The invisible theft alarm system consists of a phototransistor and an infrared LED. When the infrared beam is unobstructed, the alarm remains silent. However, if the infrared beam is interrupted, the alarm is triggered, emitting a sound. The system has an effective range of approximately 1 meter, provided that the phototransistor and infrared LED are properly enclosed in black tubes and correctly aligned.

How It Works

When the infrared rays hit the L14F1 phototransistor, it keeps the PNP transistor (BC557) in a non-conductive state, resulting in silence from the alarm. However, when the beam is interrupted, the phototransistor is deactivated, allowing the PNP transistor to conduct and activate the alarm sound.

To ensure proper functionality, the phototransistor and infrared LED must be securely positioned to avoid false alarms. Additionally, adjusting the variable resistor is critical to correctly bias the PNP transistor.

Although other phototransistors can be used as substitutes for the L14F1, this particular model is preferred due to its higher sensitivity.