Little Door Guard

If some intruder tries to open the door of your personal home, this circuit sounds an alarm to warn you against the tried intrusion. The circuit (Fig. 1) uses with no trouble on hand, reasonably feed components. For compactness, an alkaline 12V battery is used for powering the unit. Input DC provide is additional regulated to a steady DC voltage of 5V with the support of three-pin regulator IC 7805 (IC2).

Fig. 1: Circuit of the door guard

Assemble the unit on a general-purpose PCB as shown in Fig. four and mount the identical on the door as proven in Fig. 3. Now mount a section of replicate on the door body such that it is exactly aligned with the unit. Pin configurations of IC UM3561 and transistors 2N5777 and BC547 are proven in Fig. 2. 

Fig. 2: Pin configurations of UM3561 and transistors 2N5777 and BC547

Initially, when the door is closed, the infrared (IR) beam transmitted by means of IR LED1 is mirrored (by the replicate) back to phototransistor 2N5777 (T1). The IR beam falling on phototransistor T1 reverse biases npn transistor T2 and IC1 does now not get positive provide at its pin 5. As a consequence, no tone is produced at its output pin 3 and the loudspeaker remains silent. Resistor R1 limits the operating current for the IR LED.

When the door isopened, the absence of IR rays at phototransistor T1 ahead biases npn transistor T2, which gives supply to  sureIC1. Now 3-sirensound generator IC UM3561 (IC1) gets power via resistor R5. The output of IC1 at pin three is amplified by means of Darlington-pair transistors T3 and T4 to supply the alert tone via the loudspeaker. 

Fig. three: Back view of the door assembly

Rotary switch S2 is used to choose the three preprogrammed tones of IC1. IC1 produces hearth engine, police and ambulance siren sounds when its pin 6 is attached to point F, P or A, respectively.

Fig. four: Suggested enclosure with major elements layout

Author : T.K. Hareendran