Hall sensor for motor RPM

To know the RPM of the motor is a good thing. As the throttle control is done by a Arduino, I can use the RPM value to prevent the motor from overheating and adapt the throttle to different propeller configurations by software.
I made some research how to install a RPM sensor in a model motor. There are basically three posibilities:

• getting a signal right from the 3 wires of the synchronous motor --> that needs some soldering and signal processing
• putting a optical sensor for the propeller RPM --> possible, but too much components for electronics
• Hall effect sensor with magnets glue to the motor --> needs a least 2 additional mangets

I than realized that the motor has yet a lot of magnets inside. There are 40 of them ... so why do not use the Hall efect sensor with them?
This magnets are glued to the inner side of the moving motor housing. I have to get as close as I can to get a good signal. I tried some configurations with a standard Hall effect sensor and it worked.

The Arduino software checks now every 200ms the counts of the magnets and calculates the RPM.
Nice and simple. The value can be used to adapt the ESC signal to control motor speed.
The photo shows the position of the sensor, glued to the motor mounting. You can also see the motor mounting rod, made of carbon fibre. There are some wires left for 2 additional temperature sensors: one for the motor and one for the ESC.