Petrol engine RPM sensor.

Some glider planes have a motor to get airborne and to return if no lift can be found. Motor running time is recorded for flight evaluation and billing. The flight computer has a digital input for RPM pulses.(12V or 5V). The interface produces these pulses from a pick-up near the spark plug line. A 555 timer is triggered by an antenna wire near the spark plug line. It produces 6 μs wide 5V (12V) pulses that go into the flight computer.