Re: [GPSL] Geiger Counter
- Hi Paul,
Looking over the SparkFun device, I don't see any means for the uP to
control the G-M tube HV bias; Q1-Q2 comprise a free-running ~50 Hz power
oscillator driving a step-up xformer that feeds a voltage tripler,
D2-D4. G-M tube discharge current is limited by R7-8 (11 meg).
The R7-8 x C6 time constant is about 110 ms, which is a lot longer
than the G-M tube quench time.
But I think you have a good idea to use the uP to control the HV
supply. Not sure I've seen that approach taken before.
BTW, I still think you should run it thru your altitude chamber
before flying it. Corona discharge could as a minimum ruin your
data, if not your hardware. I've seen it happen!
73 de Mike W5VSI
On 1/31/2010 19:33, L. Paul Verhage wrote:
> Perhaps I should use a PICAXE-08M in place of the 555 timer. The PICAXE
> can then shut off when at a detection to let the tube quench. That
> would make a smart Geiger counter like it looks like Sparkfun is doing.
> The PICAXE could also do the calculations of counts per minute and send
> the data to a BalloonSat flight computer.
> I'll send a copy of the schematic of the current circuit tomorrow (need
> to scan it).
> The circuit connects the transformer to the GM tube with a diode and
> 100k resistor. So only DC pulses get to the GM tube.
> When the GM tube discharges, it triggers the piezo speaker and
> darlington connected to the indicator LED.
> On Sun, Jan 31, 2010 at 5:35 PM, Mike Manes <mrmanes@...
> <mailto:mrmanes@...>> wrote:
> Yeah, lacking a schematic, I'd assumed that there was a many-meg R in
> series with the tube, across which the arc pulse signal is developed.
> I'm guessing that the slight negative bias between the +HV pulses is
> intended to quench the tube, but maybe not, or it's not doing the job.
> 73 de Mike W5VSI
> On 1/31/2010 12:40, James Hannon wrote:
> > I would not put the capacitor across the gm tube itself. This could
> > damage the tube by supplying excessive energy when the tube does
> > You really want is to filter the 600 volt supply then feed it to the
> > tube thru a high value resistor. For example:
> > notice the 10 meg ohm resistor. You need to limit the current
> into the
> > tube to allow it to quench and prevent damage. Sounds like your
> > does this by supplying the voltage in pulses. If that is the case I
> > would seriously consider redoing the power supply to make it more
> > the sparkfun circuit.
> > Jim Hannon
> > http://www.fmtcs.com/web/jmhannon/
> > 42,11.90N,91,39.26W
> > WB0TXL
> > -----Original Message-----
> > From: "L. Paul Verhage" <nearsys@...
> > Sent 1/31/2010 11:19:00 AM
> > To: "Mike Manes" <mrmanes@... <mailto:mrmanes@...>>
> > Cc: firstname.lastname@example.org <mailto:email@example.com>
> > Subject: Re: [GPSL] Geiger Counter
> > I wrote about this on my blog (nearsys.blogspot.com
> > <http://nearsys.blogspot.com <http://nearsys.blogspot.com/>>).
> Those pulses are from the 555 timer. The
> > dead time on the GM tube is around 50 ms, so there's time for six 555
> > pulsesto show up.
> > I'd like to filter that out with a cap and see if it also helps to
> > reduce the GM tube's dead time.
> > Paul
> > On Sat, Jan 30, 2010 at 9:47 PM, Mike Manes <mrmanes@...
> > <mailto:mrmanes@... <mailto:mrmanes@...>>> wrote:
> > Hi Paul,
> > That 8 ms rep rate is intriguingly close to the period of the
> 128 Hz
> > DC-DC
> > converter switching freq, so it appears that the 600Vdc
> supply may
> > not be
> > filtered at all. Is that what the 33 nF 600V cap is for? I'm sure
> > I've got
> > one in my junk box, but it would still cost some $$ to ship it to
> > you. If
> > you can dig up a stash of old tube-type radios or TVs, I'll
> > you could
> > find what you need in the chassis (that's where my junk box
> got its
> > start).
> > Otherwise, you could simply run that output signal thru a
> simple R-C low
> > pass filter, with maybe a 10 - 15 Hz pole freq before it gets
> to the
> > PICAXE - or you could run it thru a comparator to detect only
> the first,
> > high-amplitude pulse. Or both.
> > If I had a schematic, I might be able to be of greater help.
> > Oh yeah - be SURE to run it up to altitude, looking for
> corona (dim blue
> > glow) in a darkened room. If you find some, try filing off
> any sharp
> > metal
> > points carrying the HV and/or applying corona dope (thick, clear
> > nail polish)
> > to the offending points. If PWB clearance is shy, there may
> even be
> > a frank
> > breakdown, which will be a lot more obvious - and potentially
> > to LV
> > parts nearby.
> > 73 de Mike W5VSI
> > CTO EOSS
> > On 1/30/2010 09:45, L. Paul Verhage wrote:
> > I'm experimenting with the Electronics Golmine Geiger counter
> > kit. It
> > uses a 555 timer (set at 128 Hz) and small transformer to
> boost nine
> > volts to 600 volts for the GM tube. Each time a cosmic
> ray passes
> > through the GM tube, there are 5 or 6 voltage pulses of four
> > volts high.
> > Here's the video so far,
> > I used a BalloonSat Mini flight computer to count the
> pulses and it
> > worked well (it just needs to divide the count by 6). The kit
> > cost me
> > $70, so it's less expensive than the Aware RM-60 Geiger
> > I use
> > today.
> > However, I'd like to get rid of the pulses and smooth out
> the DC. So
> > I'm looking for a .033 uF cap with a voltage rating of
> over 600
> > volts.
> > I see Digi-Key carries them, but I'd have to pay $6
> shipping on a 75
> > cent piece. Does any one local to Topeka have a cap like
> this in
> > their
> > junk drawer?
> > The time it takes to get the six pulses per detection is
> 48 ms. That
> > means during that time, no other detection can be make
> (this is
> > referred
> > to as the GM tube's dead time). I'm wondering if I smooth out
> > the voltage spikes with the cap across the GM tube if
> that will help
> > shorten the dead time. compare this to the dead time of a
> > which
> > is 20 ms (50 ms is not excessive for near space use,
> since the
> > highest
> > counts I've seen is 800 per minute - it becomes a problem
> as the
> > dead
> > time approaches 120 ms)
> > Electronic Goldmine sells a smaller GM tube suitable for
> > measuring gamma
> > rays. So after I complete this experiment, I'll look into
> building a
> > kit around this tube for near space use. I think I can
> keep it
> > to less
> > than $50 - which will make it easier for schools to measure
> > cosmic rays
> > during a near space mission. I could also see it being
> used in
> > robotics
> > competitions (say simulating the clean up of a nuclear
> > --
> > Onwards and Upwards,
> > Paul
> > --
> > Mike Manes mrmanes@... <mailto:mrmanes@...>
> <mailto:mrmanes@... <mailto:mrmanes@...>> Tel:
> > 303-979-4899
> > "Things should be made as simple as possible, but not more so."
> > A. Einstein
> > --
> > Onwards and Upwards,
> > Paul
> Mike Manes mrmanes@... <mailto:mrmanes@...> Tel:
> "Things should be made as simple as possible, but not more so."
> A. Einstein
> Yahoo! Groups Links
> Onwards and Upwards,
Mike Manes mrmanes@... Tel: 303-979-4899
"Things should be made as simple as possible, but not more so."
- Here is an interesting site for homebuilt Geiger counter and other cosmic ray detectors, power supplies ect.