A battery-operated sensor device has a sensor circuit with a dormant mode and an active mode. A self-powered wake up circuit uses power harvesting for generating a wake up signal for switching the sensor circuit from the dormant mode to the active mode. In this way, the dormant mode may not have any live circuits draining power. The invention is of interest for any low power sensor where it is desired not to have to tamper with the sensor for prolonged periods (i.e. to change the battery). This is for example of interest for implantable sensor devices.
