Embedded Systems Conference
Breaking News
Design How-To

How to make a Geiger counter count

5/24/2011 04:40 PM EDT
34 comments
NO RATINGS
More Related Links
View Comments: Newest First | Oldest First | Threaded View
<<   <   Page 2 / 4   >   >>
Jay Sinnett
User Rank
Author
re: How to make a Geiger counter count
Jay Sinnett   6/3/2011 6:48:22 PM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
David Ashton   5/27/2011 11:05:56 AM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
Flurmy   5/27/2011 8:10:42 AM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
David Ashton   5/26/2011 10:51:42 PM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
Duane Benson   5/26/2011 3:44:52 PM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
pchow   5/26/2011 2:50:09 PM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
finevlad   5/26/2011 2:44:41 PM
NO RATINGS
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
User Rank
Author
re: How to make a Geiger counter count
Max The Magnificent   5/26/2011 12:22:21 PM
NO RATINGS
Good catch Peter -- David sent me an updated schematic, which I just added to the article -- Max

Max The Magnificent
User Rank
Author
re: How to make a Geiger counter count
Max The Magnificent   5/26/2011 12:21:32 PM
NO RATINGS
Hi David -- thanks for the revised schematic -- I've updated the article -- Max

daveismissing
User Rank
Author
re: How to make a Geiger counter count
daveismissing   5/26/2011 12:21:27 PM
NO RATINGS
This is the kind of article that will appeal to your target audience and make a successful portal. More please. :)

<<   <   Page 2 / 4   >   >>
Flash Poll
Like Us on Facebook

Datasheets.com Parts Search

185 million searchable parts
(please enter a part number or hit search to begin)
EE Times on Twitter
EE Times Twitter Feed
Top Comments of the Week