The Radio Shack Safe House Infrared Photo relay/Alarm sounds a tone any time something blocks its invisible infrared beam. You can use the alarm across a doorway or any opening up to fifty feet wide. The invisible bean has to be setup to bounce into a reflector that can be setup up to 50 feet away. There is an alignment indicator (red light on top) that helps you install and test your alarm and a sensitivity control (dial on back) that lets you adjust the alarm for best operation.
The unit sensor and reflector have to be aligned to work. Most automatic garage door openers have a safety beam, an invisible light beam across the garage door opening that will automatically reverse the door if anything interrupts the beam while the door is going down. Garage door beams come in pairs while the alarm uses the reflector to bounce the beam back into itself.
The alarm is virtually unaffected by room lights, indirect sunlight, or other light sources, so false alarms from these sources are not likely.
There are 3 Alarm Modes that let you set the alarm so that when someone blocks the beam, the alarm can sound for 5 minutes, sound only while something blocks the beam, or trigger relay contacts - 12-Volt DC output terminals let you connect and power and external device.