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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   4/19/2015 7:23:52 PM
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@DaveB - you have certainly gone into this in some detail - which Max's kit designers do not appear to have done.   Yes with flyback the turns ratio is not tooo important, but will still help.   Good luck with it and if you can come back and let us know how you did, that would be good.  Even better, do a full write-up and get hold of Max, he can post it as a full article.

davebirdieee
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re: How to make a Geiger counter count
davebirdieee   4/18/2015 8:52:39 PM
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Re pulse width. I constructed the entire circuit on a breadboard, except made the PW continuously variable with constant frequency. Used a pot for the RA, RB timing resistors and a couple of diodes. Ckt in app notes somewhere. One could calculate turns ratio using volt-second product as long as core is not saturating. With the pot ckt, I could watch the primary waveform and see where the core started to saturate. On the breadboard, I used an old audio transformer of about the same core size. Later I got the same part from Electronic Goldmine and used that. Didn't seem to make much difference. Not surprised.

David Ashton
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re: How to make a Geiger counter count
David Ashton   4/18/2015 5:48:17 PM
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@DaveB...100GΩ  R...$5 is pretty good....where I was looking $20 semed about the norm.  Useful thing to have.  I tried slight changes to the Pulse width on Max's circuit without much difference.  The 1nF HT smoothing cap did seem to make things better though, I suspect you could get away with even less.  Do you know what turns ratio your transformer is?  I have some small transofrmers scrounged off an ISDN card form a PABX that seem to have a huge turns ratio which I might try if I ever build my own one.....but it is fairly low on my list at present.....

davebirdieee
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re: How to make a Geiger counter count
davebirdieee   4/18/2015 3:52:48 PM
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Yes. I bought a 10 Gohm resistor and that seemed to do the trick. About $5 on ebay. So, with instrument input impedance of 10Meg, the attenuation ratio is 1000. Mazimized the output voltage by changing the pulse width. 10us gave the max. That seems to be close to where the transformer saturates. The circuit is a flyback. Suspect that the reason the output circuit is isolated to to get the overall capacitance right empirically. With the original circuit, grounding the secondary side destroys the functionality.

David Ashton
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re: How to make a Geiger counter count
David Ashton   4/18/2015 5:43:41 AM
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@DaveB - the 555 is operated without the usual discharge resistor so the ON pulse is very short - saves battery I suppose.  I was intrigued as to the transformer turns ratio but did not at the time have anything to measure it with.  The other strange thing is that there is no DC connection between the HV and LV sides of the circuit except through R4 & R5 which connect the GM1+ to the battery + (via about 5.7 M .

I thought of getting a 100MΩ  or 1GΩ resistor to make a voltage divider probe to measure things like this - they are available but not cheap.  An ordinary DMM even with its 10MΩ input resistance just does not cut it here.

Thanks again for the comments.

davebirdieee
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re: How to make a Geiger counter count
davebirdieee   4/18/2015 5:01:17 AM
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Thanks Dave for the reply. I agree with your approach. When I built mine from the Electronic Goldmine kit, while it did click and light the led, both were very weak. So, I was motivated to see what was going on. The first challenge was just measuring the high voltage. But, I found other oddities too. For instance, not all 555 timers will work. All in all, a fascinating little circuit.

David Ashton
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re: How to make a Geiger counter count
David Ashton   4/18/2015 4:02:14 AM
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@Davebirdee.... if you click on the Post Message link at the bottom of any previous post, your comment appears as a new thread rather than as a reply to a previous comment.    Confusingly, the COMMENT link at the bottom of the article does NOT let you comment.....

As it's my article, I get an email on any new threads or any replies to my comments, but not on replies to other peoples comments.....

David Ashton
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re: How to make a Geiger counter count
David Ashton   4/18/2015 3:58:28 AM
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Hi Davebirdee...as your comment was a reply to a previous comment I didn't get advised of it, came across it by chance - on the very day you posted it!  You're right about Q3, it must fire on the positive going recovery of GM1 anode.  While I agree with your sentiments about exceeding the VBER of transistors, in this case it is through a 10MΩ resistor so the BE junction will just zener at a low current, however point taken.

As I said in the article, had I designed this circuit I would have done it a bit differently, but in this case I was just trying to make an existing circuit work.....

davebirdieee
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re: How to make a Geiger counter count
davebirdieee   4/18/2015 2:37:25 AM
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After some adaptations for probing high impedance circuits I have found the answer to my question. When the Geiger tube discharges, it also discharges to some degree capacitor C3. Then on the positive portion of the pulse, C3 delivers a positive pulse to Q3 to turn it on. The circuit works better if the resistor R7 in the improved circuit is replace by a diode that clamps the base to prevent it from going lower than the VBE breakdown. In this case one diode drop below ground. I don't like challenging the breakdown voltage of transistors under normal operation even when the current is limited.

davebirdieee
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re: How to make a Geiger counter count
davebirdieee   3/21/2015 3:19:21 AM
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Found this thread just this month. Bought and assembled a very similar circuit from Electronic Goldmine. While the circuit does work, I'm having trouble understanding how some parts do indeed work. Specifically the Q3 circuit. Q3 takes a positive pulse to turn on, but a gieger tube pulse will appear as a negative pulse, at least the leading edge of it. So, it would seem that Q3 only turns on when the tube recovers (trailing edge) and/or the high voltage line charges back up. I suspect the recovery time may be faster than the charge-up time. Has anyone investigated this? Hope someone besides me is still interested in this.

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