An sensor unit has an analog Hall-effect sensor coupled to a microprocessor containing an analog to digital converter. It is mounted to the frame of a cell door. A magnet is mounted to the cell door so that when the door is closed, they are in close-proximity. A calibration initiates the storage of digital voltage signal presented by the Hall-effect sensor, via an analog to digital converter, when the cell door is closed. This door-closed value is constantly compared to the current value based on the position of the cell door. If this value deviates within preset limitations of the stored value, the sensor unit signals a fault condition to a connected central control panel which sends a “door ajar” notification to a designated user's cell phone as well as activating a local alarm.