The Reporter Wireless Driveway Alert System comes with one sensor and one receiver. Now you can extend your system by adding up to three of these Extra PIR Transmitters to each receiver, allowing you to cover four different locations simultaneously, each with four distinct beeps. Never miss an important visitor or package delivery again with the Reporter Wireless Driveway Alert System that detects and reports people or vehicles and reports the results to a receiver unit inside the house. Just place the sensors near any location needing notification or protection — including driveways, business entrances, garages, and barns — and when it detects activity, the receiver unit inside the house will sound a tone. It's ideal for monitoring boats, RVs, trucks, mailboxes or anything else you can think of. Other features of the Reporter Wireless Driveway Alert System include: Amazing range; the sensor has a 30' range and can transmit a signal to the receiver up to 1,200' away Detects humans and vehicles, but ignores animals Waterproof sensor assembly Weather shield included to protect sensor from the elements Receiver features sound control and has an output to trigger external alarms or bells Microprocessor code-learning technology eliminates false alarms — receiver responds only to transmitters assigned to it One sensor can communicate with multiple receivers in larger installations.