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How to make a Geiger counter count

5/24/2011 04:40 PM EDT
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Max The Magnificent
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Max The Magnificent   5/24/2011 5:01:05 PM
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This is a wonderful article David thanks so much for all of your efforts on this this -- Max

David Ashton
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David Ashton   5/24/2011 7:52:15 PM
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Phew! This has been a mission. As above, I had taken 10 days off over easter (in mid-April) but alas Max's counter did not arrive until just after I was back at a very busy work. Couple that with a wife demanding that the lawn get mowed and how come I fix other guys things before ours?.... and I was left with precious little time to get into Max's baby. @ The SPEAKER FELL OFF?.... Don't worry Max, I treated it like a newborn and it is now cosseted in a nice bed of double sided tape so it won't fall our of the cot again.... All in all a very instructive fix though, I'm very grateful to Max for giving me something interesting to do in my non-existent spare time....

Max The Magnificent
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Max The Magnificent   5/24/2011 8:08:24 PM
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@David: "... I'm very grateful to Max..." Please, don't mention it -- I live to serve :-)

spyderjacks
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spyderjacks   5/24/2011 9:09:09 PM
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David: Excellent result. Many thanks for your efforts. I've been away on business travel for many weeks and dying to get back and bring my Geiger counter back to like. In the mean time I ordered spares and studies every blog entry. Everything is waiting for a free Saturday. Maybe next month! By the way - TIG welding electrodes, 2% Thoriated tungsten, are a very excellent source. I was just getting ready to capture that video when the kit gave up the ghost!

old account Frank Eory
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old account Frank Eory   5/24/2011 10:08:42 PM
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A great example of a broad range of engineering skills -- testing, debugging, circuit analysis and creative PCB modification to add the new components as neatly as possible. Very nice work David. I got a good laugh at the quote from your wife. Mine would've said pretty much the same thing :)

David Ashton
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David Ashton   5/24/2011 10:35:02 PM
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Thanks Frank. Yep - electronics and wives don't always mix.....

Max The Magnificent
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Max The Magnificent   5/25/2011 4:07:14 PM
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@Frank: How is it that your wife knows enough about David to say the same thing? :-)

David Ashton
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David Ashton   5/26/2011 10:38:05 AM
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It's called universal Female intuition, Max....

David Ashton
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David Ashton   5/24/2011 10:17:01 PM
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One further comment here. My solution to the LED ON problem was a 1 Megohm in parallel with a 68pF capacitor, across Q3 Base-Emitter. As in the video, the LED tends to go off at high count rates. Pchow's solution was a lower resistor (330K I think) and no cap. I think his solution might work better at high count rates but by the time I saw it I had already done my test. It would be interesting to redo the test with Peter's mod instead of mine and compare - I have a suspicion that his might work better for high radiation counts - my addition of a cap would tend to kill high freqs. If anyone's in a position to test further, let us know.

pchow
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pchow   5/26/2011 12:27:10 AM
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David, et al., Great work you have done on this project and the documentation and write-up - and the time you spent on it! I have a few notes and comments: 1) I noticed that your modified schematic (Figure 6) shows R7 and C5 connected not across base-emitter (BE) of Q3, but to the G-M tube "Gnd". Although your text indicates these two components are connected to BE of Q3 - I assume the text is correct? Which means that the G-M tube is only referenced to the positive side of the battery via the speaker-resistor network. 2) I also measured the voltage across the G-M tube with a Fluke DVM a while back, and also got about 70 VDC. Prior to that, I had a friend who had an old Beckman Int'l scope in which we measured 400+ volt charge-up spikes (prior to installing the HV smoothing capacitor). 3) This past weekend I was able to buy an old travel wind-up clock at a flea market, which turned out to be pretty radioactive (w/radium, I presume). When placed directly on the G-M tube, the digital readout on my C6981 kit gave a reading of 700-900 CPM (counts per minute)! I quickly headed for the hills. :-) The LED had no problems keeping up, but I don't have source like in David's video, which probably had CPMs in the 1000's. I also tested a smoke detector source (Americium 241), which gave about 200-300 CPMs. 4) I would also like to give credit to a fellow ham (amateur radio) person who initially suggested a HV smoothing capacitor during our initial debug session together back in March. Given her EE background and intuitive debugging skills, she was instrumental in getting the kit going. I will give her name credit on my PicasaWeb page. 5) Thanks for all those who provided their experiences with their problematic kits. I'm glad some folks got their kits working (better) using the modifications I had. It was good to see that both David and my modifications closely concur based on David's independent investigation of Max's kit. - Peter

