A compact size wireless sensor for sensing a change of state that includes a sensor switch, a microprocessor, a wireless transmitter, a timer (e.g., a low power clock circuit), an antenna, and a coin cell battery power source. The coin cell battery, which is positioned in a stacking arrangement with the microprocessor, switch, and transmitter, allows the sensor to be of a significantly reduced size. Moreover, to provide long life despite a small battery, the microprocessor is run in a standby mode in which the microprocessor draws little power unless it actually samples the state of the sensor switch during select intervals. Various electronic components individually, or in combination, assist in the sampling (monitoring mode) in such a way as to reduce current consumption from the power source. The compact size makes the sensor ideally applicable for wireless intrusion systems embedded within hollow frames of windows and doors. Not only are such wireless sensors concealed and not readily seen by intruders, but the size allows the sensors to be installed within conventional sized window and door frames without piercing outer walls of the frames (thus avoiding nullification of window and door manufacturers' warranties).