Electronic watchdog is used for your home that sounds to inform you that someone is at the door or pass through it. The circuit comprises a transmitter unit and receiver units which are mounted face-to-face on the opposing pillars of the door or door such that the IR beam transmitted through the IR LED1 of the transmitter is interrupted when someone is standing in the Door or passing through. Circuit Operation
The transmitter circuit shown in FIG. 1 is constructed around timer NE555 (u1), which is wired as an unstable multiple vibrator producing a frequency of about 38KHz. The infrared (IR) beam is transmitted through the IR LED1.