Doorbell with Security Feature:
This doorbell security system is designed to function as follows: when someone presses your doorbell switch at night, not only does the bell ring, but the connected bulb also illuminates. To turn off the bulb, simply press the reset pushbutton switch included in the circuit. Position the bulb in proximity to the doorbell switch, allowing you to identify the person pressing the doorbell before opening the door. This feature enables you to exercise caution when dealing with unfamiliar individuals. During daylight hours, the bulb remains unlit, with only the doorbell chime activated.
Upon closing the doorbell switch S1, the doorbell rings, and simultaneously, the transformer X receives an AC power supply. This transformer provides a 6V-0V-6V/500mA step-down voltage. The output from the transformer undergoes rectification through diodes D1 and D2. Subsequently, the rectified output is smoothed by a 1000µF, 25V capacitor labeled as C1, and then directed to the collector of transistor BC547 (T1) via a 2.2k resistor designated as R1. An integral part of this circuit is a light-dependent resistor (LDR) connected to the base of the transistor.
In daylight conditions, the LDR exhibits extremely low resistance due to the continuous exposure to light. Consequently, when the doorbell switch is pressed, the transistor enters a conducting state, causing its collector to be grounded. As a result, the subsequent portion of the circuit remains dormant, allowing only the doorbell to ring.
The subsequent section, which comprises IC 7408 (IC1) and IC 7473 (IC2), receives an independent power supply voltage of 5V from the regulator IC 7805 (IC3), as illustrated in the diagram.