USB PPM Interface for Windows in SMT - RC Groups
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Feb 14, 2006, 11:34 AM
now that's a wattmeter...
simingx's Avatar

USB PPM Interface for Windows in SMT

Following the schematic here: (he also has an excellent circuit for a 64K CamPAC which works with my FF8), I decided to build this very useful circuit in SMD.
Only 4 holes to drill (2 for the crystal, 2 for the PPM input).

Changes to the original circuit layout:
1) A STMicroelectronics LE33CD (SMT 3.3V regulator) was used instead of the LP2950, because that's what I had on hand
Any LDO 3.3V regulator should work too, as long as the pinouts are the same.

2) The SMT version of the ATMEGA8 has a different pinout from the DIP version, but that is taken care of by the PCB. Note that I forgot to connect one of the VCC pins in the prototype PCB, but I've since corrected that. The chip seems to work fine without it, though.

3) A PDTC114YT (NPN transistor with built-in bias resistors) was used in place of the BC547, 10k and 200k resistors. Saves 2 components and reduces the space needed greatly.

4) USB connector is Molex P/N 54819-0578. I got mine from Farnell (order code 476-8322). Any equivalent will do (example:

5) You'll have to somehow program the firmware file into the microcontroller, I just tack-soldered some prototyping wire onto the appropriate pins on the uC while programming and removed them later. I didn't see the need for a ISP header on the PCB.

The interface appears in Windows as a standard joystick.

Will try to upload a PDF of the PCB layout if anyone's interested.
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Feb 14, 2006, 12:44 PM
Registered User
Whats the physical size of the PCB ?

Looks good
Feb 14, 2006, 12:54 PM
now that's a wattmeter...
simingx's Avatar
IIRC the size of the PCB used is about 0.8in x 1.4in... could be smaller but I happened to have this scrap of PCB lying around...
Feb 14, 2006, 01:14 PM
My member is not
Zlatko's Avatar

Thanks for sharing.
Very nice find on the program and nicely executed PCB.

Thank You
Feb 14, 2006, 01:39 PM
Registered User
I always thought SMD meant smaller PCBs [/toung in cheek mode]

The attached image shows the USB interface I built using descrete devices and a Microchip 16C745 PIC.. board size is 1.7" x 0.9"
Feb 15, 2006, 12:48 AM
now that's a wattmeter...
simingx's Avatar
@Malc C: It's smaller, right? And the USB connector is on board...

Here is the PCB layout of the board in Acrobat format.
Make sure you do not enable any "fit to page" feature.
The board should measure 20 x 35mm. This is very important, as the Mega8 will not fit otherwise.
The printout is already mirrored, so expose the PCB with the toner side down.
You can compare your board with the photos to see if you got it right.

I assume anyone attempting to build this has reasonable electronics skill, so I didn't include a component overlay. That should be clear enough from the photos.

The input cable from the TX is secured with a dab of hot glue. Alternatively, you could make the PCB longer so that you can drill 2 holes to use a cable tie.

The PPM input is on the big square pad, while the signal input is on the round pad.
The other 2 holes are for the 12MHz crystal.
Nov 10, 2008, 11:38 PM
Registered User
I know this thread is a bit dated. But, i am attempting to build this circuit with a VEX Robotics PPM receiver. Unfortunately i am having a really hard time figuring out if i have the fuses right for the atmega8. I use avrdude to program. Any help is very appreciative.


Feb 27, 2011, 09:55 AM
I don't speak German ....Yet !
GrootWitBaas's Avatar
I see this is some years old, have any one got the file to flash ? the site above says forbidden so can't get it from there
Feb 27, 2011, 02:51 PM
Registered User
Originally Posted by GrootWitBaas
I see this is some years old, have any one got the file to flash ? the site above says forbidden so can't get it from there
Hope that helps.
Another one that might be better....
Feb 27, 2011, 04:13 PM
I don't speak German ....Yet !
GrootWitBaas's Avatar
I tried the one from Thomas, but it is not working (windows sees only a error not a device)
I am unfortunately at work now, and forgot to bring the boards I'll have to wait until morning to test, but thanks for the files.

Feb 27, 2011, 04:56 PM
Registered User
I had problems with VSUB when i built usbtinyisp. Windows kept seeing error.
Never did find problem for sure. Was either poor soldering or caps for crystal and res for usb input i did change.
Which is pretty much everything ......doh!

I did use diodes too.
Feb 27, 2011, 05:22 PM
I don't speak German ....Yet !
GrootWitBaas's Avatar
it is pre build, well actually it is 2 working programmers, one configured to be self programed. I can program it as usbasp and windows sees it no problem. I can do the same for USB joystick (one taking 8 analog and some sw inputs) and windows sees it, but as ppm no go

All I want is a ppm to USB able to handle at least 8 channels. If it can take more I'd be happy, but for now 8 will do. I know the atmega8 can handle it, and for circuit you don't need more than the program circuit and a buffer between ppm and atmega8 chip. The problem I have is most hex images out there does not seem to work. Maybe because they are all done for the dip, not the SMD, but I am not sure.

Thanks again for the help, I will leave this old thread now to go to rest again

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