[Swlug] DIY Geiger counter

[Rhys Sage]

My latest development is that using the circuit as it was, I replaced the 1uF elecrolytic with a 0.1uF electrolytic and raised the voltage from 3v to 4.7v. I put an eBay buck converter and powered the buck converter from my onboard solar system (14.2v). 

I noticed that the voltage range on the oscilloscope from the circuit was varying by 0.01V. That looks like I'm at least refining what I have. 

The amplifier circuit came from Instructables and I put it as it was: https://www.instructables.com/BC547-Double-Transistor-Audio-Amplifier/ - I don't know whether the link will be scrubbed so I'll replace all the dots with asterisks: www*instructables*com/BC547-Double-Transistor-Audio-Amplifier/ I do know that the BBS in use won't do photos or videos. HTML gets scrubbed too. It would not surprise me if there were mistakes in the circuit design - repositories like GitHub and Instructibles are very much caveat emptor.

After checking with the power removed and with the power applied and with both my solar 14.2 and with a 9v PP3 I can confirm the graph on the scope was still flat so I'll agree that it does look like one of the transistors isn't doing very much. That'll be the next adjustment. What I saw today was much more encouraging.

I would use a lower value capacitor but my lowest electrolytic is 0.1uF. I do have some metal film capacitors down to 0.01uF. 

I'm very much blundering around in the dark, researching and asking questions. I have my  goal and I'm trying to get there despite a sketchy knowledge of electronics and rusty programming skills. Progress is shown in snippets on my YouTube channel (British Tech Guru). Today's short videos on the Geiger counter will be online on the 16th and 17th. As it's now 6pm on Sunday evening, I won't be doing more on the project until next weekend. My work day starts with a 4AM alarm and I'm at work from 5:30am until 4:45pm every day. I do get an extended break in the middle from 10am to 1:15pm, most of the time. Last week I had Wednesday free because of a storm. We don't work in high winds or when snow is forecast.

Right now I'm just hoping the random interruptions in the almost straight line on the 'scope represent background radiation. I would expect background radiation to be in the range of 20-30cps. I didn't open the smoke alarm because my crystal is in a rectangular plastic block which I hope at $20 is real and not just a cute lump of Perspex; thus the crystal would never receive alpha rays from americium-241.

The big thick leads in the photo go to the scope/ (I'll include a photo just in case it doesn't get scrubbed) https://yt3.ggpht.com/N5oH38hpOLriH41kEZjayIbiYw65nYwIMqWoLIOaCWdUpa9pAD4HZbiW3NYEVf3sRpn-OgLh27rgn7I=s640-c-fcrop64=1,20000000dfffffff-rw-nd-v1

The yellow/green pair go to the photodiode. Yellow is the emitter. The other yellow lead is +3v. The blue lead on the left goes from batt -ve to circuit GND. 

We really need to set up some place where we can post photos and videos. I don't do social media other than Mastodon. I do have a Peer Tube account but it takes way too long for videos to be approved. Mastodon only does still images on the instance I use.

Rhys Sage






On Sunday, 9 March 2025 at 14:00:08 GMT-4, James R. Haigh (+ML.LUG subaddress) <jrhaigh+ml.lug at runbox.com> wrote: 





I notice that the electrolytic that you have there is already a low-ESR one, and the way you are using it is only a diode away from being an envelope detector, pretty much.  I've just thought that if you leave the resistor of the envelope detector out for now you will force the circuit into a mode where it will be much more sensitive and much more likely to "get stuck on" as soon as you get some activity.  This could be a useful milestone to know that you are getting somewhere.

    Here's a schematic (in monospace ASCII-art) where the amplifier is a pair of NPN BJTs, like what you have there with the BC547s:-

                    +        +
                    |        |
                    |        >
                    >        < R
                    < R      > 3
                    > 4      <
          fast      <        |      OPEN-COLLECTOR
        Schottky    |    R2  C      AMPLIFIED
CRYSTAL --|>|--o----o--\/\/\--B  C-- OUTPUT (INVERTED)
              |    |        E--B  TO PULLED-UP ADC OR
              |+  >            E  CURRENT-LIMITED LED
          low ---  < R          |
          ESR ---  > 1          |
              |    <            |
              |    |            |
              -    -            -

    I'm thinking that it would be a good idea to leave R1 disconnected for now, because any charge that gets pumped onto the capacitor will be force to drain through your amplifier, thus sustaining each signal for a much longer time, which should help to get it working.  Once you get it working and find that it's on for way too long to be able to count pulses accurately, then you can experiment with the value of R1 until it behaves correctly.

    You may find that it takes multiple pulse events to get enough charge on the capacitor for the amplifier to detect anything.  In fact, yeah, the voltage on the cap has to rise by 2 diode-drops, probably about 1.2vDC, before you get any current flowing through the bases of the NPNs.  Therefore, I have added biasing resistor R4 to the schematic.  It'll raise the resting voltage as a potential divider with R1.  You could even replace R1 and R4 with a trimpot so that you can tune the circuit to the "bite-point" of the amp, so-to-speak, at about 1.2v, or maybe a little more.

    So you could try with neither R1 or R4 and see whether you get anything at all (which should be quite hopeful because if charge keeps punching through the Schottky, it will eventually spill over the 2 PN-junction-drops of the amp), and then you can try with both of them in there or a trimpot to see whether you can fine-tune the sensitivity and hopefully start detecting individual pulses.

    How are you testing the crystal?  Did you crack open that smoke alarm, with all the hazards that that presents if inhaled or ingested, or did you find some other way to test the crystal?  Is there a way to test your circuit without any radioactivity, e.g. by maybe swapping your crystal for a piezoelectric crystal or something that responds to percussion (e.g. from a broken lighter), so you can test how well your circuit works and how sensitive it can be to the slightest tap, and then when all that works well with the piezoelectric crystal, swap-in the Geiger crystal and cautiously test the real thing.


Kind regards,
James.
-- 
Wealth doesn't bring happiness, but poverty brings sadness.
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