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Old Dec 03, 2005, 12:07 PM
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Las Vegas
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PCB layout for Mr.RC-Cam's Servo tester

I was wondering if anyone is good at PCB layout that can translate this to a PCB layout for me? It is Mr. Rc-Cam's servo tester(I hope he doesn't mind), I wanted to make it nice and clean and etch my own board, but don't know anything about Eagle cad to do so. Can someone that is very proficient with this whip up the PCB layout??

I'd appreciate any help
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Old Dec 03, 2005, 12:52 PM
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Sterling Heights, Michigan, United States
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UPDATE, the schematic is correct, my bad. The board works now thanks to Rc-cam. Thanks Thomas.

OLD POST:
I've already made a board, but it didn't work right. For some reason his schematic seems to be wrong or perhaps the software. The pot function does not seem to work. Everything else is ok. I see the rc time constant change on the input with changes to the pot, but the pic doesn't respond with servo commands.

I have a few more boards left if you are interested in one, but I would look into way the schematic or software is wrong first.

Kin

http://www.embeddedtronics.com/


Quote:
Originally Posted by duhaas88
I was wondering if anyone is good at PCB layout that can translate this to a PCB layout for me? It is Mr. Rc-Cam's servo tester(I hope he doesn't mind), I wanted to make it nice and clean and etch my own board, but don't know anything about Eagle cad to do so. Can someone that is very proficient with this whip up the PCB layout??

I'd appreciate any help
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Old Dec 03, 2005, 01:03 PM
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I guess we need Mr. RC-Cam to chime in here and give us his wisdom do you have the file that you could print out of the pcb layout??

Thanks
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Old Dec 03, 2005, 01:53 PM
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I'm no expert, but it could be either the hardware or software that's the problem as Kin has stated. A PIC should be able to generate the 1 - 2ms pulses on a 20ms time frame without any external timing circuitry (assuming thats what the two capacitors and resistor is). Hopefully Mr RC-Cam will shed some light with a full CCt discription and help you out.
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Old Dec 03, 2005, 02:47 PM
Xtreme Power Systems
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12C508 PIC chips don't have an A/D converter. Looking at the schematic (I don't know where the code is), I would assume that the designer intends to set GP5 to an output charge the capacitor by setting the output high for 'x' length of time, and then time the discharge by setting GP5 to an input and using the timer to determine how long it takes to for input to become TTL low. This time will vary with the resistance of the pot. This is a very simple trick that is used quite often when you don't want to go to a higher end PIC.
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Old Dec 03, 2005, 03:18 PM
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Jim, thanks for informing us why there are additional components. I would of thought that it would be better to of used a 12F675 as that has A/D functionality - still an 8 pin package, and not what I would really call "high end" of the PICs available. You could then use a simple 10K pot wired as a divider could be used to adjust the servo position.
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Old Dec 03, 2005, 07:25 PM
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I mailed Mr. RC-CAM a PIC12C508 for programming. I assume that your problem must be in the hardware then. If he shows that it works on his website then I wonder what you did wrong? or maybe something was changed by Mr.RC-CAM.

Hmm, I hope he chimes in soon
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Old Dec 03, 2005, 09:11 PM
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Sorry for the problems. Hopefully I can help.

The PIC12C508 does not have a A/D. The RC time constant comprised of R1+R2/C1 is the solution. Several times a second the PIC charges up the CAP, then discharges it through the variable POT. It carefully measures the discharge time, which will change depending on the POT resistance. This effectively emulates a A/D function and is decently accurate.

The circuit is not fussy. It has been built by MANY hobbyists and no problems have been reported. The C1 cap should be a 2% part (or hand picked) to keep things honest. If the circuit does not work then my first guess (assuming it was wired correctly) would be that the OSCAL value was over-written during programming. If the PIC was obtained from me then we can cross this off the list.

