PCB layout for Mr.RC-Cam's Servo tester

[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:)

[kfong]

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/


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:)

[mrbaseballny]

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

[Malc C]

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.

[JimDrew]

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.

[Malc C]

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.

[duhaas88]

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:D

[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.

[duhaas88]

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

[westfw]

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 :-)

[westfw]

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.

http://www.geocities.com/westfw/electronics/srvtest.png

And the Eagle .sch and .brd files, with a PDF that might work as layout.
rccam-servotest.zip (http://www.geocities.com/westfw/electronics/rccam-servotest.zip)

[duhaas88]

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:D

[kfong]

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/


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.

[Mr.RC-CAM]

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"?

[kfong]

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/


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"?

[Mr.RC-CAM]

westfw: Thanks for posting your artwork. I agree that the layout is a slave to the component footprints that will be used. The POT will be something with a knob, so a chassis mounted part is probably going to be used. No need to change what you have created since a wire harness (just two wires needed) will easily interface to the R1 footprint you used.

kfong: I PM'd my address to you. I have used Imagineering and they do good work. I liked their 5 pcs for $15 each 2-sided deal.

[westfw]

The POT will be something with a knob, so a chassis mounted part is probably going to be used. No need to change what you have created since a wire harness will easily interface to the R1 footprint you used.
Actually, that was the intent, despite the use of an existing trimpot "package outline."

Hmm. **WARNING** The PCB pattern I sent out is apparently set up for airtronics
style servo connectors (sig, gnd, v+ instead of sig, v+, gnd.) It wasn't supposed to
be; I suspect this means I have a bug in my part library...

BillW

[westfw]

I've fixed and updated the schematics and board layouts for use with Futaba style
servo connectors. I left the old schematics there too, only renamed in the zip file.

[duhaas88]

Mr.RC-CAM,

I got the PIC in the mail and was wondering if I need to solder in a ic socket to protect the PIC from heat when sodering it in?? Also, I got all the parts from Digikey, except the spst/spdt switch and the pot, I can pick these up from radioshack right??

[Mr.RC-CAM]

The socket is handy, but not needed to protect the PIC from heat. I'm not sure what you'll find at Radio Shack -- their selection of parts is limited.

[Mr.RC-CAM]

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.Kfong, your nice looking stuffed PCB arrived today and has been repaired (simple cap installation problem). It will go back to you ASAP.

duhaas88, how did your PCB turn out?

[duhaas88]

MY PCB didn't turn out so well, I have to buy a laminator and try it that way. The Iron didn't seam to adhere the photo paper well enough

[Mr.RC-CAM]

"Kitchen sink" etched boards have always been a love-hate affair with me. I never found it a real joy to do, but the instant gratification is nice. However, I've retired from the chemical mess and now just use professional fab services. The cost sux, but at least I don't ruin my clothes anymore. :)

The RCST project is really simple and a perfboard works fine. Construction time is well under 2 hours, so a PCB is just not essential. So keep that in mind.

[duhaas88]

the layout that westfw so graciously provided for us has two holes for each of the switches that are used for cycle and center, what if a SPDT switch is being used like the BOM calls for in part # EG1903??
Thanks

[Mr.RC-CAM]

...what if a SPDT switch is being used like the BOM calls for in part # EG1903?? If you want it to use the EG1903 on his PCB then you'll have to modify his PCB artwork. Poor man's trick: If there is room on the PCB for the SPDT switch, just drill the required holes and then jump the Sw's leads to the PCB's related pads with wire.

[duhaas88]

otherwise a SPST will work fine??

[Mr.RC-CAM]

I just realized that the EG1903 is the wrong part number. The switch is a SPDT on-off-on (three position) type. The correct part number is EG2377. I'll fix the docs ASAP.

Edit: Ignore this -- see post below.

[duhaas88]

ahh, OK tht explains it. I ordered and rec'd the EG1903 I'll have to ordere that part and then I should be ready to go on this servo tester:) I'll post photos when I get it completed

[Mr.RC-CAM]

Oops, I'm thinking of the wrong project. The parts called out on the bill of materials are correct. Two separate SPST or SPDT switches are fine, but one must be a momentary type.