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Jul 05, 2005, 11:29 AM
For Sale-everything over 250 g

PWM to servo signal conversion?

Is it possible to make a device to convert PWM from a micro Rx to the servo signal used by a brushless controller? And what would be involved? I do believe it's possible with a micro chip controller but am not sure how to proceed or if it has been done before.
I have a very small brushless motor I would like to put in a plane but a GWS Rx and assosiated servos are way too heavy.
Any ideas or comments?
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Jul 05, 2005, 11:38 AM
Registered User
What type of micro Rx are you considering which has outputs which will not drive servos or a speed controller ? Most of the ones I know are available with standard servo outputs.

Jul 05, 2005, 11:52 AM
For Sale-everything over 250 g
Hi Steve,
What I want is only the servo output for throttle. Most micros are available with actuator control and PWM for the motor OR servo outputs for everything. I am not aware of any that have actuators outputs for control surfaces and servo output for motor.
I have a RFFS100 (in a plane) and an available actuator JMP. Also have a one cell brushless controller.
Jul 05, 2005, 12:16 PM
Micro Flyer, Big Poster
Pete P's Avatar
You mean control a servo with an H bridge (actuator output)? I believe that isn't possible without heavy duty PICing :-P I believe there is a radio (new) with one PWM channel and actuator outputs... I know I've read of it.
Jul 05, 2005, 12:24 PM
For Sale-everything over 250 g
Hi Pete,
No, I do not want to use an actuator output for the motor. The micros have built in speed controls for the throttle channel. It is this PWM for the throttle that I want to convert to a signal that will be accepted by the brushless controller.
Jul 05, 2005, 01:13 PM
Registered User
Perhaps it would be possible to pickup the pulsed PPM signal after the RF frontend, just as it's fed into the signal processor on the RX. Feed that into a PIC which plucks out the approriate channel and outputs that. One of the Microchip 10F PICs would be a pretty small and light solution.

Only problem I could see would be that the PIC and RX controller might disagree on signal quality, and you could end up with a bit of oddity at extreme range (e.g. motor control but no attitude, or v/v).

Jul 05, 2005, 03:22 PM
Registered User
If you talk to JMP I'd imagine he can tell you where to find the PPM signal in his Rx. It may be possible to get directly to the throttle signal but even if you can only get at the composite (all channels) signal it would almost certainly be easier to decode that as darkith suggests than to try translating the PWM back to a 50Hz servo signal.

Jul 05, 2005, 04:25 PM
Registered User
Mr.RC-CAM's Avatar
If you utilize the raw PPM signal out of the Rx's demodulator IC then you could use Bruce Abbot's PIC based PPM decoder to recreate the throttle channel's PWM "servo" pulse. The decoder is discussed here:

Jul 05, 2005, 05:51 PM
For Sale-everything over 250 g
Thanks, I vagely remembered something like that but didn't have the link. Do you think there will be any problems like Darkith suggested? This really sounds like the way to go, most of the work has been done by Bruce Abbot and I even have the pic chip.
Thanks guys
Jul 05, 2005, 06:03 PM
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Mr.RC-CAM's Avatar
Do you think there will be any problems like Darkith suggested?
Interference is, what interference does. Having DSP protection on the throttle channel is not a bad idea, at least for the typical model.

Jul 05, 2005, 07:34 PM
Registered User
I once designed a similar device that was intended to be installed on the "Draganfly" four-rotor helicopter thingy electronics board. A fellow aeromodeller wanted to design a similar lifting platform only much bigger and using stronger motors (brushless), using the original electronics with all its gyros and stabilization stuff. Hence he needed to drive four brushless motor controllers by the Draganfly electronics. What I did was actually using an Atmel microcontroller to evaluate the PWM of all four driver signals (directly from the PIC on the Draganfly electronics) and converted them back to PPM suitable to drive the ESCs. I also added a momentary switch so it was possibly to provide full throttle PPM to all four outputs so the ESCs could be "learned" to the available PPM range. I don't know if this lifting platform ever flew since I lost contact with this individual (I doubt it due to the delay most of the brushless ESCs have...).

If anybody's interested, I may provide more detailed information.

Jul 05, 2005, 07:42 PM
My member is not
Zlatko's Avatar
Hi All,

I was looking at something similar a while back ( after reading your wonderful work on the 6mm brushless motor ) . I found a few chips that actually used PWM signal to drive a 3 phase brushless motor. I remember that none of them would work below 5V though, however I can't remember the chips or the manufacturers now.

I'll try and search for it again if this is what you are after.

Also, you could try what darkith and Mr.RC-CAM suggested. I've used Bruce Abbotts code ( for 12C508 ) into a 12F509/MS without a problem . The MS chip weights 0.025gm !!

Mr.RC-CAM also has DSP code but for 12C683 SOIC ( ~0.08gm ) and is not publicly available. Mr.RC-CAMs code has “insane” resolution .

With the SOIC chip you can piggy back new one on top of the old actuator decoder and just connect +ve, -ve and PPM in for the JMP RX and take the servo/throtle output from it. I've done this myself .

Jul 05, 2005, 08:20 PM
For Sale-everything over 250 g
I would love to see the code to covert the PWM to servo signal but I'm afraid I won't know what I'm looking at let alone be able to adapt it to my need.

I'm aware of the chips your talking about but the 5 volts is the problem. There isn't much need in the industry for a lower voltage chip so I doubt they will be made.
I have some 12c509, any idea if they would work with Bruce Abbotts code? I also have some 10f200 and 10f206, really small and light but I really don't want to get into programming at this point.
Next version of my micro brushless will be molded and have a little more power. I have a test mold made and when I get the time to make a motor I'll post some pictures. Still needs some work to get it more efficient but that will drive my cost sky high.

Please correct me if I'm wrong. I believe the 12c509 chips are the same as the 12f509 except the c is one time programable where as the f chips are flash and reprogramable, so the code will still work, I just need the right programmer.
Last edited by mcross; Jul 05, 2005 at 08:49 PM.
Jul 05, 2005, 08:48 PM
My member is not
Zlatko's Avatar
Hi Mike,

That sounds very interesting Can't wait to see the pictures.

I havent tried the 12C varieties but by Bruces notes, 508 code will work in the 12C508 and 12C509 .

You have to check the PPM signal to make sure you burn the corect code ( positive / negative ), or just burn one of each and see which one works. His code is also JR / Futaba specific ( if you want throtle arming ).

code here

You can use the LED to see that the PIC works properly and is receiving PPM signal.

Good Luck


Please correct me if I'm wrong. I believe the 12c509 chips are the same as the 12f509 except the c is one time programable where as the f chips are flash and reprogramable, so the code will still work, I just need the right programmer.
In one of the Microchip Aplication Notes I saw they recommend 12F508 for doing developing work for 12c508, and 12F509 for 12C509.
The F versions can be reprogrammed many times, I bought the PICkit 1 programmer from DigiKey, I think they are $36 and you also get software and 1x DIP 16F684 and 1 x DIP 12F675 . The programmer uses USB for programming and power. It will also programm the 10F20x chips but you will need a adapter AC163020.

PICkit1 $36
AC163020 part number AC163020-ND $36
Last edited by Zlatko; Jul 05, 2005 at 09:17 PM.

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