Patent ID: 7741838

Claim:
A rotation detecting device comprising: a rotation detector having a capability of detecting the rotational speed and including an electric power generator having a rotor and a stator; a transmitting device to wirelessly transmit a rotation signal indicative of the rotational speed detected by the rotation detector; an electric power supply circuit to utilize an electric power generated by the electric power generator to electrically power the transmitting device; a wireless switching device to selectively switch on or off supply of the electric power from the electric power supply circuit, said wireless switching device including a tuning circuit to detect a radio wave of a particular frequency and a semiconductor switching element interposed in the electric power supply circuit and capable of being selectively turned on or off in response to a detection signal of the radio wave detected by the tuning circuit, said wireless switching device in its entirety being electrically powered by an electric power of the detection signal of the radio wave detected by the tuning circuit.