Schematics for Standard Counter

Computers and stuff like that

Schematics for Standard Counter

Postby Doug Coulter » Thu Jun 30, 2011 1:57 pm

This could go in several places, but since most of this is a digital project, I'm putting it here, with the software under embedded software.

This is actually the analog part of this project. This is the HV power and signal conditioning for the standard counter based on some NOS Russian pancake geiger tubes we got surplus. Hope we find more, they're really nice and extra sensitive to betas, which is what most activation products give for detection, a nice match. The tubes seem very stable and repeatable, making them ideal for this use.

Here's the schiz:
STDPWR.gif
Schematic for analog/power part of Standard Counter.


Edit: Due to real world testing there have been a couple of changes to the above. The 1 meg resistor from the HV supply has changed to ten megs, and the order of the 1 meg resistor and 47 pf cap after the geiger tube has reversed so there's less stuff up at HV on the board to attract dust and damp. Also, we found that by tying a 1 meg resistor (not shown) across pins 1 and 3 of this tube, the sensitivity goes up 25% or so, so I added that. I'll redraw this and re-post when I get a chance. /edit

There is more skull sweat in this than meets the eye at first. Sometimes it takes extra work to get to real simplicity. I am using an off the shelf CCFL inverter well below its normal supply voltage (which would have been 12v) and even then, with only a half wave rectifier, it gives a little too much voltage. Also, these are noise generators, and we'd rather not kick a ton of noise back into the main 5v power, nor put a big capacitor directly across the USB power bus - the spec says that's a no no. So a 47 ohm resistor handles both of those requirements. This one needs to be a half watt so it can't fry no matter what, and it gets very slightly warm in use. More could have been done to reduce power draw, but hey, that's coming from a computer that presumably has plenty, and it takes a lot more parts to do it -- a voltage doubler and down regulator for the CCFL are required to cut the power to about 1/4 of what it is, but really since a bigger drop in a regulator also uses some wattage, you only get to about 1/2 this unless that's an efficient switcher. Nope, we KISS around here, and this page draws right around 20 ma off the 5v bus.

I show the 1 meg resistors as quarter watt. In this case it has nothing to do with power, but size and voltage standoff ability. A too tiny package risks internal breakdowns with high voltage drops. I like the blue 1% resistors for digi here. The 47k resistor is a 5v pullup, and the load for the attenuator that takes the 100 or 200 volt pulses down to size (about 10v), so the diodes can clip them to the digital rails and hand the PIC counter a very nice, clean, waveform to count. In the prototype, I didn't add the 1n4148's at all - there are already some wimpy diodes in the PIC itself, but for production, these should be in there to protect the PIC better. A dust and humidity induced arc across the second 1 meg resistor would take out the PIC without them.

The CCFL as shipped has two capacitors in series with the transformer secondary, used as ballasts when used with a CCFL lamp. We don't need and can't allow them here with a simple half wave rectifier, so I pulled them out, did a little creative PCB track cutting, and put the diode and filter cap back on the CCFL board itself. You can put them separate if you like, this just saved space and made my sorry bench kludge a little more robust during development. As the geiger tube draws almost no current even when counting, and the CCFL runs about 50khz, this simple power supply is actually quite noise and ripple free -- way better than actually needed for this, and since the geiger tube is very unpicky about voltage, no further regulation is needed -- it would just add parts that could fail, and definitely cost money.

I'll get the digital part here as another post as soon as I get it drawn. It's pretty simple too.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Schematics for Standard Counter

Postby Doug Coulter » Thu Jun 30, 2011 6:40 pm

And here's the digital part. Again, as simple as I could make it. I've shown some useful options here that might be desireable on any layout. You could simply not stuff the optional parts for projects that don't need them. I didn't give pin numbers because:
This is so generic it will work with many different PIC chips -- and some have from 28 to 64 pins -- numbers are meaningless even within the PIC18F4550 as it comes in two packages with different pinouts. I also show only one Vdd and one Vss pin, but most pics have more than one -- so hook them up too.

This is also so generic that with teeny changes, it will work with almost any PIC. Some might not support USB...and some might want a different Xtal frequency. That's about it.
I've not set the pinouts for the programming pads (it's not very important except that they all be alike). The plan here is to use a 2x3 set of "stakes" on the end of the cable from the programmer, and just hold it to the pads while programming. This can be facilitated by making the holes in the pads the right size for that -- a little too small for the stakes to fit in fully, say .030" (stakes need .035 or more to go through). For uses where you'll be programming over and over, you can always make a cable with these on one end, and the standard RJ11-6 on the other end -- minding the wiring pinouts.

RS 232 parts are optional. They're nice to have for debugging. Since this is completely powered by USB, they're not real useful by themselves, however.
I made provision for another counting input, and some of the more "valuable" and "multipurpose" PIC pins to be brought to pads, so that you can use them for custom things or other designs.
It's a sort of compromise, as for something really different, you'd just lay out another board, but for diddling around and testing things, a couple spare pins are nice to have.

Here it is:
StdCPU.gif
Standard counter CPU part.



I show the pullup 47k for counter 0 on both sheets, only one is needed in the real build. The power led is obvious. By putting the other led on a PIC IO pin, we can use it for anything. For this, connection and activity status come to mind. A fancier design might put this between two IO pins and use a bi-color led.


