Relay Module with Pixhawk 4 v5 FMU

Hi There,

My setup: Pixhawk 4 FMUv5, Taranis X9D + 19, XR8 receiver all happily bound and talking to each other. I am running Ardupilot 4.0.5 for now.

My end goal is to setup a electromagnet powered by 12v from the power distribution board with a relay switch controlled by my flight controller & transmitter.

Where I am in that process:

I intend to use channel 6 on my transmitter as the on/off switch. I have set up the switch and I can see it operating as intended in QGroundcontrol.
As you can see below, In QGroundcontrol, I set Channel 6 switch option in flight modes to relay on/off:

To simulate the electromagnet, I have a 9v battery connected to an LED. The positive wire is cut midway with the new ends connected to the common and NC (normally closed terminals).

In QGroundcontrol, I have mapped Relay_PIN to Auxout1:

Relevant Documentation I have found:
Pixhawk 4 Wiring Diagram
Pixhakw 4 Pinout

Issue: Despite having connected a servo cable between FMU Out CH1 & my relay and doing the above mapping, my relay is not responsive.

My Theories:

  • My relay requires a higher trigger power
  • I am connected to the wrong pin
  • I may need to power the FMU pins directly


  1. Which physical pin out of Pixhawk 4 FMUv5 corresponds to Auxout 1? (Based on the quick start guide, I believe this is FMU_CH1)
  2. Do I need to separately power the FMU pins for this to work?
  3. Is there additional mapping I need to do? (So far I have mapped SF to Channel 6 on my taranis x9d+2019, mapped channel 6 to relay on/off in Qgroundcontrol, mapped relay_Pin to Auxout 1)

Additional possibly helpful images:

Different pin boards:

General Setup:


For assistance setting up with ArduPilot, best to ask at

Hi erik, did you find a solution to this because i am experiencing the same problems as you.
Waiting to hear from you.
Kind regards