A wireless sensor device including a sensor configured to sense a target object and provide a sensor signal of varying levels indicative of condition of the target object, a signal processing circuit configured to amplify the sensor signal and provide an amplified electric analog signal, and a detection circuit configured to receive the amplified analog signal and provide a detection output when the electric analog signal goes beyond a predetermined detection threshold. A radio transmitter transmits a radio detection signal in response to the detection output. A power supply is configured to provide an electric power to the signal processing circuit and the radio transmitter and includes a power generating element which converts an external energy into the electric power to be accumulated in the power supply. A controller activates the radio transmitter only in response to the detection output, permitting the radio transmitter to generate the radio detection signal.