Can not get 3.3 volts from Pixhawk relay pin

Dear Pixhawk community,

I am trying to connect a 3v relay to Aux5 on Pixhawk 2.4.8 in order to operate a pump, but the relay does not work. The multimeter shows between 0 and 1.4 - 1.8 volts when the switch on my transmitter is Off and On, respectively. Same story when I press “Low” and “High” tabs in “Servo/Relay” section in Mission planner.

I have the following parameters in the Mission Planner:
BRD_PWM_Count = 4
Relay_Pin = 54

I have no idea what I am doing wrong. Any help would save my project. Thanks.

If you have the RCx_option set to 28, x being the transmitter channel you configured, it should work.

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I have Channel 7 set to trigger the relay. I have set RC7_Option to 28, but no result, same 1.4 volts. Can you please take a look at the attached pics. Thank you.

On that pixhawk, the servo rail is not powered (only the RC + pin has power, the one on the edge). The aux act as a bus bar that you can power the flight controller from, but you cannot power servo devices from that rail without applying power to that rail from an external BEC. A diode should be used also. More info at

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I didn’t know that. Thank you so much!

This community is amazing!