Relay Switch

A “Relay” is an digital output pin on the APM or Pixhawk that can be switched between 0 volts and either 3.3V (Pixhawk) or 5V (APM2). Similar to a servo it allows the flight controller to invoke some action from another device on the vehicle. In Copter, Plane and Rover, up to 4 pins can be defined as relays.

Relay pins on the Pixhawk

On the Pixhawk AUX OUT 5 (pin 54) and AUX OUT 6 (pin 55) are setup as the “First” (or #0) and “Second” (or #1) Relays respectively.

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The number of available Relays can be increased to a maximum of 6 by reducing the number of AUX pins used as Servo outputs. This can be accomplished by reducing the BRD_PWM_COUNT from 4 to 2 or 0.

Relay pins on the APM2

On the APM2, A9 (pin 9) is the pin that is recommended to be used as a relay. In fact any of the pins from A0 ~ A8 may also be used as relays but to use these you must manually set the RELAY_PIN parameter (to “0” ~ “8”) through the Mission Planner’s full parameter list.

../_images/Relay_APM2.jpg

Defining the relay pins through the Mission Planner

The First ~ Fourth relay pins can be defined most easily using the Mission Planner’s Config/Tuning screen’s Standard Params list as shown below. Alternatively if you know the exactly pin number to use, the RELAY_PIN parameter can be set directly through the mission planner’s Full Parameters List.

../_images/Relay_SetupWithMP.png

Pilot control of the relay

On Copter 3.2 (and higher) the “First” relay can be controlled with the Ch7/Ch8 switches.

../_images/Relay_SetupCh7WithMP.png

Mission control of the relay

Similar to a servo, the Relays can be activated during a mission using the Do-Set-Relay mission command. This is described on the Copter Mission Command List wiki page.