QA060 Geiger-Muller radiation detector teardown - Page 1
Forum section: electronics / projects

author: d18c7db
author_rank: Newbie
timestamp: May 23, 2022, 02:29:49 pm
content: I bought one of those cheap $40 GM radiation detectors and when it arrived it didn't detect sh1t!On power on it initially starts displaying a fixed 0.16uSv value that slowly over a minute trends to 0 and stays there even in the presence of radiation.It's not worth the time and effort to me to return it and ask for a replacement plus I really wanted to take it apart right now instead of waiting.If anything I'll buy a different type next time, if I can't fix this.Removing the 4 screws detaches the back and we initially see a 1Ah marked LiPo and the J302 GM tube on a PCB with SMD components.Removing the battery and GM tube reveals the rest of the PCB with a HK32F030C8T6 micro made out of pure chinesium. A TP4056 LiPo charger can also be seen next to the beeper.The unpopulated components might be an EM field detector (these gadgets can be bought as a combined Nuclear + EM radiation detectors)The other side of the PCB doesn't have much except the buttons, an LED and a LCD connector.There seems to be space for a bluetooth controller which is not fitted.The LCD is a Z180SN019 1.8 inch TFT based on a ST7735S controller with SPI.I believe the reason it's not detecting anything is because the voltage across the tube is too low.The J302 GM tube needs a nominal 400VDC supply but I can only measure about 250VDC across it (with a Fluke 75III 10M input impedance).I'm not even sure what topology they have used for the voltage multiplier, I expected a typical Cockroft-Walton multiplier but tracing the circuit I end up with thisDoes anyone even recognize this topology? I haven't seen it before.

author: mikerj
author_rank: Super Contributor
timestamp: May 23, 2022, 07:06:38 pm
content: I believe that is a Dickson charge pump.

author: CaptDon
author_rank: Super Contributor
timestamp: May 23, 2022, 08:34:32 pm
content: Another piece of chinese garbage going into the U.S. landfills. China has a new toxic waste dump, formerly known as the United States.

author: Gyro
author_rank: Super Contributor
timestamp: May 23, 2022, 08:44:45 pm
content: It looks worth a repair attempt- check whether they've put the GM tube in the right way round!Worst case, keep the bits and make a simple clicky one. I don't need a fancy display to tell me when there's a radiation source close at hand.EDIT: The tube look strange, I can't see the internal structure properly but shouldn't it have a cathode cylinder around the anode wire?https://www.atcg.ltd/products/j302by-metal-gm-tube.html

author: d18c7db
author_rank: Newbie
timestamp: May 23, 2022, 10:54:27 pm
content: @Gyro The tube was mounted properly as the side marked with the + on the tube was up where the PCB also marks the electrode as positive. The tube seems to be verysimilar as these found on AliexpressThe positive end connects to a thin electrode that runs down the certer of the tube and the negative side metal cap connects to a thin wire that starts on the outside of the tube and then penetrates the glass and seems to connect to a thin and transparent metallic layer deposited on the inside of the outer glass tube.@mikerj You may well be right, though most multi-stage Dickson charge pumps I can find seem to have a two phase driver. Perhaps if the coil input went positive and negative with respect to ground you could consider the two capacitor banks (top and bottom) as being driven out of phase but without a scope I can't see how the coil output could go negative, The transistor shorts it to ground then when the transistor opens the coil will snap to some higher voltage riding on top of 3V3. The transistor has an absolute maximum Collector-Emitter voltage rating of 65V.I measure the following voltages with respect to GND at each diode junction starting with that connected to GND:6V8, 95V, 72V, 172V, 130V, 222V, 182V, 258V and 227V at the tube's + contact. The voltages don't change much with/without tube connected.