David Ashton
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David Ashton   5/26/2011 10:53:41 AM
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OOPS!! Well spotted my deliberate mistake, Peter :-)! I will provide Max with a corrected schematic to insert if possible. Thanks for that. As you advised, my highest count rate would have been pretty high, and it was only when it was right near the source that the LED dimmed. Not an easy test to repeat. I went for the cap based on the facts that (a) it worked and (b) the power supply pulses were considerably sharper than the event pulses, so a cap would attenuate them more. As I said, I think if I was building one from scratch I'd do it a bit differently. One article I saw (think it was somewhere else, not here) suggested using a fluorescent lamp inverter transformer - as it happens I have quite a few of these (I pick up the emergency exit signs at work when they renew them every couple of years!). However they are designed for powering an 8 or 10 W tube and would probably be overkill - size and power wise- for a geiger counter. The Maxim AN ref in the article looks really good. See also comment on ArekZ's post below. Thanks again Peter for your input - I was also happy that our results were similar (proves I'm not such an idiot!!)

Max The Magnificent
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Max The Magnificent   5/26/2011 12:21:32 PM
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Hi David -- thanks for the revised schematic -- I've updated the article -- Max

David Ashton
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David Ashton   5/26/2011 11:18:43 AM
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Peter - a further comment ref your point 1. There is also a good AC capacitance coupling between the HV and LV sides through primary to secondary capacitance of T1. I think this is also responsible for introducing some AC voltage as well - at one stage I shorted the tube and still got fairly strong power supply pulses at the BE of Q3. I tried putting a 0.01 uF cap between the primary and secondary "ground" ends of T1's windings, but it did not make much difference. I was reluctant to short it because then the HV DC would have been directly coupled to Q2 and would probably have introduced further problems....

Max The Magnificent
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Max The Magnificent   5/26/2011 12:22:21 PM
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Good catch Peter -- David sent me an updated schematic, which I just added to the article -- Max

ArekZ
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ArekZ   5/26/2011 9:48:27 AM
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It's nice to see a happy end. 8) My only comment regards the "strange thing" number 2 - a G-M tube is a high voltage capacitor (with some side effects) and perhaps this is why a smoothing capacitor was omitted in the original design.Also 300Hz is much higher than 900CPM.

David Ashton
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David Ashton   5/26/2011 11:11:34 AM
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Good point - as I noted, Max's circuit started clicking for me with no mods at all, so as you say the capacitance of the tube does provide reasonable smoothing. The tube data diagrams I linked to give tube capacitance as only a few pF. However, with the low duty cycle 300 Hs power pulses from the transformer, I wonder what would happen to the tube voltage if several ionisations occurred between power "top ups" as would certainly happen with the source in the video. The best info I could find in the tube data gave "dead time" as 100-200 uS for most tubes, impying a maximum count rate of 5000 counts / second - ie more than 10 counts between each 300 Hz power pulse. I will freely admit I was also influenced by the fact that almost every other schematic I saw had a smoothing cap of a few nF or so. And I'm not keen on re-inventing wheels! Again, this is a relatively low-cost kit not intended for professional measurements, but Cheney certainly could have improved the design considerably for only a couple of dollars worth of extra parts.

daveismissing
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daveismissing   5/26/2011 12:21:27 PM
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This is the kind of article that will appeal to your target audience and make a successful portal. More please. :)

finevlad
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finevlad   5/26/2011 2:44:41 PM
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Great to hear it is working! My note is about the background count of the tube. Don't be confused trying to link it to background radiation. This should be mostly the self discharge of the tube itself.

