This document describes how to build a portable anti-theft alarm using common electronic components including an op-amp, resistors, transistors, a piezo buzzer, IR photodiode, and IR LED. The alarm consists of a transmitter circuit and receiver circuit that are triggered when an object breaks the infrared beam between the IR LED and photodiode, causing the piezo buzzer to sound. Proper setup requires placing the circuits in parallel and ensuring an object interrupts the infrared beam to prevent false alarms.