DJI Mini 3 and 4 Pro Payload Drop Mechanism operated via Meshtastic

This payload drop mechanism attaches to https://www.printables.com/model/1400517-rak-19003-case-for-dji-mini-3-and-4-pro-with-hinge . Check the instructions on https://www.printables.com/model/961658-rak-19003-case-for-dji-mini-3-and-4-pro first!

For this build, you'll also need:

• an ES9051 servo
• a ⌀1.8 mm, 25 mm long metal pin (optional, but recommended)
• two 20 mm M3 screws and nuts

You don't need RAK 12500 GPS used in the original build, it wouldn't work with the RX0 pin connected to the servo anyway.

Connect the servo as follows:

• power wire to the battery “+” pin or wire
• ground wire to the battery “-” pin or wire, or the GND pin
• signal wire to the RX0 pin

You might be tempted to connect the power wire to the VDD pin, but that only provides 3.3V and a very minimal current, not enough to drive the servo. Connect the wires in a way that will allow them to lie flat on the board.

You will need a modified version of the Meshtastic firmware to control the servo, you can grab it from this github repository. 

After flashing the firmware, you'll also need a modified version of the Meshtastic CLI to configure the servo, you can find it here. Set it as follows:

• servo_control.enabled: true
• servo_control.gpio_pin: 19
• servo_control.open_position: 0
• servo_control.closed_position: 90
• servo_control.authorized_key - public key of a Meshtastic node you plan to use to control the servo

The open and closed position should be 90° apart, as the servo only needs to rotate this much.

Do not mount the servo just yet! After reboot, the servo will move to the open position. Use the other node to send OPEN, CLOSE and STATUS commands to confirm it's working as expected. The servo should rotate clockwise when going from the open to the closed position. If that's not the case, swap the two settings.

⚠ The servo can become very hot after being powered for a while.

Move the servo to the closed position before mounting. Insert the metal pin into the plastic track (or print it as a single part), lay the track into the slot and push it so that the pin goes through the hole and lands in the recess on the other side of the gap. Now push the spur gear onto the output shaft of the servo and mount the servo.

In the open position, the pin should be retracted and the track should not extend beyond the end of the slot.

Lay down the wires securing them with little pieces of double-side adhesive. If your servo wires end up being too short, you can print a mirrored version of this model to put the servo closer to the RAK board. Don't forget to update the settings as the servo needs to rotate in the opposite direction with this setup.