Even if the cap and POT were horribly messed up, the switch selectable "Servo Center" feature should still work. If it doesn't, then the PIC is toast or wiring errors are in the way.
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Old Dec 03, 2005, 10:51 PM
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Quote:
Originally Posted by kfong
I've already made a board, but it didn't work right. For some reason his schematic seems to be wrong or perhaps the software. The pot function does not seem to work. Everything else is ok. I see the rc time constant change on the input with changes to the pot, but the pic doesn't respond with servo commands.

I have a few more boards left if you are interested in one, but I would look into way the schematic or software is wrong first.

Kin

http://www.embeddedtronics.com/
Can you post the pcb layout that you used so I can etch one??

Thanks
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Old Dec 04, 2005, 01:59 AM
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The schematic has three components that are very annoying from a PCB layout
perspective: the pot and the two switches. These come in SO many different
packages that the would-be PCB designer has to either know/guess exactly which parts
you're planning on using, or put standard pads on the board for connecting to off-board
parts (which is nice and general but just SO inelegant :-)
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Old Dec 04, 2005, 04:36 AM
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Ask and you shall receive. 40x40mm, rather relaxed in its design rules. I added
the 2940 LDO regulator in case someone needs it.

I didn't see anything fishy with the schematic while in the process of
transcribing it, BTW. I can't tell whether the schematic matches the
firmware, of course, and this PCB is NOT tested.



And the Eagle .sch and .brd files, with a PDF that might work as layout.
rccam-servotest.zip
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Old Dec 04, 2005, 09:27 AM
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Westfw,

Thanks for the PCB layout. I am going to give it a test tonight. I have all the parts except the PIC from MR.RC-CAM. So I'll populate everything but that. I'm sure that others will appreciate you doing this as well
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Old Dec 04, 2005, 10:09 AM
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Hmm, I'll check the OSCAL, Servo center works when button is pressed, even the cycle function. I can see the time constant changing as it goes into the input pin. I had programmed the chip myself, so I will try programming it again when I get the chance.

Here is a picture of the board, only did a small run.

Thanks,

Kin

http://www.embeddedtronics.com/


Quote:
Originally Posted by Mr.RC-CAM
Sorry for the problems. Hopefully I can help.

The PIC12C508 does not have a A/D. The RC time constant comprised of R1+R2/C1 is the solution. Several times a second the PIC charges up the CAP, then discharges it through the variable POT. It carefully measures the discharge time, which will change depending on the POT resistance. This effectively emulates a A/D function and is decently accurate.

The circuit is not fussy. It has been built by MANY hobbyists and no problems have been reported. The C1 cap should be a 2% part (or hand picked) to keep things honest. If the circuit does not work then my first guess (assuming it was wired correctly) would be that the OSCAL value was over-written during programming. If the PIC was obtained from me then we can cross this off the list.

Even if the cap and POT were horribly messed up, the switch selectable "Servo Center" feature should still work. If it doesn't, then the PIC is toast or wiring errors are in the way.
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Old Dec 04, 2005, 12:59 PM
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Quote:
Here is a picture of the board, only did a small run.
That is a nice looking board. I can see the servo centering push switch, and the connector for the pot. Do you support the cycle switch? {Don't readily see it in there}.

If you want, you can mail the assembled board to me (along with the pot you will be using) and I will check it out to see what is going on. All I ask is that you include enough stamps to cover the return postage.


BTW, what fab house is "EW"?
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Old Dec 04, 2005, 01:46 PM
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Thanks, Imagineering is what this board was from and even then I think they farm some of them out, but most boards houses these days do very similar work.

Yes the cycle switch is there, but since I don't intend to use it. It wasn't installed. I tested it by just shorting out the connector and it works.

Sure I can send it to you, just pm me your address. I'll send you a blank board for your troubles.

Kin

http://www.embeddedtronics.com/


Quote:
Originally Posted by Mr.RC-CAM
That is a nice looking board. I can see the servo centering push switch, and the connector for the pot. Do you support the cycle switch? {Don't readily see it in there}.

If you want, you can mail the assembled board to me (along with the pot you will be using) and I will check it out to see what is going on. All I ask is that you include enough stamps to cover the return postage.


BTW, what fab house is "EW"?
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