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Theremino PMT 3.3 based radiation detector

Theremino "the modular in out" software, PMT driver and detector with USB sound card analog in.

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This one's probably even more of a doozy than than the other projects, since it's based pretty much straight off the Theremino PMT project files. However, once again it was little difficult to find all the pertinent information from different sources, and there's scant personal experiences to be found, so I thought I might as well briefly document my ongoing foray.
The PMT adapter, as the acronym implies, is intended for driving a PhotoMultiplier Tube for Gamma Ray Spectroscopy. However, as I have not yet validated the functioning of my build, I have just a Soviet "new old stock" geiger tube I'm hoping to run with it. Eventually, if everything works out, I'm planning to obtain a suitable starter scintillation probe to use with it.

It's been literally ages since I've done electronics from ground up and a number of things have changed since then, not the least reasonable manufacturing of small PCB runs. Since the PCB gerbers are available off the Theremino PMT site, getting started was fairly straightforward. There is a small caveat, though; the published design features a noise-inducing ground loop which the author has helpfully pointed out, but not fixed in the published gerbers.

The advice is to file or cut the trace on the PCB manually, but since I was doing my own run of the boards, I decided to fix that ground loop at the source, on the gerbers. With a little pointers from one of the electronics designers at the company I'm working at, I quickly got the spurious ground loop removed with Eagle PCB (It took a while to realize the schematic and PCB are tied together, so you'll have to make bizarre changes to the schematic), and bill of materials produced for the item. I then sent them to OSHPark, one of the better hobbyist-directed PCB manufacturers.

Here's the first digression of the project, while getting small PCB runs and components at reasonable prices may have gotten easier, in general if you roll your own, you still can't always get components or PCB's one by one. This is generally good, since messing up one PCB for example won't delay your project if you have a spare. However, if you're figuring the cost of building something, the final price will likely be at least triple what you'd calculated from BOM because you order everything in batches (and sometimes extensive shipping costs to double even that final figure). This also means I now have components for at least two more Theremino PMT adapters, though I'm not sure if we're allowed to advertise here :)

While I was waiting for the PCB's to arrive from the foundry, it was time to turn to the component requirements. Okay, I'll confess, I was doing these things in somewhat wrong order. If I'll do something similar again, I will order the components I can get from eBay first, then when most of them have arrived, revise the PCB to accommodate the parts I have and send it in. Then when confirmation of shipping PCB's comes from foundry, I'll set up an order with Mouser or similar for the components I could not source off eBay, "got lost in mail" or won't fit on the PCB layout.

But this time I was over-eager and ordered everything at once. This meant I got the Mouser components within a few days, the PCB's after a few weeks before any eBay sourced components had arrived, and had to wait couple of months for some of the electrolytic capacitors. Luckily, all the eBa sourced components arrived. Getting everything, including the eBay components for which there was often no size specs, to fit on the pre-built PCB took some ingenuity.

For the Theremino PMT adapter, there are several components that can't be sourced off eBay, and some which probably shouldn't be sourced off eBay. The BSP300 Mosfet at the core of the voltage booster I could only find at Mouser, and the 1kV and 1.5kV components, including the RPG02 voltage doubler diodes, belong firmly in the former category that can't be sourced off eBay. I also opted to build the 4 x 1Mohm output stage from 1kV SMD resistors, even if the voltage drop through each one individually is likely to be significantly lower.

The temperature componsation zener diode 1N757A is another Cold War surplus relic and quite expensive, so I decided to substitute it with BZX55B9V1 from Mouser, which side by side appears to have better specs and certainly cheaper if already ordering from Mouser. In retrospect, I should have ordered the 3.3mH boost coil from Mouser as well, but the eBay listing had limited specs that led me to believe it'd be a good option. The eBay one appears to have quite high series resistance, and there was no way to determine which is the inner core lead for higher efficiency.

  • Component choices

    BadgerBadgerMushroom03/07/2015 at 16:56 0 comments

    See the weird "bent" component unfocused at the back of the board? Technically, that was my failure in reading the schematics and thinking those 4 x 1Mohm resistors were all identical SMD components. Turns out the first one is meant to be a "normal" barrel type. I like the end-result though, so I'm going to claim I always planned to use one of those high-quality 1kV SMD resistors there.

    It has short leftover resistor leads soldered on so it serves as a barrel type resistor. However, I soldered the leads on the board first, and then the SMD in between them. This way you don't end up bending and cracking the delicate SMD component. So no soldering leads to it first.

    The Theremino PMT adapter shows the L1 flyback coil having a current drop of 120mA, possiibly more, though the specification doesn't state a requirement for high current, just "L1 must have a low series resistance (A few ohms, maximum 5, absolute maximum 15)". The eBay descriptions unfortunately didn't list series resistance for these components, but I opted for the highest current rating available, expecting series resistance to lower alongside it: "3.3mH 300mA 6x8mm 10% Ferrite Core Shielded Radial Lead Inductor Black". With a standard multimeter, they measure about 10.5 ohms. The 1mH chokes that eBay specs said are 0.91Ohm @ 1Khz measure 1.6 Ohm at DC multimeter. So I am thinking these components might be better obtained somewhere else than eBay, but I'm making do with them for now.

    The choice of resistors is one issue; in the Theremino PMT prototype pictures a curious mix of metal film and carbon film resistors can be seen, leading me to assume the designers didn't consider resistor choice to play much of a part, so I've followed suit with getting whatever resistors I could best get. For the unpolarized condensators there is bit of an issue with physical sizes. In particular, 2kV cap instead of the specified 1.5kV was the most affordable choice for the signal filter capacitor, and also best considering the target voltage range, but on assembly it turned out I had to bend the lead dangerously and position it leaning over against the noise shield to get it fit within intended encolosure. I worry what this might do for noise, so sticking with 1.5kV capacitor may prove for easiest assembly and casing.

    And speaking of noise shield, I built mine from electric guitar copper tape. In the end it may not have been as clever as I expected, and I still soldered the connection posts to it all the way down of course. Mostly the copper adhesive tape seems to ensure no sheet metal cutters are needed, and if there's a danger it makes contact with the casing, it's easy to crumble into lower profile. At worst I've introduced more ground loops by the doubled over (lenghtwise) copper tape, although I'm hoping that isn't a real issue here.

    All other components have been populated here, except for the T1 which is alternative to Mosfet BSP300. Since I had the BSP300 from Mouser, T1 isn't populated (and in fact wouldn't fit because BSP300 is blocking the holes on the flipside of the board). The RC circuit intended for T1 is populated, because instructions said it could be left on. Beyond that there were a few gotcha's, like the eBay sourced trimmer potentiometer had to be stretched to max to fit the PCB footprint and the pin-strips are difficult if not impossible to cut without the final pin breaking free. I also worry about how good connection the pin-strips are making; they seem to be failing the test set forth in the build instructions, despite carefully choosing what looked like quality parts.

    As of right now, the board powers up successfully with no lost magic smoke, and provides voltage. There's an entirely expected problem, however: The voltage is beyond what multimeters will measure, and the current output is below what the multimeters tend to demand. More on that in another update.

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