We have two sensors, the RPM Sensor and the Spektrum RPM Sensor Adaptor.

The RPM Sensor is for every radio system except Spektrum - do not purchase the RPM Sensor if you have a Spektrum radio. The sensor has opto-isolation and ESD protection. The sensor performs the RPM measurement and interfaces to the telemetry bus of your radio.

The Spektrum RPM Sensor Adaptor is for Spektrum radios. You need an RX with an RPM or a combined RPM/Temp port. The adaptor provides opto-isolation and ESD protection for the RX making it safe to connect the ignition, via the adaptor, to the RX. The RX itself performs the RPM measure from the converted signal.