author: Gyro
author_rank: Super Contributor
timestamp: May 24, 2022, 09:31:20 am
content: Quote from: d18c7db on May 23, 2022, 10:54:27 pmAh, that would explain it then. As long as the contact to the metalic layer is intact (it seems unlikely that it wouldn't be) and the seals are ok then the GM tube probably isn't the source of the problem.Your measurements may show up a problem. The plateau region of the tube seems to be 350V to ? (550V seems a bit long) with a recommended operating voltage on 390V. The problem is that the impedance of the charge pump is very high, and you have a 2Meg series resistor on the Anode and 3Meg on the Cathode. GM tubes run at extremely low current. At least the charge pump is being driven, based on your >100V measurements.If you are measuring the voltages with a standard DMM, it will have a 10Meg input resistance. This will be significantly loading down your readings, so it might or might not be reaching 390V on the Anode. I would firstly suggest a passive approach - do a diode test on every diode in the charge pump (you should be able to do these in-circuit) and then check whether any of the capacitors are shorted. If they are all ok, then the best way to test the voltage is to add a series string of 9 10Meg resistors in series with one of the meter probes. This will give you a 100M input x10 attenuator which will load the circuit much less. Air wiring or putting them is a bit of sleeving will be fine, this is a very low energy situation. Make the measurement directly across the charge pump, not through the Anode and Cathode resistors, these will not be dropping any voltage when the tube is fitted. 100M may still be low enough to load it a bit, but the reading should be in the ballpark.If the tube voltage is ok, then attention falls on the darlington configured sense transistors and HK320K input.

author: d18c7db
author_rank: Newbie
timestamp: May 24, 2022, 11:15:38 am
content: Thanks Chris, we have very similar troubleshooting thoughts. I have already measured forward (.53V) and reverse (OL) across all diodes and measured the caps for shorts, they all look fine.I thought about the load imposed by the DMM on the HV, but unfortunately I don't have a bunch of 10M resistors. I know, I know... not much I can do without tools and supplies. No scope, no resistors... I suck.When I measure the voltage across the tube by putting the 10M of the DMM across it, the instrument "detects" radiation so it looks very much like the tube doesn't strike in the presence of radiation, either like you said faulty tube or more likely incorrect operating voltage.I wonder how the tube voltage is supposed to be regulated, is there some kind of feedback loop, I may need to trace more of the circuit to see how does it know if it reached operating voltage.

author: free_electron
author_rank: Super Contributor
timestamp: May 24, 2022, 03:40:15 pm
content: another design where they put more thought to the lipstick ( lcd display with graphical user interfaces ) but inside it is still a pig (crappy boost pump )so much stuff these days is lipstick on a pig

author: Gyro
author_rank: Super Contributor
timestamp: May 24, 2022, 04:56:00 pm
content: Quote from: d18c7db on May 24, 2022, 11:15:38 amYou're welcome, it sounds as if you have covered the normal possibilities.It does look a bit as though the tube may be faulty (also on the basis of Sod's law and it being the most expensive part!), but the only way to tell for sure will be to buy some 10M resistor and check the voltage. If the meter registers counts when you put the dmm across the tube connections then it is a fair indication that everything down-stream of the tube is working as intended.You haven't shown the switching transistor drive circuit, but from what you've drawn, there doesn't appear to be any voltage feedback. It is being fed from a regulated 3V3 supply though, so with virtually no load, the output voltage would be fairly predictable in this case.

author: trobbins
author_rank: Frequent Contributor
timestamp: May 25, 2022, 03:02:55 am
content: What are using as an emitter? There aren't many easy to obtain emitters around nowadays.

author: d18c7db
author_rank: Newbie
timestamp: May 25, 2022, 07:09:29 am
content: The core from an old school ionization smoke alarm, it contains americium 241.

author: Gyro
author_rank: Super Contributor
timestamp: May 25, 2022, 08:15:19 am
content: Americium 241 is an alpha emitter. Alpha particles won't be able to make it through the glass of the GM tube. I have a tube with a very thin Mica end window which will detect them at close range, but they won't make it through glass.Part of the Am241 decay process will produce the occasional X-ray, which the tube should detect, but they are few and far between. Try taping the source directly to wall of the tube and see if you get a few counts.EDIT: An old 1930s - 1950s watch with a no-longer luminous Radium dial (hopefully with a burn type discolouration on the face where the hands have been stationary for a long time) is an ideal gamma test source.

author: skander36
author_rank: Frequent Contributor
timestamp: May 25, 2022, 09:23:48 am
content: AM 241 emit only alpha and X-ray particles.This types of detectors are not able to detect alpha particles.But it should trigger on x-ray if your sample emit.This cheap chinese detectors are sometimes advertised as semifunctional, but I don't know why.Those that detect alpha particles are usually expensive(starting from 500 $ )It seem that those with ionization chamber are better, but I am not an expert.