Since we love model airplanes too we made this a side project, This is an opto isolated tach interface for DLE engines to directly connect to a telemetry input on a Spektrum telemetry module. It is fully isolated so there is no risk of any kind of voltage getting back to your radio.
Tech Note: The Spektrum AR10100T telemetry module will require the J4 connector pins to be soldered together on the tach PCB. Best way to do this is to cut the heat shrink area out with an Xacto knife and use a small piece of solid wire. Other models may require this modification if the blue LED on the board is flashing but no RPM data is received.