This has been an interesting read for me. As someone who has also created a Geiger kit, I can understand some of the oddities of trouble shooting a HV circuit.
The HV will travel very well through a poor conductor - excess flux on board, etc. I even discovered (unintentionally) that black foam board is conductive enough to short out the HV!
Also, since the current available is so small, measuring it with a DMM will load it down so much you won't get a good reading, and the HV might stop working while the DMM is connected.
At the risk of plugging a competitive product, I have written instructions that apply to measuring HV in Geiger circuits here . . .
The should apply to any HV circuit.
Thank you so much for making this available. I got a new kit in today, revision 04/12/11LW, which did something similar. The schemos include a 100R resistor for the LED but everything else is the same. The sensitivity was almost nothing, probably a click every 3 minutes or so. I noticed that if I touched the + end of the tube with a single voltmeter probe, it stabilized and I got actual background level, about 25CPM. To my surprise I had disk shaped 1000pF/500V caps in a drawer (model military suitcase) and fitted one of them standing vertically on the outside of the transformer with one pin between T1 and Q1 to the jumper over to the (middle) case (-) of the GM tube, the other pin along the edge of the board to the cathode of the diode. I also changed R3 to 1MOhm (biggest I had in throughhole). The board now works beautifully with no double clicks.
(Repost from the blog)
I must have been thinking of another tranny I was using recently. And you'd never get this info off the kit maker. I've seen kits - and boards - where the maker has sanded the markings off the ICs to stop people copying their design...
As a bit of an aside, I just bought a few LCD voltmeter modules cheap from my local Dick Smith (Aussie equivalent of Tandy). They say they have 100 Megohms input impedance. So it should be possible to make a 2KV meter out of one of these with an input R of about 1 gig-ohm. Anyone know where I can get some 100 meg resistors??
Hmmm....smart maybe (even that is stretching it) but genius, no. I regularly surprise even myself with my own stupidity....
There is no information on T1 in the kit documentation and none of my photos show any markings. As I recall there were some markings on the transformer (on the tape covering the windings) but not sure what they were. Max??
D1 is indeed a 1N4007 - this has 1000v max reverse voltage so quite adequate for this job.
If you're thinking of building one of these yourself, if you can get an inverter transformer from a small fluorescent lamp it should do. A turns ratio of about 1: 50 would be needed (9V to 400V) but do have a look at the Maxim app note at
which is for a regulated supply. Needs a few more (cheap) components, but no transformer (it's a boost switchmode supply with a voltage multiplier).
Thanks Jay. The lack of DC coupling between the HV/LV did worry me as well. And I was tempted to try connecting them together. However you will note that the speaker stage is DC-coupled to the tube (ie uses the capacitance between the transformer windings for AC coupling) and if I'd DC coupled it then that would quite possibly have had other problems.
I DID try putting a large-ish cap (1 uF) between the HV/LV grounds and it did not make much difference.
The diode across BE of Q3 is a good idea. I'm not sure if the led going off at high counts was due to the charge on C3 or because of the capacitance (68pF) I put across the BE. As you can see form the trace above, the "event" pulse and the power supply pulses are quite different at low count rates, but at high count rates I think the event pulses would merge a bit and reduce in amplitude, so my cap would attenuate them to the point where they no longer switch Q3 on. So I'd also be keen to try Pchow's solution (lower R and no C) with a high rate source.
As I said, were I designing form scratch I'd do it differently, but in this case I wanted to get the existing design going with the minimum of modification. I had such fun with this that I might buy myself a tube and try that.....
HURRAY!!! My (now-working) Geiger counter has just arrived back from David.
It’s on my desk clicking away randomly – my (mildly) uranium-enriched glass marbles do seem to increase the count a bit – but I need to time things like David did to get a good feel for it.
I did just borrow a 5 uSv radioactive sample from the folks in the next bay and the little scamp started clicking and flashing furiously.
I also want to try it with that "No-Salt Salt" David used.
I’m heading out to DAC this weekend – so I’ll take the video and write the follow-up blog when I get back.
THANK YOU DAVID!!!
Your observation (3)- that the high-voltage section has no DC return - may be the key to the LED staying on. The HV section is actually pulsating, and it's coupled to the low-voltage section (as you say) by capacitance in the transformer and by C3 feeding into the base of the LED driver. So there is a continuing series of power-supply pulses charging and discharging C3 through the LED driver. If you just connected the "gnd" terminal of the Geiger tube to the -9V terminal of the battery, I am pretty sure you would eliminate the LED-on problem (no new component needed - as long as C3 has a high enough voltage rating).
Now for the LED going off at high duty cycle, that's probably because the E-B junction of Q3 carries much more current in the forward direction than the reverse, eventually building up an "off" voltage charge on Q3. I'd fix that with a simple diode across the E-B junction to carry current in the other direction.
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