Electronic watchdog is used for your house that sounds to inform you that somebody is at gate or passing through it. The circuit comprises of a transmitter unit and receiver units, which are mounted face to face on the opposite pillars of the gate or door such that the IR beam transmitted through IR LED1 of the transmitter, get interrupted when someone is standing at the gate or passing through it. The transmitter circuit is built around timer, which is wired as an astable multivibrator producing a frequency of about 38 KHz. The infrared (IR) beam is transmitted through IR LED.  The receiver circuit comprises IR sensor (IR RX1), NPN transistor, timer and some resistors & capacitors. Timer is wired as a mono stable multi vibrator with a time period of around 30 seconds. The melody generator section is built around melody generator, transistor and loudspeaker. The transmitter and receiver units are aligned such that the IR beam falls directly on the IR sensor. As long as the IR beam falls on the sensor, its output remains low, transistor does not conduct trigger pin of IC remains high. When anyone interrupts IR the beam falling on the sensor, its output goes high to transistor into conduction and trigger pin of IC goes low momentarily. As a result, IC gets triggered its  (output) goes high to supply 3.3V to melody generator  at pin2, which produces a sweet melody through the speaker fitted inside the house. Output pin of 555 IC remains high for around 30 seconds. To achieve high directivity of the IR beam towards the sensor, use a reflector behind IR LED. When somebody enters through the gate or door the IR beam is interrupted and the alarm sounds for 30 seconds. The alarm keeps sounding as long as the one stands between transmitter and receiver units. This circuit can also be used as a doorbell or burglar alarm.

Major blocks present in this system:

1. 1. Timer
   2. IR sensor
   3. Buzzer
   4. Regulated power supply

          BLOCK DIAGRAM: video: