APM2 and Rx All-in-One!


As most of you may or may not know, I've been a big proponent of using PPM Sum (aka CPPM) with the Ardupilot since I first figured out how to make it work.  The simplicity of just running a single wire from the Rx to the APM is very attractive.  It eliminates 7 possible failure modes (losing the wire connecting channels 2-8) which could result in loss of control.  There's only 1 wire to get right, but also one wire to get wrong.  Fortunately, the APM can handle complete loss of signal well, and if you set your failsafe to handle complete signal loss, all is good.

I was using this on my HK450GT FBL helicopter, along with a FrSky D8R-SP Rx. Again, as anybody who knows me is aware, I'm a big fan of FrSky radio equipment.  It is affordable, and high performance offering true diversity on the Rx, easy to use CPPM (just requires a jumper on two pins), the Rx is very tolerant of low voltage (it'll keep going LONG after the APM would have shut down, down below 3V).  And rebooting and reacquiring the Tx signal takes less than 1 second.

All of these features are superior to the big name brands, Futaba, Spektrum, etc.

And the telemetry features offered by FrSky is just the icing on the cake.  RSSI (signal strength), Rx voltage, external voltage (flight battery), and even the option for a telemetry hub with all kinds of goodies This is somewhat negated by using an APM, but still, you can put one of these on your Tx and have all the info right at your fingertips.


Anyway, back to my little 450 heli with a HUGE Rx mounted on it, the D8R-SP.


I discovered that FrSky offer a new Rx, the D4Fr which is a small, yet full-range Rx.  And while it only has 4 channels out, it can be configured to do 8-channel CPPM with the simple addition of a jumper! Brilliant, and there's no reason why any modern compact receiver shouldn't be able to do this.  This Rx does not have as many telemetry features as it's big brother.  It can only report RSSI and Rx Voltage, it does not have connectors for the telemetry hub or external voltage.  But that's fine, as the APM can be used to report voltage anyway.  So I ordered one and it arrived Friday.


I started working on setting it up, when the wheels started turning in my brain.  The "case" is just a flimsy plastic thing, almost useless anyway.  And I'm only going to have a single cable going from the Rx to the APM, and it's so small.  And the APM2 has those stand-off pins onto which the daughterboard is soldered.  And those pins are kinda long...  And I've got this piece of empty prototype board sitting around...

Pretty soon I had de-cased the Rx and stripped all the input pins off the APM2.


I soldered the proto-board to the daughter board standoffs, and then glued the Rx board onto the proto-board.  I soldered jumpers on both the Rx and APM2 in order to set both into CPPM mode.  And then I soldered a jumper wire for the CPPM signal, power and ground.  Done.

The complete unit is compact.  There's no wires to get in the way of whatever vibration-damping mount you are using.  There are no wires to come disconnected.  I suspect this setup is at least an order of magnitude more reliable than a standard 8-channel APM2 setup.

Even better, it's compact, and easy to move between vehicles.

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  • Hey guys, I guess you did miss the first time around! :)

    I am working on another version of this using an APM2.5, and a full-size Rx.  I'm trying to work in a couple more goodies onto in, including a really good 5V regulator and some voltage dividers for battery monitoring.  I'll post it up when I'm done. 

  • I love it, Fabulous work R!

  • Hey R_Lefebvre,

    Great work on this reduction! I'm not really sure how you went about it. It looks like you gave the Rx power with the tin leads, and the black wire to channel 1 or 2. Then shorted channel 3&4.

    So where does this single signal wire go to the APM? does it need to be programmed?

    I have the FRSky V8FR-II, how would you go about changing that? Or just buy another cheap D4Fr?

  • Christian,

    Please see my post in forum under APM2 for photos.

  • Steven, which SMD resistor is missing? This might be my problem on my APM2 as well.

  • I may save some of you a big headache. I trouble shot the heck out this and traced it down to a missing pull up resistor on Ch1. The signal from Ch 1 never makes it anywhere. So much for the claim that 3DR tests each and everyone of these before shipping! I mean the SMT Resistor is completely missing. This could have never passed a test. Mine may be the only one but if not, this clue will help you from spending 3 hours on it like I did.



  • Ok, Got it. My APM 2.0 works fine too in the 2/3 jumper config. but not my APM 2.5.

  • R_Lefebvre,

    Do you know where the PPM trace cut is that people recommended cutting? I hate to do that in case my unit is defective, then I may not be able to return it. The PWM seems to work fine. So all I need is a direct inject into the processor and it should work, right?



  • Darren,

    Do you have an APM 2.5? Do you really mean Sig. Ch 3&4 or 2&3?



  • Steven, sorry I can't answer your question, I don't know.  I was told the APM2.5 was the same as APM2 in this regard.  I have a 2.5 I just haven't gotten that far with it yet.  I will be attempting this soon.  I'll probably be using it with a D8R Rx, but I might decase it and hard mount it since there's lots of room on the 2.5.

    Darren, sorry, I don't know the answers to those questions either.

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