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Sep 24, 2011, 03:16 PM
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FRSky D4FR, a mod to monitor an external voltage


Intro:
The small D4FR telemetry receiver from Frsky lack all external ports that the bigger receivers are having. The only voltage that can be reported is the power for the receiver which is kind of worthless if you are using some kind of BEC to power it. This mod is about removing this internal voltage sensor and using it external to monitor a choosen battery instead.

I also want to take this opportunity to express that I'm not pleased at all with this receiver and with FRSky that never answers my polite mails that asks about those bugs:
* My CPPM out differs about 50 ms per channel compared to the transmitter
* The RSSI out continues to send the last "good" value on connection lost instead of 0. (What were they thinking..!)


Credits:
I took the internal sensor up to discussion on the OpenPilot forum and user "C2PO" found out where on the card the internal sensor was placed. Finding this was the big work and most of the glory for this Mod should be given to him.
http://forums.openpilot.org/topic/62...d4fr-receiver/. The original picture is also from him.

Warning:
Since its a four layer card, my solution might have been a little drastic but it seems to have worked. I have not had any problem with the receiver (Other than the bugs already present..). Of course, all forms of warranty vill be lost when doing this mod.

Instructions:
More or less: Same type of voltage sensor/voltage divider that is used external on FrSky receivers is placed onboard for the internal voltage aswell. By cutting the lines for this onboard sensor I could solder cables and measure an external voltage instead. Be aware, the ground should probably not be cutted as I did. You atleast must be sure that the same ground is used for the supply power of the receiver and the external batttery you are measuring.

I used a little saw and cut a notch into the side of the card to remove the internal connection from the supply voltage (A sharp knife could probably be used also). Then used tiny single wire cables and soldered directly on the resistors. A good solder iron with tiny tip is probably a must for this. A picture tells more than words:
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Sep 25, 2011, 10:18 AM
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The good news is this receiver is firmware upgradable 9 (the four vertical lined pads area left in the lower left)
So, the RSSI issue can be fixed with a firmware, now that the cat is out of bag .
The 50ms delay I am afraid is known and by design and you must live with it.
Also, I suggest a further improvment, add a diode between MCU ADC input and receiver plus power line, as in this schematic.
Sep 25, 2011, 12:53 PM
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Quote:
Originally Posted by RENATOA
The good news is this receiver is firmware upgradable 9 (the four vertical lined pads area left in the lower left)
So, the RSSI issue can be fixed with a firmware, now that the cat is out of bag .
The 50ms delay I am afraid is known and by design and you must live with it.
Also, I suggest a further improvment, add a diode between MCU ADC input and receiver plus power line, as in this schematic.
Nope, the 30-50ms (on each channel) seems to be a firmware bug as well (if they didn't change the hardware after the first batch) since newer batches don't have it. It's not a delay, its each channel that having the pulses of by a certain ms, look at
https://www.rcgroups.com/forums/show...highlight=d4fr


Frsky says (not to me since they not talking to me) that D4FR is not software upgradable. Might it be that its a one-write MCU? (they need pads to do that write as well)

What should that diode fix?
Sep 25, 2011, 01:24 PM
Registered User
OMG, that was MICROseconds, not MILI, and was really a non issue, easy fixable on any computer radio with digital trims.

Diode will add overvoltage protection.
Sep 25, 2011, 06:00 PM
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Quote:
Originally Posted by RENATOA
OMG, that was MICROseconds, not MILI, and was really a non issue, easy fixable on any computer radio with digital trims.

Diode will add overvoltage protection.
Yepp microseconds not milli, my wrong. However, its far from a non-issue for me since I change models a lot. (Changing receivers but not controller card and tx).
Not fun to connect the controller card to a computer and trim it every time. (autoleveling with gyrostabilized camera so quite important to get right)
I think its terrible how you can release a receiver where the output differ that much from the input...
Jan 30, 2012, 11:20 PM
CD-ROM Junkie
Art Newland's Avatar
Thanks to you all for this thread and instruction. I was terribly disappointed that external voltage tap wasn't included in the design. Who cares that your BEC is outputting 5v?

I took your advise, drilled a small hole instead of sawing, leaving the ground in tact. Now need to figure a way to secure a battery connection to it. Any good ideas about that?
Feb 10, 2012, 08:14 AM
Registered User

easier solution?


Hi-
Sorry- but why not only desolder R12 and R13 (guess the cap will do no harm) and connect external voltage with an external voltage divider directly to the top of R12 (which I would assume is the input of the DAC)? Only would need to reference to the same ground as the receiver- piece of cake...
Might be I REALLY am missing something here but this way no brute force needed ;-)
Greets Roman
Mar 30, 2012, 10:45 PM
devIAtion..one TX to bind them
Hexperience's Avatar
You guys saved me... I bought the D4FR and wasn't paying any attention that it wasn't the D4FR-II...

Thanks!


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