How to set VR1 + think up some correctness tests

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icknay
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Joined: Sun Dec 04, 2011 7:43 pm

How to set VR1 + think up some correctness tests

Post by icknay »

This kit looks fantastic, thanks so much for making it. I'm a teacher, so the lens though which I see everything is "where are the students going to get stuck", and that leads to my questions....

1. It would be great to add a step in the instructions to spell out how to set VR1. Actually I imagine having a few different instructions, depending on how much the user cares and what hardware they have available. (Putting answers in the forums is great in the short term, but ultimately this should be spelled out in the actual instructions.)

a. you can just rotate VR1 to the middle of its range , and that will actually work fine (I don't know if this is true, I'm just guessing)
b. if you have a multimeter and want to set VR1 more closely, the procedure is (something)
c. (as above, but the user has access to a 1 GigaOhm resistor)

Also maybe document the relationship between VR1 and the SBM-20 voltage, and in turn the effect of the SBM-20 voltage -- does that increase or decrease the min event it can detect and how many events per second it can handle.

2. I would love the docs to have short list of correctness-measurements one could make on an assembled, powered off board that would check for common errors -- either swapping components around or bad solder joints. Imagine having a room full of kids putting these together, and when one raises their hand and calls out "hey it's not working" ... it's nice to have a clear list of checks to point them to, even if the checks only catch some errors. It's ok if the checks are labor intensive, or tedious, or require jumpers to short across some components or whatever. If the thing is not working, they don't have anything to do anyway, so give them something to do. (I'm happy to contribute a diagram or writing for this, but the electronics is not my area.)

Just thinking out loud here, but I wonder if there a check that could be implemented where the SBM-20 is replaced by some easy to get resistor, like say 1 MegaOhm, and then power the thing up and check the voltage .. would that work as a check on the HV side of things, or would that just instantly fry the whole thing? I'm guessing that the HV side is the most common source of problems, so I'd love for someone to come up with some sort of end-to-end functional check of that side.

Thanks!

Nick
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mightyohm
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Re: How to set VR1 + think up some correctness tests

Post by mightyohm »

Hi Nick!

1. The instructions do include 2 methods for adjusting VR1 - rotate until you hear clicks or use a 1GOhm resistor to more precisely set the voltage. The use of a multimeter without the resistor is an interesting idea, but the problem is that different multimeters have different input impedances so it would be impossible to come up with a single set of numbers that would work for all meters.

The important thing to remember here is that the SBM-20 can operate over a very wide voltage range and the CPM values do not have a large dependence on the bias voltage on the tube. You can verify this yourself by measuring a test source and playing with VR1. You should not see a strong dependence.

2. A list of checks and debugging tips is an excellent suggestion! I will add this in the next major rev to the instructions.

I'll think about basic checks that could be done to verify functionality. My sense is that for most people, the kit goes together very smoothly.

From my experiences giving workshops, the most common problems are (in no particular order):
- Chips installed backwards or not seated correctly
- Bad solder joints
- Tube inserted backwards

And that's about it.

The two best indicators that things are working:
1. You can hear a faint whine from the speaker - this means the HV supply is working
2. If you use your fingers to short the + and - terminal of the geiger tube, you should hear a click and see a flash from the LED. (And I have never received even a mild shock this way despite the high voltage. The series resistance is huge and the HV supply is not capable of producing any serious amounts of current anyway.)
icknay
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Joined: Sun Dec 04, 2011 7:43 pm

Re: How to set VR1 + think up some correctness tests

Post by icknay »

Thanks Jeff -- ah oops, I see we're talking about different instructions.

I looked at the ones linked from geiger counter page here http://mightyohm.com/blog/products/geig ... tructions/

I think you must have accidentally let that copy of the instructions get out of date. I guess ideally you want the web site and what's in the package to show literally the same PDF or whatever to avoid such snafus.

That's interesting what you say about the voltage and the sensitivity, suggests a natural little classroom experiment. I'd encourage you to add background info like that to the instructions, as I expect we're all naturally curious about how it works and variations and whatnot when assembling the thing (and of course someone teaching a workshop can crib off your list of interesting observations). Heh, or maybe that's all in the paper instructions already which I don't have in hand yet.

For the finger-short test ... I suspect that would work with a banana too. This gets around the voltage fear, plus "banana test" has fun-science-lab written all over it. Testing with fingers could be sort of part (b) if you want to go into an explanation of amperage. And aren't bananas a bit radioactive .. it writes itself!

Thanks again,

Nick
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Re: How to set VR1 + think up some correctness tests

Post by mightyohm »

icknay wrote: Thanks Jeff -- ah oops, I see we're talking about different instructions.

I looked at the ones linked from geiger counter page here http://mightyohm.com/blog/products/geig ... tructions/

I think you must have accidentally let that copy of the instructions get out of date. I guess ideally you want the web site and what's in the package to show literally the same PDF or whatever to avoid such snafus.
Yeah, this is my fault. I need to remove the "placeholder" instructions from that page. The full text PDF you are looking for is linked at the top. I'll remove everything except the link. I don't actually ship any instructions with the kit - it's all online.
icknay wrote:
That's interesting what you say about the voltage and the sensitivity, suggests a natural little classroom experiment. I'd encourage you to add background info like that to the instructions, as I expect we're all naturally curious about how it works and variations and whatnot when assembling the thing (and of course someone teaching a workshop can crib off your list of interesting observations). Heh, or maybe that's all in the paper instructions already which I don't have in hand yet.

For the finger-short test ... I suspect that would work with a banana too. This gets around the voltage fear, plus "banana test" has fun-science-lab written all over it. Testing with fingers could be sort of part (b) if you want to go into an explanation of amperage. And aren't bananas a bit radioactive .. it writes itself!

Thanks again,

Nick
I'll look into expanding the instructions - this is on my list, so is creating a web version of the instructions to replace what is there now. Bananas are faint emitters (alpha I think) and unfortunately are not detectable by the kit, at least the bananas I used were not.

Take look at the PDF and let me know if that is more in line with what you are looking for...
icknay
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Joined: Sun Dec 04, 2011 7:43 pm

Re: How to set VR1 + think up some correctness tests

Post by icknay »

Sounds good -- I'll keep notes during assembly about where I get confused or have questions.

If I work on the correctness-test angle ... are there any limitations on what resistances one could temporarily put in place of the SBM-20 in terms of damaging the device?

Thanks,

Nick
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