Geiger detectors
Peter KG6LSE
Posts: 1,383
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 tocase ground the tube .
So we use it in a voltage divider Configuration .
Attachment not found.
This way I can use a BNC and a external probe with out fuss.
Now 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 .
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 LNDINC can run at 500V
Thankfully the current needed is super small in the µA range at times.
Theory of operation of the circuit .
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 ..
I will have more data to come as well as a PCB layout and a few other goodies .
Peter KG6LSE
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 tocase ground the tube .
So we use it in a voltage divider Configuration .
Attachment not found.
This way I can use a BNC and a external probe with out fuss.
Now 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 .
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 LNDINC can run at 500V
Thankfully the current needed is super small in the µA range at times.
Theory of operation of the circuit .
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 ..
I will have more data to come as well as a PCB layout and a few other goodies .
Peter KG6LSE
Comments
Interesting! So is your particular 300V PSU from a disposable camera? Does the PSU provide a continuous supply of charge to the tube or is this intermittent or.... how does that work?
I am going to document this cam hack to see what can be done .
My strobe puts out 310 V ..
Peter