Electronic Watch Dog

by KNS Pvt Ltd
Save Rs. 50.00
Rs. 300.00
Rs. 250.00


The Electronic Watch Dog produces an alarm sound when a person crosses the gate. The circuit comprises a transmitter unit and a receiver unit, which are mounted face to face on opposite pillars/walls of the gate such that the IR beam gets interrupted when someone is standing at the gate or passing through it. When such a condition is detected, an alarm sound is generated for audible indication. No. of ICs used: 3


Things which you can learn:

*Application of IR transmitter and receiver.
*Application of the NE555 timer in the astable mode of operation.

Product Description

A simple circuit which acts as a Watch dog, informing the house owners about the forced or unauthorized entry.


The transmitter end of the Electronic Watch Dog is built around the NE555 timer (IC1) which is wired as an astable multi-vibrator producing a frequency of about 38 kHz. The infrared (IR) beam is transmitted through IR LED1.

The receiver end of the circuit comprises of an IR sensor TSOP1738 (IR RX1), NPN transistor BC548 (T1), NE555 timer (IC2) and some resistors and capacitors. IC2 is wired as a monostable multi-vibrator with a time period of around 30 seconds. The melody generator section is built around the melody generator IC UM66 (IC3), transistor T2 and loudspeaker LS1. The transmitter and receiver units are aligned such that the IR beam falls directly on the IR sensor. As long as IR beam falls on the sensor, its output remains low, transistor T1 does not conduct and trigger pin2 of IC2 remains high. When anyone interrupts the IR beam falling on the sensor, its output goes high to drive transistor T1 into conduction and pin 2 of IC2 goes low momentarily. As a result, IC2 gets triggered and its pin3 goes high to supply 3.3V to melody generator IC3 at its pin 2, which produces a sweet melody through the speaker fitted inside the house. Output pin 3 of IC2 remains high for around 30 seconds. To achieve a high directionality of the IR beam towards the sensor, use a reflector behind the IR LED.

After both the units have been built, connect the 6V power supply to the receiver circuit. You should hear a continuous melody from the speaker. Now connect 6V power to the transmitter also and orient IR LED1 towards IR receiver. The melody should stop after about 30 seconds.Now the transmitter and the receiver units are ready for use. When somebody enters through the door, the IR beam is interrupted and the alarm sounds for 30 seconds. The alarm keeps sounding as long as one stands between the transmitter and receiver units. Using preset VR1, you can set the volume of the loudspeaker.


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