1. Install each pair at the opposite positions.Then there is an infrared area between these two detector parts. When someone passes the infrared region, the detector will send alarm signal to the security alarm host 2. Install Distance (Max.) in the above chart means the maximum install distance between the two detector parts. Maximum range of 20 meters or 65 foots. 3. Great to be installed for security system in school, office building, hospital, home yard, etc. 4. Detecting range can be reduced to 30% in bad weather conditions, like rain, dust, etc. 5. English Usage Manual included in the pack. It will give you detailed instructions.With the latest in photocell technology, this photocell communication system reveals obstacles situated on the optical axis between the transmitter and receiver. These safety components ensure children, pets, and other objects stay out of harm's way. This advanced gate safety system is easily installed with a mere two wires, and maintenance is minimal. These photocells do not require any special work aside from checking them once every six months to evaluate their condition. The units are vertically wall-mounted.The photocell detector is for gate and door automation systems (type D according to EN standard 12453), consisting in a “TX” transmitter and “RX” receiver. Technical Parameters Detection Range: indoor 20m (65ft), outdoor 15m (50ft) Work voltage:12/24V Work current: 50MA Temperature: +50~ (-30) Size: 49.2 x 76 x 21.6MM Water Proof, UV proof