Install each pair at the opposite positions.Then there is a infrared area between these two detector parts. When someone passes the infrared region, the detector will send alarm signal to the security alarm host (which not included here).

Install Distance(Max.)in the above chart means the maximum install distance between the two detector parts. When interrupt the infrared, the relay will output switch signal, it can work for parking barrier, walking gate, vehicle gate as body induction. It also can work as window, door and passageway anti-burglar detection. Great to be installed for security system in school, office building, hospital, home yard, etc. Detecting range can be reduced to 30% in bad weather conditions, like rain, dust, etc. English Usage Manual included in the pack. It will give you detailed instructions.