Electronic Watchdog

CIRCUIT IDEAS

ELECTRONIC WATCHDOG
TAPAN KUMAR MAHARANA

I VED DWI S.C.

H

ere’s an electronic watchdog for your house that sounds to inform you that somebody is at the gate.

The circuit comprises a transmitter unit and a receiver unit, which are mounted face to face on the opposite

Fig. 4: Mounting arrangement for transmitter and receiver units

Fig. 1: 38kHz IR transmitter circuit

Fig. 3: Pin configurations of TSOP1738 and UM66

Fig. 2: Receiver circuit

pillars of the gate such that the IR beam gets interrupted when someone is standing at the gate or passing through it. The transmitter circuit (see Fig. 1) is built around timer NE555 (IC1), which is wired as an astable multivibrator producing a frequency of about 38 kHz. The infrared (IR) beam is transmitted through IR LED1. The receiver circuit is shown in Fig. 2. It comprises IR sensor TSOP1738 (IR RX1), npn transistor BC548 (T1), timer NE555 (IC2) and some resistors and capacitors. IC2 is wired as a monostable multivibrator with a time period of around 30 seconds. The melody generator section is built around melody generator IC UM66 (IC3), transistor T2 and loudspeaker LS1. Fig. 3 shows pin configurations of IR sensor TSOP1738 and melody generator IC UM66. The power supply for the Fig. 4 shows mounting arrangement for both the transmitter and receiver units on the gate pillars. To achieve a high directivity of the IR beam towards the sensor, use a reflector behind the IR LED. After both the units have been built, connect 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
NOVEMBER 2004 ELECTRONICS FOR YOU

transmitter is derived from the receiver circuit by connecting its points A and B to the respective points of the receiver circuit. The receiver is powered by regulated 6V DC. For the purpose, you can use a 6V battery. The transmitter and receiver