Geiger
Detector
Geiger
Detector
Spring 2011
Well radiation is a HOT topic so I decided to make a GM detector .
The operation of a GM tube is really easy .
Its a switch ..
So if we think of Pull up or pull down we can make it do what we want .
For internal detectors a Series tube is ideal as its easy to wire .
For external however we need to case ground the tube .
So we use it in a voltage divider Configuration .
This way I can use a BNC and a external probe with out fuss.
The care and feeding of a GM tube.
GM tubes need steady HVDC to run so a PSU is needed .
The best way to do it quick and dirty is to use a 12V strobe or a disposal cam charger (1.5V).
Both put out 300 V or so . Plenty to run a cascade to crank that voltage to useful amounts.
You will need to remove the diode and run the raw HV AC to the cascade .
My tube uses 800 to 900 V but some new tubes Like from LND INC can run at 500V
Thankfully the current needed is super small in the µA range at times.
Theory of operation.
The cascade shoves out 800V or so to "charge" the GM tube via a 10 and 1MΩ resistor.
This brings the tube voltage up to that 800 V mark .
Now the tube is ready to accept a particle or ray .
the 10nF cap is also charged up .
When a particle hits the tube a current flows from the Middle electrode to the shell and for a moment shorts the tube to ground like a pulldown switch ( or somewhat close ) .
This puts a wonky charge on the 10nF cap and in turn runs that pulse to the LM386 Amp to be amped up to useful levels .
And out to the speaker or headphones the pulse goes as a click .
The 10MΩ resistor is to help quench the tube after each hit as to prevent it from becoming a Neon light ..
Below is a PDF of a small PCB for the Circuit above .
NOTE the parts on the PCB are not labeled !
And the file is from the top down.
You
The circuit is super simple but needs care as there is HV at points on the board so be aware.
There is a 3rd way to make a HVPSU for the tube that bypasses the cascade all together
CCFT display tubes run at 900 to 1200VAC so they can be pressed in to this duty too.
But you will need to do a few things .
First you Need to rectify the AC to DC with a diode .Yes one diode is fine . No bridge needed.
A 1N4007 may NOT work as its PIV is 1KV and the recovery time is too long for the HF used in a CCFT driver.
But once you have DC then you MUST use a Zener stack or string to cut the Voltage down .
You may not need a load resistor as the coupling Cap on the CCFT driver board provides a good current limit anyways .
Then you can feed the voltage right to the tube at the point where the 10MΩ connects to the cascade .
The power source is a issue too as 9V are crappy batts and they are not cheap .
If you use a Low voltage LM386 and a 5V based CCFT or a
modded disposable cam strobe you can get by with 5V or so .
This allows for 4 ALK or 5 NiMh AA cells to be used in leu of the 9V for better life span .
This is my completed unit.