I'm using digi part number 151-1081-ND for the type B USB jack. Here's the factory drawing.
USBConnector.pdf
USB jack data sheet
(149.62 KiB) Downloaded 423 times

And here's a link to the pinout for USB. Link

The PIC 18f4550 datasheet is huge. But it's obviously available at microchip.com and quick to find if you type 18f4550 into the search box.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Schematics for Standard Counter - corrections

Postby Doug Coulter » Sun Jul 10, 2011 5:01 pm

And updates. I've now built a proto (simple, through hole for easy changes) for this to get my development board back for the next project. This made it easy to run all the geiger tubes through and see how they agree (very well, which is great!), see if there was anything in the design "on the edge" and so on. This resulted in some minor changes. The series R between the 416v HV and the geiger tube went up to 10 megs instead of the 1 meg shown above. The 1 meg was just too much current, and could hold the tube "on" in a glow discharge or get into relaxation oscillations, not good for something we want as stable as the hand of man can make it. The 10 meg slows it down a little, pulse down time (25 us) plus full recovery is about 75us total, so these can't count super fast, but are still as fast as most geiger tubes. That third pin is an extra outer anode ring, and hooking it to the inner one via 1 meg (not shown above) makes the counter about 20% more sensitive -- about equal to the increased area "under field", and slightly slower, which is why we now have a 75us inter-pulse time. The threshold as set will detect a second pulse inside that time should it occur after the first enough for the voltage to rise enough, so no changes needed there.

It was cool to see that tube blinking inside when I held up a source, but too bad it doesn't work well that way.
That's probably real hard on the quench gas as well.

StdProto.jpg
Bench prototype for ugly bud box here


Here's the bench prototype I'll keep and use here -- looks won't matter to me, and it'll just go in an ugly bud minibox. Mine needs that extra jack so I can reprogram it.

A much more professional version will be built and will be for sale here. Let me know if you want one. Estimated price will be a bit under $200 (not much under probably). These will be calibrated here and shipped with a calibration/test source. I'm now writing some fancy Linux software to make plots and put things into a database. I may also do windows software as it's pretty hard to install all of cygwin, gnuplot, glade, a couple perl modules, etc on Windows to make this Linux software run, I'll base that on demand. But all you really need is any terminal program to show the data and save it in files. So far, there is just one command from PC to this unit -- send it a ! and it resets its counters and runtime, then resumes running. It shows up as /dev/tty ACM? in linux, and com? in windows. It's completely powered off the USB bus.

Edit: Here's a shot of what the output looks like after counting an hour on my bench upstairs (the quietest place I have). The higher sensitivity boosted the average count rate from 22.xxx to what is shown here, averaged over an hour. Not bad at all. This counter is much more sensitive to betas than my older standard, which counted more like 100-120 cpm over the same interval, so the signal to noise here for the intended purpose will be better yet.
StdLongCount.png
Screenshot


2nd edit: result of 5.0 hour count was 8221 counts of background or 27.403 cpm. This included both the late afternoon high counts we usually get here as well as late night quiet times. A pretty good average, and about 5 cpm more than before I wired in the extra pin on the tube.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Schematics for Standard Counter

Postby Doug Coulter » Fri Sep 02, 2011 10:07 am

Some progress here. Joe Jarski has been beavering away to take this from a one-off proto to something we can build and sell. Arranged around my ugly prototype (but hey, it works perfectly and I'll keep that one), are the new boards, and the lamp mantle calibration sample, with a piece of silver foil we'll be shipping with these. We'll select the cal samples so everyone gets the same activity and verify/cross calibrate them with other sensors here for recording purposes.
stdctrpre.jpg
std prototype with new PC boards and extras

We did the CPU board very general purpose, and the HV board modular, as this is planned as only the first use of this type of platform. There's a pump station controller in the near future, as well as a multichannel analyzer. Both will use this same CPU pcb, plugged onto a larger board that will contain the project-specific other stuff.

We'll sell these to all comers for $200 -- built, tested, and in a nice box (Joe's working on that now). Contact me for the board member discount if you're interested in this. The goal is to get a few out there so we're not confused when we compare numbers across labs. We're not exactly going to get rich off this, but I think the people who did the work ought to at least get a little for their effort upfront (risk, as we say in trading markets).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Schematics for Standard Counter

Postby Alex Funk » Tue Sep 06, 2011 3:02 pm

Apparently GIF files as attachments are not allowed in private messages. TBD are part number for high speed HV diode, uP count input pin number, and the pin numbers for at least 2 G-M tube terminals.
Std_counter_analog_PS.gif
Std_counter_analog_PS.gif (9.85 KiB) Viewed 8464 times
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Re: Schematics for Standard Counter

Postby Joe Jarski » Tue Sep 06, 2011 3:14 pm

Nice looking schiz Alex. Doug might have a better suggestion, but I've using UF4007 for the high speed (1kV) diodes when the output voltages are low enough.

One correction on your schematic though, the R across pins 1 & 3 is correct, but pin 2 is the ground - it kinda looks like you have pin 3 as the ground.
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Re: Schematics for Standard Counter

Postby Doug Coulter » Tue Sep 06, 2011 4:26 pm

Also, the coupling cap between the tube and the uP is 47 pf @ 3kv -- I went ahead and ordered the higher voltage so the leftovers are useful in other projects. The 1 meg goes between pins one and two to pick up the outermost ring in the tube. I'm still working on the tube supply series R. 1 meg turned out to be way too low -- the tube actually lit up during pulses (cool but very hard on the quench gas). 10 megs works, but is on the slow and noisy side, so I'm trying 4.7 megs on the next build (had to order some to get them in 1/2 watt, not required for power but for standoff voltage in humid air).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Schematics for Standard Counter

Postby Doug Coulter » Thu Sep 08, 2011 7:50 pm

I'm very happy to report that the boards Joe Jarski laid out for the STD counter work as designed, and on the first try!
Good show Joe :D :D :D :D :mrgreen: :ugeek:

Won't be long before we have these to ship to anyone who wants them! A tweak here and there, a nice box, and a .inf file mod to make windows realize what it is, and we're there.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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