David Ashton
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David Ashton   5/26/2011 10:51:42 PM
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Good point Vlad. I think some of it WAS background as it did seem to vary a bit depending where I was - in building or out, etc. Funny story - when I was at the nuclear medicine place, one of the staff said "Come and test me!" From a background count well above the usual 20 CPM, when I held the counter against him I did not get one click for about 20 seconds. "I reckon you're dead" I said. "No", he replied "I'm just made of lead!" The other point here is that as the supply to the tube is unregulated, it might go a bit above the "Plateau" region, at which point apparently the tube will frequently self-ionise.

pchow
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pchow   5/26/2011 2:50:09 PM
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David and Max, Thank you both for the clarification and update. BTW, I was also able to verify last night that Morton Salt Substitute (Potassium Chloride, w/o NaCl) gave similar results as in David's article. The 3-1/8 oz. salt container, when laid on top of the G-M tube, produced 2-3 times the CPM compared to normal background. e.g. background CPM was ~20, and the salt substitute was 40's to low 60's CPM. Thanks again for the great investigative work, and the EE Times support of this interesting subject! - Peter

Duane Benson
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Duane Benson   5/26/2011 3:44:52 PM
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It's refreshing to see that, with all of the multi-GHz processors, DSPs, MEMs and everything else complex and advanced, basic electronics theory still exists. Maybe it's a little in short supply at the original designer of this kit, but here at eeTimes, it's not. What's more is that it's a great story of troubleshooting methodology. I think some people forget that the fundamentals are still important. Very nice job.

Flurmy
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Flurmy   5/27/2011 8:10:42 AM
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I'm very surprised: you work with a scope probe with 1 Mohm resistance? I do that only when I need to work at 100 MHz or above (wide band probes) or when the signal amplitude is very low. In all the other cases I use a 10:1 probe with 10 Mohm input resistance. And a multimeter with 1 Mohm input resistance? Even the cheapest one I have has 10 Mohm! "if I was building one from scratch I'd do it a bit differently." Were it a single case in which this isn't true? :-) N.B. uS = microsiemens; us = microseconds

David Ashton
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David Ashton   5/27/2011 11:05:56 AM
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I did use a x10 probe on the HV side, and it is 10 Megohms - on the BE of Q3 I had it on x1 and it was 1 Meg. Ref DMMs - I had it in mind that mine were 1Meg but I will check that now. In any case this circuit is demonstrably sensitive to even a x10 scope probe, I think something with 100 Meg would be better. It should be possible to build a 100:1 divider using (say) 22 Meg resistors which are easily obtainable, and couple it to a standard DMM... Ref building it differently... sometimes you see a circuit and think "That has been really well designed". This is NOT one of those... and ref uS, us, thanks for the correction.

Jay Sinnett
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Jay Sinnett   6/3/2011 6:48:22 PM
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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.

David Ashton
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David Ashton   6/5/2011 12:50:31 AM
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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.....

Max The Magnificent
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Max The Magnificent   6/3/2011 6:53:18 PM
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HURRAY!!! My (now-working) Geiger counter has just arrived back from David. Its 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. Im heading out to DAC this weekend so Ill take the video and write the follow-up blog when I get back. THANK YOU DAVID!!!

benbojangles
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benbojangles   6/9/2011 10:02:12 AM
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Dave you're a genius. Just wondering, there is no information about the transformer (T1) product number. Am I right in guessing that it is a Tamura TTC-105 Microtran? And the diode (D1) is a 1N4007?

David Ashton
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David Ashton   6/9/2011 11:05:47 AM
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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 http://pdfserv.maxim-ic.com/en/an/AN3757.pdf 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).

Max The Magnificent
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Max The Magnificent   6/10/2011 2:45:20 PM
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I'm back in the office -- I can't see any markings on the tape on the transformer -- sorry...

David Ashton
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David Ashton   6/11/2011 12:31:52 AM
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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...

David Ashton
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David Ashton   6/9/2011 11:10:29 AM
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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??

Anakron
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Anakron   6/30/2011 1:26:53 AM
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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. Kent (Repost from the blog)

Bhogan
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Bhogan   7/9/2011 4:40:57 PM
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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 . . . https://sites.google.com/site/diygeigercounter/home/assembly/adjusting-the-hv The should apply to any HV circuit.

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