You can easily monitor a door or window with the Napco Gemini Wireless Recessed Door/Window Sensor designed for use with a Napco GEM-series wireless system. When your door or window is opened the transmitter will send a signal to your GEM system to trigger an alarm. It s that simple. When battery voltage drops below normal the transmitter will send a low-battery report to your receiver. You can replace the battery with a Duracell DL123A or Varta/Power-One CR123A). Each transmitter has a unique factory-programmed code (printed on the board and also on the transmitter tube) that distinguishes itself to the receiver. FEATURES: For recessed use in both windows and doors. Powerful wireless transmitter. White housing magnet and hole cover. Long-life 3V type DL123A lithium battery supplied.