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How to make a Geiger counter count

5/24/2011 04:40 PM EDT
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David Ashton
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re: How to make a Geiger counter count
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.

Flurmy
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re: How to make a Geiger counter count
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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re: How to make a Geiger counter count
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.

Duane Benson
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re: How to make a Geiger counter count
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.

pchow
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re: How to make a Geiger counter count
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

finevlad
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re: How to make a Geiger counter count
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.

Max The Magnificent
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re: How to make a Geiger counter count
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

Max The Magnificent
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re: How to make a Geiger counter count
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

daveismissing
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re: How to make a Geiger counter count
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. :)

David Ashton
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re: How to make a Geiger counter count
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....

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