Motor arming

Props can cause serious injury! In this article I'll demonstrate a framework for creating safe motor arming systems without the need for switch checks at startup.


The following terms are used in the text:

The Sticky logical switch

The basis of the framework is the Sticky logical switch. Think of it as an on/off switch where the 'on' and 'off' actions can be customised. Here's the general idea for an arming switch:

L1: Type=Sticky V1={log switch for On} V2={log switch for Off}

L1 is the arming switch - it controls whether the motor can run. V1 and V2 are physical or logical switches representing the On (arming) and Off (disarming) gestures.

This method provides great flexibility since the arming and disarming gestures can tailored individually.

How to guarantee that the motor is disarmed at startup

By default, sticky switches are False at startup. However, if V1 is True at startup, then the sticky will see an On command and flip to True ('armed') immediately after startup. This will put the motor in an unsafe state.

To avoid this possibility, V1 should always represent a user action not a physical switch. That way, the motor is guaranteed safe until the user action is carried out. That way there is no need for startup switch checks.

Example 1: safe arming system

As an example, we'll design an arming system with the emphasis on safety. For this kind of system, (a) arming should be difficult and (b) disarming should be easy (but not so easy that you can do it accidentallly whilst flying the model).

We'll use the following gesture to arm the motor:
  1. Throttle to idle
  2. Elevator stick back
  3. Pull SH
And the gesture to disarm the motor:

The logical switches are shown in the Companion screenshot below.


Logical switch scheme

How it works:

Note that 'Thr' and 'Ele' refer to sticks (not inputs). In the mixer menu, inputs are prefixed with 'I', sticks have no prefix.

Example 2: 'smart' arming switch

Many pilots like to use a simple 2-position switch for arming. The problem is that switch checks are necessary to ensure the switch starts in the disarmed position. However, by making it 'smart', we can do away with switch checks, by making it report 'Off' at startup, regardless of the actual position of the switch.

Here's the code, using switch SF.
SF↑ = Off and SF↓ is On.

L2: Edge SF↑ [0, infinite]

L3: Sticky (L2, SF↑)

The first line, L2, triggers an On pulse when SF transitions from Off to On. If SF starts in the On position, it will need to be moved Off then On in order to arm. Therefore, regardless of the position of SF, arming switch L3 will always be false at startup.

Using the arming switch to control the motor

With the arming logic done, we can apply it to the motor channel. Here's an example. CH7 is the motor channel, and it has two mixers:


Source=MAX Weight (−100%)

Source=Thr Weight (+100%) Switch (L3) Multiplex=REPLACE

How it works:

Using Channel Override special function

Instead of the Replace line, one could use the Channel Override special function to disarm the motor. Channel Override bypasses all the mixers and outputs affecting a channel, so it might seem an easy method. However there are some drawbacks:

I don't recommend this approach.


Arming/kill switch demo


  • For OpenTX 2.2 and above


Safety first!