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. Technical Parameters Detector Area: indoor 20m. Outdoor 10m work voltage:12V,24V DC ( can NOT working with AC power, the user manual is wrong ) Work current : 50MA temperature : +70~ (-20) Size : 76 x 49.2 x 21.6MM Water Proof, UV proof Wide range of uses: Garage doors, Electronic gates, Barrier gates, sliding doors, automatic doors, rolling doors, shutters Electrical appliances/Home automation i.e. lights, Alarm system, access control system Terms Of Sale: Please read and understand before bidding or buying. Return of Faulty & Non Faulty Goods All products are individually inspected before being dispatched, to ensure items are sent out in perfect condition. Item must be returned within 15 days of After receiving Item must be returned unmarked and in the Original Packaging Shipping: Please ensure your delivery address is correct. Items will be shipped within 48 hours after payment is received / cleared (except weekends and holidays). Contact Us If you have any questions please ask me. I will reply within 24 hours. In the rare event that you have a problem with your item we strongly recommend contacting us before leaving negative or neutral feedback. We will be happy to help with your problem.