DX18

There is an issue with the DX18 radio, It has been burning out the Nano33BLE when attached via PPM. The voltage levels can be much higher than 3.3v

The current recommended method to attach a Headtracker is by adding a zener diode and resistor to the PPM output pin

To attach the DX18, you will need a 3.3v zener diode and a 100ohm resistor.