An entry sensor, which supports either wireless or wired operation, facilitates installation and is adjustable for gaps between a building entry barrier such as a door or window and the surrounding frame. The sensor, which may be implemented as a single piece design, includes an adjustment mechanism that enables an installer to vary the extension of the sensor to match the actual gap so that the sensor properly secures the building entry when closed. The sensor includes a detector that determines the state of a switch that is responsive to the movement of a plunger mechanism, where the state is indicative whether the building entry barrier is opened or closed, and that may determine whether the sensor is tampered with. The sensor also may facilitate battery replacement that protects the associated circuitry during the replacement.