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Re: [GPSL] Geiger Counter

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  • Mike Manes
    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
    Message 1 of 34 , Jan 31, 2010
      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.
      > Paul
      >
      > 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
      > fire.
      > > You really want is to filter the 600 volt supply then feed it to the
      > > tube thru a high value resistor. For example:
      > >
      > http://www.sparkfun.com/datasheets/Sensors/Radiation/Geiger_Counter-v18.pdf
      > > 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
      > circuit
      > > 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
      > like
      > > 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@...
      > <mailto:nearsys@...>>
      > > Sent 1/31/2010 11:19:00 AM
      > > To: "Mike Manes" <mrmanes@... <mailto:mrmanes@...>>
      > > Cc: gpsl@yahoogroups.com <mailto:gpsl@yahoogroups.com>
      > > Subject: Re: [GPSL] Geiger Counter
      > >
      > > I wrote about this on my blog (nearsys.blogspot.com
      > <http://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@... <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
      > betcha
      > > 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
      > fatal
      > > 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,
      > http://www.youtube.com/watch?v=lc0dirkliMo
      > > 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
      > counters
      > > 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
      > RM-60
      > > 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
      > accident).
      > >
      > > --
      > > 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:
      > 303-979-4899
      > "Things should be made as simple as possible, but not more so."
      > A. Einstein
      >
      >
      > ------------------------------------
      >
      > Yahoo! Groups Links
      >
      >
      > <mailto:GPSL-fullfeatured@yahoogroups.com>
      >
      >
      >
      >
      >
      > --
      > Onwards and Upwards,
      > Paul
      >
      >
      >

      --
      Mike Manes mrmanes@... Tel: 303-979-4899
      "Things should be made as simple as possible, but not more so."
      A. Einstein
    • James Hannon
      Here is an interesting site for homebuilt Geiger counter and other cosmic ray detectors, power supplies ect.http://www.cosmicrays.org/Jim Hannon
      Message 34 of 34 , Feb 24, 2010
        Here is an interesting site for homebuilt Geiger counter and other cosmic ray detectors, power supplies ect.
        http://www.cosmicrays.org/

        Jim Hannon
        http://www.fmtcs.com/web/jmhannon/
        42,11.90N,91,39.26W
        WB0TXL



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