentation! You pays yer money, you takes yer choice. I looked at some of the offerings from Banggood to see if I could match it. Maybe I should have paid an extra £5 and got someone in China to assemble it for me but the board looks the same. Bangood’s board says RadiationD v1.1 (CAJOE).

CAJOE Geiger counter board top
CAJOE Geiger counter board top
CAJOE Geiger counter board underside
CAJOE Geiger counter board underside

It’s a tough life in Chinese electronics, even the copiers get copied. The write up is on Github – it’s a little bit obscure, the PDF overall schematic isn’t that useful but the more detailed schematic in blocks is.

Not a kit for beginners

If you are new to electronics, this kit is not for you due to the amount of guesswork and component ID required, buy it built, the difference isn’t that much. There’s nothing that special or earth-shattering in the parts however, all are easily available, and the board is a reasonably decent construction and easy to solder. There was some satisfaction in second-guessing our Chinese friends but really, how hard would it be to provide the link in a slip of paper or even on the original ad? Investigation of the original designer CAJOE with the Wayback machine at the URL indicated on the PDF didn’t deliver much enlightenment, but there’s enough in Github to give it a go. I used the block schematic with values, thankfully the screenprinting reference numbers on the board match the components in the PDF, which the exception of C5 which is MIA on the schematic, at a guess a 100nF decoupling cap.

The kit prep is a bit slapdash. The classic way to drive a mechanic bonkers is tossing some spare screws in his tin of screws taken out of the engine, and here they lob in some spare components – on populating the resistors I was left with a few over, same for the caps and a couple of transistors. Continue reading “CAJOE ebay geiger counter kit”

Maplin geiger counter

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.

Counter part of Geiger counter
Counter part of Geiger counter

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

Continue reading “Maplin geiger counter”