DIY "Geiger-Muller" counter - "proof of concept" circuit - Page 1
Forum section: electronics / projects

author: FlyingDutch
author_rank: Regular Contributor
timestamp: January 17, 2021, 02:59:18 pm
content: Hello,I have bought an "SBM-20" Geiger-Muller tube counter detector. I would like to test this counter - it needs 400V powering (for tube). I found such prooof of concept circuit.It is schematics from book titled "Raspbbery Pi - Awesome projects (Insane Genius). In general I understand this circuit - it is generator with transformer and "voltage multiplier" (Villard cascade). And there also is (tube) simple circuit shaping impulses from Geiger tube - in order to count them on microcontroller.I have an query regarding the "Villard cascade" (voltage multipiler) on diodes and capacitors: Is in this attached schematic the multiplier factor of cascade equal eight times (8x)?I am asking because original schematics was for "K2645" geiger tube wich needs 600V powering and I have "SBm-20" which needs only 400V.Gates on schematics are CMOS 4000 series. I have to properly select transformer and "Villard cascade" (voltage multiplier). What should be permisible voltage for capacitors and their values? And also types of diodes in voltage multiplier? Could somebody help me with this a little?Polish goverment is going to build "Nuclear Plant" in Poland so I would like to have a "Geiger Counter" on my own (just in case).BTW:here is link to SBM-20 Geiger tube:https://www.ebay.pl/itm/NEW-SBM-20-SBM20-20-an-STS-5-SI22G-Geiger-Muller-Tube-Counter-TESTED/191831193846Thanks and Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: January 17, 2021, 04:04:12 pm
content: The cascade should give 4 times the peak to peak voltage, maybe + a little extra. If in double run a spice simulation. How much cascade stages are needed depends on the transformer.The diode should be relatively fast small (not too much capacitance) diodes for suficient voltage. With so many stages 200 V diodes may be enough. BAV20 may be suitable one, but many choices in this range.For the capacitors I would keep them relatively small, like some 10 or 22 nF for the cascade and similar for the final filter cap (maye one more RC filter stage).The point is to keep the energy at a safe level - it would still hurt, but would not be lethal to thouch.The small russian GM tube would one detect really high level of radioactivity. So it would not really help much in case of a nuclear accident.

author: jmelson
author_rank: Super Contributor
timestamp: January 17, 2021, 04:57:56 pm
content: One issue with this circuit is the signal is picked off the case of the GMT. I've never seen this. For capacitive and shielding reasons, the signal is usually picked off the center wire of the GMT. This may require a filtering stage between the power supply and the GMT, and the signal polarity will be reversed. Generally, a modest preamp is also required, you only get a few hundred mV of signal.Jon

author: Gyro
author_rank: Super Contributor
timestamp: January 17, 2021, 05:58:36 pm
content: It's fairly normal to sense the discharge (or more properly, recharge) of a GM tube on the Cathode. The problem with picking off the signal from the anode is that it increases capacitance there. The life of a GM tube is heavily dependent on the magnitude of avalanche pulse discharge current within the tube, any added capacitance at the anode increases this. For this reason it is important that R3 (in the OP diagram) is placed as physically close to the tube pin connection as possible.Yes, sensing across a cathode resistor does require more gain and increases the possibility of noise pickup, but for an expensive GM tube, it is usually considered worth it. With a small cheaper tube, you can usually get away with an anode signal pick-up, as long as stray capacitance is minimised.

author: doktor pyta
author_rank: Frequent Contributor
timestamp: January 17, 2021, 06:11:23 pm
content: Hi,1. the HV power supply section is too complex for the low power it provides.2. I don't see any HV regulation mechanism.3. I agree with jmelsonSearch for better schematic. Example here:http://www.fdscience.org/uneko/geiger.htmEDITHV PSU section is drawn here in much more clear way:or from Theremino website:

author: Skashkash
author_rank: Regular Contributor
timestamp: January 17, 2021, 07:23:47 pm
content: I also feel that there are simpler circuits available. And they may be easier to prototype.I've used a variation of this 555 based circuit with an SBM-20, and it's been running non-stop for years.https://www.electroschematics.com/diy-geiger-counter/Just be sure to use a cmos 555.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: January 17, 2021, 07:38:28 pm
content: I've the same project somewhere flying on my workbench with LND712.taking the counts from the cathode is clearly the best way to do, a simple transistor delivers the necessary gainbut I was rather unhappy with the HV cascade; to run such a setup from a small 3,7V LiPo is not really feasible, too much current drainso I'm trying to get a HV9150 from Microchip running, that turned out to not be very easy as well.the LND712 requires 500V anode voltageas radiation source I got me a ion chamber smoke detector from ali which contains a tiny quantity of Americium; certainly that's of no help for a non-alpha tube