Modify FrSky D4R-II for external AD1

FrSky D4R-II receiver

The FrSky D4R-II is a miniature conventional 4-channel or 8-channel CPPM telemetry RC-receiver weighing less then 6 grams. But in contrast to its bigger brother FrSky D8R-XP, only one external analogue value can be applied.

This little hack allows both analogue voltages AD1 and AD2 can be supplied externally.

external AD1 and AD2 on FrSky D4R-II

Attention: You should have very good soldering skills as the parts on the PCB are extremely tiny.

By default AD1 is internally tied via R13 (330Ω) to a voltage divider between the supply voltage and ground consisting of R21 (4.5kΩ) and R20 (1.5kΩ). For this hack the connection between R13 and the R21/R20 voltage divider has to be disconnected with a cutter knife. Two of the pins of the .1" header have to be cut by diagonal cutter from the PCB.
The now open pin of R13 has to be wired using a thin solder-able enamelled wire to one of the rededicated pins. To have both analogue signals easily available the AD2 signal can be connected to another pin on the .1" header.

To enable CPPM (Combined Pulse Position Modulation) output on CH1, CH3 and CH4 have to be shortened with a jumper.
Unfortunately the frame standard frame period of the D4R-II and D8R-XP is only 18ms which isn't enough time for 8 channels with maximum throttle. Luckily FrSky provides an alternative firmware with 27ms framing.

Alternative FrSky firmware with 27ms CPPM timingAlternative FrSky firmware with 27ms CPPM timing

D4R-II manual
D4R-II 27ms firmware update howto
D4R-II 27ms firmware and update tool
D8R-XP manual
D8R-XP 27ms firmware update howto
D8R-XP 27ms firmware and update tool