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).

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.

More importantly, they didn’t include the 100µH series inductor L1 to reduce EMC going back into the power supply, so I scavenged one from the junkbox. At least they included L3, the 10mH flyback inductor where the value matters. It was unmarked, but I measured it at 9.8mH, which is within the groove. The transistors were marked a bit weird, the MPSA42 at least had 42 in the marking

whereas the 2N3904 replacement was anybody’s guess

I measured these on a transistor tester to compare with a known 2N3904 and there were within the ballpark, so I ran with these,  the 2N3904 is the classic jellybean NPN. The MPSA42 is pushed a bit harder and needs to take the back-EMF of the 10mH inductor, so it needs the 300V VCE of the MPSA42 (datasheet), I measured the spike on a scope at about 250V. The first NE555 IC1 runs about 14kHz, with the reset pin set at the right point via the pot and transistor Q1 to make narrow pulses into the 10mH flyback inductor.

I’d say the HV section owes an unacknowledged debt to the Nuts & Volts Pocket Geiger from Jan 2004, which predates CAJOE’s work by two years, and has a description of how the flyback converter is supposed to work. CAJOE missed some of the subtlety, particularly getting enough Q from a wave-wound inductor to achieve the step-up in a single pass rather than using a tripler, and getting the 555 discharge pin to shut off the MPSA42 faster. Perhaps the economics of a fancy-schmancy inductor didn’t work out for them compared to two more diodes and two more caps.

Setup instructions for the CAJOE aren’t bad, though subtly wrong, of which more later. The onboard 50MΩ series resistor to your DVM (made of 5x10MΩ in series) for setting up is a nice touch.

It’s not the lowest power design, but it worked first time, once I had sourced the missing L1. It pays to note that the fuseholders used for the tube have to be mounted the right way round (pip to the outside) which I only just spotted in time. I omitted the power LED as a needless power drain. The audio out is fairly quiet, and the second NE555 seems to stretch the pulses a lot, I can hear the double knock of the rise and fall of the click that is differentiated somewhat by the poor LF performance of a 1cm wide speaker. However, that doesn’t compromise the output to the Arduino which is a lot shorter, and it does make the LED flash easy to see. Pulling J1 lets you shut the audio click up, which is handy.

Voltage setting: the indefatigible ullix/GeigerLog has looked at this² and suggests going lower than the ‘calibration’ suggested in the Github documentation., rather than 57V at the test point with a DVM@10MΩ back off to ~10x desired voltage = 400V to allow for the unknown source impedance. The fellow has a point, since R33 after the tripler ensures that the source impedance of the supply is at least 10MΩ, which sets CAJOE’s calculation off, the open circuit voltage would be higher than the calculation assumes, which could be detrimental to the GM tube’s service life. I have a secondhand 1GΩ 1000:1 Testec HVP-40 EHT probe on order from Ebay, as I don’t want to wreck Geiger tubes by overvolting them needlessly. That’s bad enough with a single tube unit but even worse with a multi-tube rig. A couple of 200V zeners in series across the tripler output is tempting.

Update: I have the Testec 1GΩ 1000:1 probe, and I had lined up the original voltage at > 500V at the tube!  Bringing it down to 412V at the tube anode with 1GΩ across it, the counter keeps running, and measuring at the test point with the DVM (10MΩ) on its own shows 47V, and 141V straight across the anode loaded by 10MΩ, the counter naturally stops counting at such a low anode voltage. So Ullix’s criticism is justified, and I was running the tube 11% over the probable maximum spec lining up to CAJOE’s recommended 57V with a 10MΩ DVM. The Testec’s 1000:1 ratio on the DVM was confirmed on a 26V source, it was good enough at 26mV o/p.

Unlike the Maplin Geiger counter from the 1980s which used a Zener diode string to stabilise the voltage the CAJOE is open loop. The J305 tube wants 400V though the Geiger plateau makes it reasonably tolerant. Unfortunately it’s the devil’s own job to get any spec sheet for this tube – from here I see 380-450V and I have read the same somewhere else, but for all I know it is the same hearsay which I have just added to!

The problem of light and glass Geiger tubes

Is that while all are sensitive to light³ to some extent, some of the blighters are very much so, and this appears to vary from not very much to hellaciously much for Chinese tubes that look identical. The reason for this variability is given in the abstract⁴ of Albrecht, 1958

The photosensitivity is traced to formation of a surface film upon the inner wall of the cathode; that is, a quasi-metallic alkyl coating is assumed to be formed.Relatively short wave radiation of sufficient energy to form free radicals in hydrogen and organic quenching gases is required to increase (“activate”) the sensitivity.

Which is a bear, as it seems this film varies with manufacturing and it’s the luck of the draw on the Chinese tubes. You have been warned. Until I came across these Chinese tubes I didn’t realise you get glass Geiger tubes, I had only come across the metal packaged LND712 from the old Maplin kit and a Black Cat Systems GM-10⁵ which I have, which also uses the LND712. I have a couple of Russian STS-5 tubes, again metal enclosed.

I shone a UV several cell torch that I use for curing Bondic rather than the gutless CR2032 single LED device they provide; I observed no immediate jump in audible count rate so my tube is one of the better ones. But I am sure it is light sensitive to some extent, this is why a genuine Russian SBM-20 is in a brass tube! You lose β sensitivity through the screening, though not so much for gamma.

I plan to use this for surveying differences of gamma radiation in granitic regions (in the open air, not radon) so I can use opaque material, indeed I could take a leaf from the SBM-20 and use brass, or a small ally box, or sheet PCB material.

I am considering paralleling some of these tubes up to present a larger volume and increase sensitivity, since the standard error for a Poisson distribution is the SD ÷ √N, N being the number of counts, and the SD of the CPM is √(avg CPM) for a Poisson distribution. This is miserable for a typical background count of ~ 12 CPM⁶ – 3.4/√N – in 1 minute that’s a standard error of 1 in 12 or nearly 10% and to get it down to 0.6 (5%) you need to wait three minutes, which is a long time in the field for one measurement. An array of four tubes in parallel would halve this. OTOH this table seems to indicate the J305 is twice as sensitive as the LND712 I am used to, (44 CPS at 1 mR/h as opposed to 18 CPS at 1 mR/h for the LND712)  so I may have an easier ride, I have only bench tested this. to see if it works so far, a board with 400V on the underside can’t be chucked in a rucksack as it is!

On the whole, this is a win for not very much money and a few weeks wait for the slow boat from China.