You can easily make this Invisible Burglar alarm using Infrared LED and a Phototransistor. When there is no obstruction in the path of IR rays, alarm will be silent. When somebody crosses the IR beam, alarm sounds. If the IR LED and Phototransistor are enclosed in black tubes and aligned perfectly, the range of the circuit is around 1 meter. When the IR beam falls on the Phototransistor L14F1, it conducts to keep the PNP transistor BC557 out of conduction. Buzzer will be silent in this condition. When the IR beam breaks, Phototransistor switches off allowing the PNP transistor to conduct and the Buzzer sounds. Fix IR LED and Phototransistor on the opposite sides with correct alignment so as to make the buzzer silent. Adjust VR1 to set the biasing of PNP transistor. Other types of Phototransistors can also be used. But L14F1 is more sensitive .