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34.1 Relay Pins on Autopilot with IOMCU
34.2 Define Relay Pins via Mission Planner
34.5 Mission Planner Control Relay
foreword
The " relay " is a digital output pin on the autopilot that toggles between 0V and 3.3V or 5V , depending on the autopilot . Similar to a servo, it allows the autopilot to invoke certain actions from other devices on the vehicle. Up to 6 pins can be defined as relays.
34.1 Relay Pins on Autopilot with IOMCU
Autopilots with an IOMCU (the IO coprocessor that provides the "master" PWM output ) can configure their AUX outputs as GPIOs instead of servo/motor outputs and use them as RELAY outputs.
For example, the Pixhawk defaults to AUX OUT 5 (pin 54 ) and AUX OUT 6 (pin 55 ) as the " first " (or #0 ) and " second " (or #1 ) relays respectively .
By reducing the number of AUX pins used as servo ( Servo ) outputs , the number of available relays can be increased to a maximum of 6 . This can be achieved by reducing BRD_PWM_COUNT from 4 to 2 or even 0 .
！Note
In firmware 4.2 and later, the method of setting PWM/SERVO/MOTOR output as GPIO function has changed. Instead of using BRD_PWM_COUNT , set the single SERVOx_FUNCTION parameter to " -1 ". If set to " 0 ", it is still a PWM output with no function assigned, and outputs the trim value of this output when the board safety is not activated. If the servo function is " mirrored " to a remote device, such as a DroneCAN or KDECAN ESC, then in order to change the corresponding output pin of the autopilot board to GPIO , but allow SERVOx_FUNCTION to still be assigned to the remote device, the SERVO_GPIO_MASK parameter can be used to assign the board pin as GPIO without affecting the remote device's SERVOx_FUNCTION assignment.
Likewise, on other controllers with only PWM outputs and no IOMCU , setting BRD_PWM_COUNT to a lower number will free up its higher numbered output for use as a GPIO to control a relay .
34.2 Define Relay Pins via Mission Planner
The first to sixth relay pins are most conveniently set using the Mission Planner's CONFIG/Standard Params list , shown below, after " Finding " the relays. The dropdown box shows the pin names for several different autopilots .
For other boards, you'll need to find their hwdef.dat file here ( here ) and identify the GPIO pin numbers listed next to their output numbers, like this:
In the above case, you can set BRD_PWM_COUNT to 8 , release PWM9 and PWM10 for GPIO use, and then use GPIO pin 58 (that is, PWM output 9 on the board ) as a relay pin. Set the RELAY_PIN parameter in the Mission Planner Parameters item above using its GPIO number, ignoring the dropdown selection or using the CONFIG/Full Parameter List screen directly in the parameter itself .
！Note
Any changes to relay pin settings will require a reboot to take effect.
34.3 Pilot Control Relay
The relay can be controlled by an auxiliary switch. Settings can be made through the CONFIG / User Params screen as follows:
！Note
This screen allows RC5 to RC14 to be set , but any RC channel ( 1-16 ) can have its RCx_OPTION set to RELAY , provided that channel is not already using the other controls of the CONFIG / Full Parameter List screen.
34.4 Mission Control Relay
Similar to servos, relays can also be activated during missions using the Do-Set-Relay mission command. This is described on the Copter Mission Command List wiki page.
！Note
In MAVLink , relays are numbered 0 - 5 , not 1 - 6 , so RELAY 0 is the first relay .
34.5 Mission Planner Control Relay
The Mission Planner allows the user to set the output of any of the first four relay pins to LOW, HIGH, or set LOW and briefly toggle HIGH through the Data screen and the Servo/Relay subwindow using buttons, as shown in the image below:
