Auxiliary Functions Swithces - Lost Copter Alarm (issue!?)

The idea is to have the Lost Copter Alarm activated in case of communication problems between the TX and Rx.
I have set CHx_OPT=30 (in this case CH7) to sound the Lost Copter Alarm. It’s working fine when I push the radio switch.
I have also configured CH7 to F/S = +100%. It’s working OK as well, monitored via Mission Planner. The problem is when I simulate a communication failure Tx-Rx (turning off the TX). The CH7 get moved to +100% (as expected - I can see it via MP telemetry), but the alarm does NOT sound at all. Any idea what is going on and how to make it work? The FC is a Pixhawk 2.1 (PX4-v2), Firmware 3.5.0-rc6 and MP 1.3.48. Thanks!!

Hi Fabio,

Unfortunately I’m not seeing any way currently to do what you want. When you lose connection between your RC receiver and transmitter you’ll enter radio failsafe and that means we don’t read the aux switches.

An enhancement request to the issues list I guess:

The request could be reworded to request, “play Lost Copter alarm when RC failsafe triggers”.

There is a slight danger of course that the sound of the alarm could upset the IMUs. A careful physical setup of the vehicle would protect against this though (i.e. attach the buzzer furhter from the IMU).

Good advise about IMU interference! I did not realise that.
I will move the buzzer as far away as possible and add some foam pad to enhance the isolation between frame and buzzer.

What about the activation of the “alarm” trigged by radio Rx (pwm) channel?
The radio was set to activate a channel to +100% under lost communication. This is working properly. The FC receive this information from the Rx, but it does not sound the buzzer. However, under a normal communication Tx-Rx, the “switch” configured to sound the alarm woks just fine. ?!??!?!

Thanks again,

Hi Francisco, When I turn off the TX, the RX put out the PWM values programmed on the RC. And the FC see these values.
I am monitoring the RC Channels via Mission Planner! I am not understanding your commented “don’t read the aux switches”?!?!?! Sorry if I miss anything, but how the RTL command if activated? It suppose to follow the same principle, doesn’t it? Thanks in advance

It depends on the RC setup. Some systems stop outputting signals on all channels, others drop the signal on a specific channel (usually the throttle channel) and others have programmable channel position.

The FrSky have one mode that you program each channel to a specific output and can do what you want. On some Spektrum systems you can also set channels to specific positions, but is a bit more complicated than on FrSky.

What you want to achieve can be done on a FrSky. I used this method to lower a landing gear (same principle as the buzzer) on RC loss.

When I did a similar setup on Spektrum I only managed to get the first 8 channels on a specific position.

Luis, I understand your comments and agree with all of them. This is exactly what I am using, “programmable channel positions”. I have Futaba 14SG. The channels are responding accordingly when Tx is turned off and Pixhawk2.1 can see these channels changing their values. When I toggle the switch with the Tx-On, the Lost Alarm works normally, but when TX is turned Off, even the channel output goes to the programmed value, the alarm does not sound.

We generally don’t process any input from the receiver once the radio has gone into failsafe including channels used for the auxiliary switches (7 ~ 12). The reason is that the users may have setup other features such as tuning or flight modes (for example you can setup Flip on an auxiliary switch) and it could be dangerous to have these triggered.

Thanks a lot to clear this up! According to this information, I have a wrong setup on my bird!!! Hopefully I did not need it so far. My thanks, I will change my setup right away…