Recently I developed a timer with the SEEED Xiao nRF52840 Sense.
Reason. It incorporates acceleration/gyro sensors, so no additional electronics are needed.  Originally, I incorporated a start button on the printed circuit-
Since he Sense version also has a tiny Bluetooth antenna, I checked the range when you contact it with an iPhone.
There is a simple Bluetooth app for  iPhone.  Lightblue (also for Android). at 21 meter distance, no problem.
Starting  by Bluetooth is allowed by the FAI, since it uses the 2,4 Ghz band.

I only use it to start, as soon as the signal is given, the connection is disabled.
 Comments welcome,

Wolfgang

Hello Christ,

the timer is programmed with Arduino. After pressing DONE on the iPhone there is a 10 seconds delay, to put the iPhone in your pocket. Then the timer starts regulating the motor, trying to keep the line pull at 3g. Only the acceleration sensors are used.
That regulating system has one drawback: Flying horizontally downwind, the motor speed would become very low, and the airspeed would be too low. To solve this, during horizontal flight the minimum pulse duration is set higher.

Please note that the only setting is the desired line pull. The propeller should be chosen, so that at maximum load the ESC is not overloaded.
Adjustment for line length and lap time is done automatically.
If you want to try the system. I can build one for you.

Best regards,

Wolfgang