How to set up the GPIO?

Hi folks.
I am trying to set up a relay on a Holybro 6c with software 4.3.
Servo16 = -1
On the user parameters, RC16_Option = Relay On/Off
It feels like I’ve missed something.
Do any of you have experience setting up GPIO??

Per the FMU pinout, it looks like you need to set RELAY_PIN=57.

Pixhawk 6C Flight Controller — Copter documentation (

Be sure your radio actually supports 16 channels. You don’t need to use channel 16. Whatever channel is set to RCx_OPTION=28 will operate the relay.

You may prefer to map RELAY2 instead, since the first relay is generally used for camera operation.

I’ve written an article that may be helpful.

Thank you all friends for the answer, I will test further next weekend. Happy weekend everyone


Hello all friends
I have set up a three position switch on my TX16S and see on Missionplanner that the set channel gives the correct response. RELAY2_LOW, RELAY2_MIDDLE and RELAY2_HIGH.
Is there any way to control two different relays with the same channel?.
Have a nice flight //YB

How are you using a relay with 3 output levels? Relay’s are on/off. Not sure what you are trying to do. You can easily mix 2 RC channels from the same switch on the transmitter.

Sorry if I was unclear with my question.
Mission planner shows three different states for the RC channel. The idea was that I could somehow activate two different relays…

Just wire them both to the same output pin.

Or if you are intending to use all 3 states, then do as Dave suggests and use a transmitter mix.

That shows 3 different PWM levels depending on how you have mixed the channel to the switches but it’s not useful for a relay. Yuri’s suggesting is the simplest if you just want to trigger 2 relays together.