This circuit may be used to auto- matically switch on a light at the enterance gate to a premises, at night, by sensing the presence of a person. In addition, it sounds a musical bell to signify the presence of the person. The lamp is switched on only for a short interval to save electricity. This circuit has two stages: a transmitting unit and a sensing unit. The transmitting unit consists of two infrared LEDs while the sensing unit consists of an IR sensor and its associated circuitry. The IR LEDs emit a beam of infrared light when switch S1 is put on. This infrared beam falls on the IR sensor D3. As a result transistor T1 gets forward biased while transistor T2 is cut-off. When any person tries to enter the gate, the IR beam falling on the IR sensor is momentarily interrupted. As a result NE555, configured as monostable flip-flop, gets a trigger pulse at its trigger input pin 2. Its output goes high for a predetermined time period. The period can be adjusted by varying the value of resistor R5 and / or capacitor C1 as T = 1.1 x R5 x C1 sec. Output pulse from IC1 forward biases transistors T4 and T5. As a result the musical bell is switched on, but the bulb is switched on only at night as explained in the succeeding paragraph. The musical bell is built around IC2 (UM66). The output from IC2 is amplified by transistors T6 and T7 to drive an 8-ohm, 500mW loudspeaker. An LDR based circuit is used to switch on the bulb at night only. The bulb is switched on when relay RL1 is de-energised. During day time the LDR offers a very low resistance, and thus transistor T3 is forward biased to ‘on’ state and the relay RL1 is energised. In energised state of relay RL1, the main supply circuit to the bulb is incomplete. During night time the ‘dark’ resistance of LDR is very high and the relay is de-energised. As a result mains supply is connected to triac BT136 via relay contacts. When the IR beam is interrupted at night the output from IC NE555 forward biases transistors T4 and T5 as mentioned earlier. While conduction of transistor T5 sounds the musical bell, the conduction of transistor T4 causes firing of the triac. When the triac fires the mains supply to the bulb gets completed via the N/C contacts of relay RL1, and the bulb lights up. One can substitute any type of melody generator for IC UM66. The sensitivity of LDR circuit can be adjusted by varying potentiometer VR1.
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