Together with increasing popularity of STM32F4 flight controllers, telemetry became hard topic again. Why? Most popular telemetry protocols, SmartPort and FrSky telemetry , requires inverted signal. Zero becomes one, one becomes zero.
In case on STM32F3 that was not a big problem. Those CPUs have built in inverters. STM32F1 and STM32F4 does not. So, if flight controller designer did not put external inverters on UART ports, FrSky telemetry, SmartPort and even S.Bus would not work.
Luckily, simple inverter for FrSky telemetry and S.Bus can be build using only few electronic parts:
- Small logic level N-channel MOSFET transistor. One of the best choices is 2N7000
- 10kOhm resistor
- few cables
This device is so simple, that it can be built without PCB. Cables and resistor can be attached to the transistor, and everything can be put inside heat-shrink tube.