The FrSky D4R-II Receiver is a compact and inexpensive option for small quadcopters. While nowadays I've moved over to smaller FrSky receivers, this post is still useful so here it is.
Normally, I configure my D4R-II to send PPM control signals so that the whole 8 channels can be sent via a single wire. I also enable analog RSSI output so that this data is available to my FC or my OSD as the case might be. As is sidely discussed elsewhere, the D4R-II outputs the RSSI signal as a control pulse on the pins corresponding to channel 2. While it's easy to configure your Taranis setup to send the RSSI via an RC channel, I consider this a suboptimal solution.
When doing this, the transmitter is actually sending its recovered RSSI value via one of the RC channels, so the receiver figures out the RSSI, sends it back to the transmitter via the telemetry link and then the transmitter sends it back to the receiver via a RC channel. If you lose telemetry, nothing onboard besides the receiver actually knows what happened. Rather than this convoluted setup, I simply follow these instructions to build a simple PPM-to-analog converter that provides a 0-5V signal that corresponds to the current RSSI level. Simpler and totally onboard solution.
My first step is to verify that the receiver works out of the box. You don't want to spend your time in a receiver that arrived dead -- also this would make it impossible to return the defective receiver, not that I've ever received a dead one. Then remove the cardboard sleeve and carefully desolder the header pins. While I generally like to keep the pins, this way to mount the receiver provides for a more compact setup, specially suitable for 180 and 150 sized quads.
Carefully place a solder jumper between the signal pins on channels 3 and 4 to enable PPM output. This can be seen better in the picture below.
Choose your capacitor and resistor for the RSSI analog conversion. I chose a 330 Ω resistor paired with a 0.1 μf ceramic capacitor, which provides an analog voltage of up to 3.5V for 100% RSSI.
Solder one end of the resistor and one end of the ceramic capacitor together. Then solder the remaining resistor leg to the signal pin of channel 2, and the remaining capacitor leg to one of the ground pins on the D4R-II. It's useful to revert to the cardboard sleeve to double check your solder points. Make sure to double check with a voltmeter. Make sure to cover the solder joint using heat shrink or liquid tape.
At this stage I hooked my pocket oscilloscope and verified that the receiver performed as expected. In the picture you can see the traces corresponding to the 8 channels.
After verifying that all worked as expected, I took off the antennas and plugged them backwards, so that they come out of the receiver on the same end as the rest of the wiring. This allows me to seal one end entirely and further reduce the space required to mount the receiver. Do not apply power to the receiver without the antennas plugged in.
At this point I also applied 4 layers of 419C Acrylic Lacquer Conformal Coating. This is a clear coat that protects electronics from water and dust.
Since I tipically use the telemetry features of the D4R-II to send data back (the Flight Controller sends a bunch of data back!), I carefully removed all but the green wire from the telemetry cable that comes with the receiver. Gently lift the plastic retainer from the white connector housing and pull the wire out to remove it cleanly without damaging the crimped pin. Keep these removed wires around, as they can be used to replace the telemetry lead after a bad crash. Only the green wire is required because the ground is common.
Apply clear heat shrink to protect the receiver as the conformal coating might not offer enough protection in some cases. Since I'm mounting my D4R-II to a Naze32 Rev 6 board, I'll be wiring it to two 5 row Dupont servo housings. Some Naze32 boards include a 10 pin Dupont housing which you can also use for this purpose. Simply remove the original wires by gently lifting on the housing tab while pulling on the wire, and then place the wiring connected to the D4R-II in the proper slots. Of course, you need to crimp suitable Dupont pins to the wires connecting to the D4R-II.
My telemetry -- the green wire -- will be connected to a Softserial port in pin 8 of the Naze32. The RSSI analog goes into pin 4. Ground, 5V and PPM gots into pins 1 to 3 of the Naze32. The finished product can be seen in the last picture, mounted to my new ZMR-180 quad.
I hope these notes are helpful for you!
You can read this and other posts at my blog, https://lem.click -- Feedback is welcome! Please comment and upvote.