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

David Ashton
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re: How to make a Geiger counter count
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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re: How to make a Geiger counter count
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? :-)

pchow
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re: How to make a Geiger counter count
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

ArekZ
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re: How to make a Geiger counter count
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.

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