Adding battery voltage telemetry to FrSky RX6R with Radiomaster TX16S

Now that Combat 2 (HB1) has had its maiden flight, cut a ribbon, and survived a mid-air, it is time to add voltage telemetry to move beyond timed flight.  It turns out our ZOHD based speed controllers make this pretty easy.

Items you will need:

• ZOHD 30A ESC
• FrSky RX6R receiver
• 4.7k ohm resistor
• 1.0k ohm resistor
• The FrSky header connector (the one without the programming plug -- supplied with the receiver)
• Heat shrink, extra wire, zip ties, etc.

Important, the resistor divider above is configured to get a decent sensitivity for 3 or 4 cell batteries (max voltage should not exceed 3.3v at the A2 input,  for this divider this means max 18.8v battery voltage). Is possible to use other resistor values for higher voltage battery packs.

Here we see the ZOHD ESC as previously installed. What is nice is that extra 2 wire connector (red/black) is directly connected to the battery terminals! This sure cleans up the wiring.

Next solder a resistor divider like this.  Note that the two pins facing down will directly plug into that extra 2 wire connector. Cut them straight and of a good length for inserting into the connector.

Now locate the header cable included with the receiver and cut all the wires except green (actually double check your color code and ensure you keep the wire that will connect to A2 sensor input as seen in the receiver pinout diagram).

Insert the resistor divider into the connector (Important: the 4.7k resistor MUST be on the RED side and the 1.0k should be on the black side.  Inserting this in reverse will likely damage the receiver's A2 sensor input circuits)

Now, using a bit of a jumper wire and heat-shrink, insert the connector into the receiver header, solder an extension wire so the length is about the same as the ESC control cable, insulate, heat shrink, and zip tie as necessary.

Now, if the TX16S is already bound and sensors discovered, the A2 voltage should show some value (here we see 7.7) when the airplane is powered up.

To calibrate to the actual voltage, enter model setup, scroll to the sensor tab. Note A2 is rendering here. If you need to discover sensors, this is the page to do it on.

Lastly, calibrate the voltage divider. Connect a flight battery and also connect a cell voltage monitor (here we see 11.06V - yes an empty battery). Scroll TX down to the A2 sensor and click enter. The page shown will display, scroll down to the Ratio field, click enter and adjust the ratio so the voltage displayed in the upper left of the display matches the battery voltage.  That's it! 

Next, you can further program the transmitter to emit voltage values, low voltage alarms. It might be interesting to apply a sag-compensated curve to the voltage so you don't have to exit the combat session too soon.

Configuring FrSky RX6R with Radiomaster TX16S transmitter

Here's a short post on the steps needed to use a very nice FrSky RX6R receiver with a Radiomaster TX16s transmitter.

1. Ensure the transmitter is running compatible OpenTX firmware
2. Upload the appropriate FrSky firmware to the Transmitter (I used ACCST_2.1.1_FCC). This should be uploaded to the 'FIRMWARE' subtree. Firmware should be available here: FrSky downloads
3. Modify the wiring of the included SBus to servo-style connector to be yellow, black, red. This ensures correct wiring from the pins inside the Transmitter:
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4. Next create a new model in the transmitter (I use a dedicated one I call 'flash') which has the model set up for external RF in PPM mode. This will configures the pins in the external RF compartment to power the receiver in standalone mode:
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5. Now, connect the receiver to the external RF ports as shown (yellow on the 'bottom'). This will power up the receiver if wired correctly:
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6. Now, prepare for firmware update.  Enter 'Sys' mode, move right to the flash icon, and scroll down to select [FIRMWARE]:
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7. Enter this subtree and scroll down to the ACCST firmware (for US, the FCC version is required):
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8. Next select the firmware and see a pop-up that says "Flash External Module":
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9. At this point the radio lights should change to green and a progress bar of writing to flash should be displayed:
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10. Once the above completes, we are ready to bind the receiver. Select the receiver mode FRSKYX2 - D16
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11. Next prepare receiver for binding.  Here I set it up on an aircraft and power up the receiver while pressing the receiver button (actually when done correctly, the red light will be steady on):
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12. And lastly, select 'bind' and set for channels 1-8:
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13. The receiver led flips to blue if bind is complete.  At this point, power cycle the receiver and do a fine tune and range check. Add in hardware failsafe (set transmitter to failsafe positions, power up the receiver and press the button on the receiver within 10 seconds).  You should also set failsafe in the transmitter.
14. Then you can discover the sensors and get receiver telemetry from the sensor sub-menu of the model.
15. (TODO: describe how to prepare a voltage divider so you can get battery voltage telemetry too.)