Motor arming system

Mike Shellim 26 April 2016
Updated 9 June 2022

Motor arming is a common topic in the forums! Here's a recipe for a safe arming system. It's designed to protect the user from accidental spinups before and after the flight.

For this kind of system, (a) arming should be difficult, to avoid any chance of accidental spinups, and (b) disarming should be easy but not so easy that you can do it accidentallly whilst flying the model.

(Note: a kill switch is slightly different - it must offer an easy and quick way to stop the motor. If you want a kill switch, then the code can be modified quite easily).

The Sticky logical switch

Arming systems are typically based on a Sticky switch. Think of it as a customisable on/off switch. The Sticky switch is switched on via the arming gesture and switched off via the disarm gesture. The arming and disarming gestures are themselves implemented as logical switches.

This method provides a lot of flexibility, as you can customise the arming and disarming gestures independently.

Operation

For this example, we'll use the following gesture to arm the motor:
  1. Throttle to idle
  2. Elevator stick back
  3. Pull SH
And to disarm the motor:

The code

In the following code:

Note that 'Thr' and 'Ele' refer to sticks (not inputs) - make sure to make the correct selection (inputs are prefixed with 'I', sticks have no prefix)

Integrating in your setup

With the arming logic done, we can apply it to the motor channel:

How it works:

[An alternative method of disarming the motor is via the Channel Override special function. However, this method is not recommended as it is applied after the mixer values are calculated. This can cause problems for example if you want to query the mixer value to detect if the motor is running. The method shown above is more explicit and easier to debug.]

Download demo

The following demo shows the arming/kill switch operating on CH3.

For OpenTX 2.2 and above

MotorArmKill.otx

Customisation

The solution presented here is very safe, but cannot be used for in flight rearming. However you can substitute your own gestures for L3 and L5, to achieve the desired balance between safety and convenience.

Safety first!