Adding a Digital Counter to an Analog Geiger Counter

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Dennis P Brown
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Adding a Digital Counter to an Analog Geiger Counter

Post by Dennis P Brown » Sun Dec 03, 2017 3:05 pm

Since I have some time due to the fact that my fusor is down for cathode repairs and the new neutron detector is stalled for the same reason - i.e. no neutrons to test with - I decided to finish another small project that I have wanted to do for awhile. I have an old, surplus Ludlun analog Geiger counter. It had many issues so getting it for $10 wasn't a surprise. The batteries had to be drilled and chiseled out; the damage to the battery case was so bad that I gave up on getting it to work as designed and instead, simply drilled the case so I could wire in an external battery pack. Also,I had to replaced a voltage regulator chip and fixed some wires (a few were too frayed and their soldier connections were failing.)

I bought a $7 dollar (free shipping) Chinese made "counter" module - these are small, self contained, have a permanent internal battery and only need to add a reset switch. I wired the unit from the Ludlum's 'speaker' posts on the circuit board and connected these to the simple counter (polarity does matter.)

When the Ludlum is counting, the digital display follows the counts. The Ludlum analog meter indicated about 700 counts a minute and the digital meter recorded 753 counts in a minute. So, works well. Since the Ludlum's internal battery holder is empty, I installed the counter in this location. I cut a small piece of plastic to act as a mounting panel for the counter. The entire effort took less than two hours (including the first, bench test before I modified the Ludlum - wanted to make sure the idea worked.)

The micro-switch to the left of the digital meter re-zero's the counter's display.

I decided to do this because at some future day, I might convert this Ludlum into a neutron counter - it has the required comparator, totalizer, and a good input amplifier (I can add an external pre-amp if necessary.) All it lacked was a decent digital meter (neutron detectors rarely provide more than a few tens to up to a hundred or so counts a minute for a fusor. Hence, a digital display is necessary to read such a low count source with such a device.) Now that I have an interface box that allows the required high voltage to be applied to the neutron tube, but steps down that voltage to a safe range that the Geiger counter can handle, so this project is actually viable.

Besides, the digital display is a nice extra feature to have: both an analog meter and a totalizing digital display on the old Ludlum. Besides, for low count situations, this is useful added feature. Another nice feature is I can calibrate the analog meter to more accurately match the count rate - that is, with my known source (about 1200 Beta counts/min; I can now set the analog meter to reflect this count rate. (Not a true calibration but a useful manner to set this analog meter to at least match the count rate.)

For those with a working battery case, the digital counter can be mounted on the side of the case - cutting might be a bit involved. Or, a small external mounting bracket could easily be installed.

This was a far easier project than I at first thought and very inexpensive. The cheap counter module works very well with the speaker output of the Geiger counter; simple to splice into the circuit, too. Also, I was pleased by its excellent performance to the applied signals from the speaker circuit - hope the internal battery lasts a good while.

Below are pics of the meter and counter and the internal wiring running from the circuit board to the battery 'case'. I reused the existing battery contact points in the event I ever fixed its problems.
Attachments
MVC-019L.JPG
Ludlum with both the analog and digital displays
MVC-020L.JPG
Close up of the simple counter display
MVC-021L.JPG
Circuit board with added counter wires (brown and white) feeding into the old battery holder

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Richard Hull
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Re: Adding a Digital Counter to an Analog Geiger Counter

Post by Richard Hull » Mon Dec 04, 2017 6:25 am

How fast can it count? put a hot source where your meter is reading a rather constant 30k CPM and see if the counter in one minute reads near 30,000.
Some very cheap counter modules like pedometer's etc. can't count fast pulses.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Dennis P Brown
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Re: Adding a Digital Counter to an Analog Geiger Counter

Post by Dennis P Brown » Mon Dec 04, 2017 11:55 am

Richard, you are most likely right (when not?) I too seriously doubt that a seven dollar special can do that rate of counting accurately; however, it only needs to count neutrons (maybe) and that only requires a few hundred (not that I'd ever get that high) neutron events per minute. Yes, one should buy a far better counter for serious work but it appears very adequate for what I need and even works well up to a thousand or so counts a minute.

For most fusor neutron event counters, the count rate this cheap module does support is over kill, I'd think. Still these Ludlum meters do have totalizers and with the still working analog scale/meter the digital meter simply isn't necessary for those count rates; however, for low count rates, it is far superior to trying and read a old, sticky meter and makes my geiger counter more useful.

It is cheap but that adds to the advantage when one has a very low quality or cost meter.

Aside: I do have a counter rated up to 30 MHz but realized it would be a waste on that surplus and old meter.

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Re: Adding a Digital Counter to an Analog Geiger Counter

Post by Richard Hull » Tue Dec 05, 2017 5:28 am

I found some great Chicom pedometers with 4 digits for $4.00 each and busting them down to their undershorts showed a maximum "keep up rate" of 8 pulses/sec on evenly spaced pulses from a signal gen, but the random pulses from a 500cpm rad source (Analog)meter over a 3 minute period compared to my HP 6 digit digital counter (50mhz) were 1. HP 1628 counts 2. pedometer 554 counts.

My very large 3He tube at 1,000,000 n/s will have indicate a total of >45,000 cpm. (irregularly timed pulses from the tube come in at 750 pulses/second) That is a lot of CPM and CPS! Yes, I have a fabulous neutron counter detection device. Lesser detectors will register a lower cpm at large neutron numbers, but where is that break for that specific detector? Once the real neuts start pouring out only the best counters need apply to warrant every single pulse is detected. As an example at only 750 pulses per second, a pulse could come one every 1millisecond followed by 3 or 11 or 33 more pulses in the next millisecond. Only the best counter need apply for this job.

You might want to "proof test" your counter as noted above first with a signal gen and then at a very low rate like 500cpm on your counter's analog meter over the three minute period. In this manner you will know your "rope limit" on that counter.....The point at which it begins to lie to you.

There is a limit on all digital and analog counters based not on the analog or digital circuitry, but the dead time of the detection tube. Good neutron tubes are proportional and do not suffer horribly the way GM tubes due, but it is good to know what you have in the way of a counter limit.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Rex Allers
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Re: Adding a Digital Counter to an Analog Geiger Counter

Post by Rex Allers » Tue Dec 05, 2017 11:36 am

One other thing you might consider. I think you said you are getting the pulses from the speaker terminals.

I don't know about any details on this Ludlum but there might be some kind of single shot pulser driving the speaker with pulses big enough to hear but limiting the max rep rate to some sane audio signal. At high count rates I wouldn't be surprised if the speaker is seeing fewer pulses than the unit actually detected and is using to drive the meter rate.
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Dennis P Brown
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Re: Adding a Digital Counter to an Analog Geiger Counter

Post by Dennis P Brown » Tue Dec 05, 2017 3:54 pm

Alex, while your point is valid I did check the digital counter against the analog meter (in its collection mode.) While Richard's issues are certainly a concern for high rates (10,000 counts and up) and currently not an experiment I can check, I did run the speaker/digital hookup system using a 'semi-calibrated' sample against the confirmed analog reading on its scale; this run demonstrated that the digital counter is a good indicator of events for signals under 2000 counts/min. So, it appears that the speaker signal is a valid source for these rates. Of course, all bets are off for higher counts. Since my only interest in using the digital display is for count rates under 2000/min (and really, under 500/min), this speaker signal appears to work fine.

For low count rates that can occur, a digital meter has a lot of advantages and being so inexpensive and very easy to incorporate, I see no down side as long as one understands that high count rates are not likely to be accurate. But for my application, that is a completely irrelevant concern.

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