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wristwatch sized geiger?

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Conundrum

Fri Apr 28 2006, 08:44PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

hi all.
i've yet to find a circuit that will run a geiger tube (needs around 380v at a microamp or so) and will run off 1.5 to 3v. does anyone know of such a circuit?

i have some of those lt series driver chips from old e700's and some mc34063's

anyone?

-A

HV Enthusiast

Fri Apr 28 2006, 09:02PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

There are plenty of step-up topologies that should be able to do this. Flybacks, Resonant, Boost, Capacitor Charge Pumps, etc... For such small output current, you could make everything very small surface mount.

Does it have to fit in a wristwatch? If so, then you might have a problem . . .

Chris Russell

Sat Apr 29 2006, 12:58AM

... not Russel!
Registered Member #1 Joined: Thu Jan 26 2006, 12:18AM
Location: Tempe, Arizona
Posts: 1052

I thought that extremely small Geiger counters used a reverse-biased diode, rather than a G-M tube.

Sulaiman

Sat Apr 29 2006, 07:35AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The capacitor charging circuit from a disposable camera flash unit could be a good start.

mikeselectricstuff

Sun Apr 30 2006, 01:10AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
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Posts: 253

A camera flash supply would be a quick way to do it from 1.5v, probably not too efficient though.

Conundrum

Sun Apr 30 2006, 03:09PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

hi.
I did consider using a 12v lighter cell to make things simpler.

what i do have is a "step" counter which would be ideal for this, there's enough room in the case to include a fair few parts but the battery is the main sticking point.

I did salvage a low powered piezo transformer lighting pcb from a dead sony camcorder which is low powered enough that it might work for this, so might try it.

-A

"Bother" said Pooh, as he accidentally dialled the black hole address...

ShawnLG

Tue Nov 24 2009, 06:27PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
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Posts: 399

A wristwatch size geiger sounds interesting. I have a couple of small geiger tubes that use around 170 volts using uA of current. It could be possible.

Proud Mary

Wed Nov 25 2009, 04:42PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Very small GM tubes of the type made for pocket dosimeters would easily fit in a chunky wrist watch, as could the HV circuit from a photoflash, since you do not need the big charging capacitor.

However, nowadays, I would agree with Chris that you would be better off starting with a photo PIN diode (like BPX65 with the lens taken out) followed by an electrometer amplifier chip etc and not have to bother with an HV circuit, and high current load on your battery.

The quantum efficiency of photo PIN diodes is very high at a few keV, and falls off steadily to just a few percent at 100keV, which must be taken account of in calibration.

The problem with any very small detector head is that it's sensitive area is so small -1 sq.mm in the case of BPX65 - that you might have to wait a while until a background particle impacts on it. This lack of area can be compensated for in calibration, which thus becomes less and less reliable as counts become infrequent.

Conundrum

Wed Nov 25 2009, 06:25PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

Very interesting.. wasn't aware of the calibration issue.

would a smoke alarm chip work? these sense ridiculously small leakage currents...

I have now acquired some small geiger tubes (tested using a personal alarm as the stepup, worked fine) unfortunately this draws too much power. However during my experimentation noticed that an ultrabright LED in series with the tube (with parallel 100K resistor) flashed in time with the counts.

Could simplify the driver circuit as just need to epoxy any old surplus red water clear LED to the sanded down "detector", and coat the whole thing in black paint. Totally immune to interference and directly drives a micro's IO pin.
(tested with an infrared LED as the sensor)

Which brings me back to that pesky inverter circuit.
Someone else suggested using a piezo transformer from a dead laptop inverter (total overkill but they draw hardly any current and have the advantage of being directly driveable from any micro output) followed by a rectifier and seriesed zeners.

just needs a micro feedback loop to keep the thing stable, and maybe a PIN large area diode for backup in case the geiger gets saturated.

Interesting to note that pyrolytic graphite + off the shelf cheap CMOS camera = instant alpha detector. You need to cleave the PG down to as thin as possible then flour sandpaper down until the thickness of mica.
Works well.

-A

ShawnLG

Wed Nov 25 2009, 08:26PM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

Regarding powering geiger tube efficiently, I was thinking that the HV inverter does not need to be running all the time. There can be a small storage capacitor that will power the tube while the inverter is off. When the charge falls past a point, the inverter will kick in and charge the capacitor back up again. This would need to happen within the tube's operating range.

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