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Midnight Security Light Circuit: A 1:00 AM Intrusion Defense
Description
Most burglaries tend to occur during late-night hours when individuals progress through the second stage of sleep, termed ‘paradoxical’ sleep. Here is an energy-efficient circuit designed to deter thieves by illuminating potential entry points, such as the kitchen or backyard, around 1:00 am. It automatically resets in the morning. The circuit is entirely automated and utilizes a CMOS integrated circuit CD 4060 to achieve the desired delay. A light-dependent resistor LDR1 controls the reset pin 12 of IC1 for its automatic operation. During daylight, the low resistance of LDR1 maintains pin 12 of IC1 ‘high,’ preventing oscillation.
Following sunset, the elevated resistance of LDR1 causes pin 12 of IC1 to become ‘low,’ initiating oscillation. This oscillation is indicated by the flashing of LED2 connected to pin 7 of IC1. The values of the oscillator components – resistors R1 and R2, and capacitor C4 – are chosen so that output pin 3 of IC1 goes ‘high’ after seven hours, approximately 1 am. This high output drives triac 1 (BT136) through D5 and R3. Bulb L1, connected between the phase line and M2 terminal of triac 1, turns on when the gate of triac 1 receives the trigger voltage from pin 3 of IC1. It remains ‘on’ until pin 12 of IC1 becomes high again in the morning. Capacitors C1 and C3 function as power reserves, ensuring IC1 continues to oscillate even during brief power interruptions. Capacitor C2 maintains trigger pin 12 of IC1 ‘high’ during daylight, mitigating the effects of minor variations in light intensity on the circuit’s performance.
A preset, designated P1, allows for adjusting the sensitivity of LDR1. Power to the circuit is derived from a step-down transformer T1 (230V AC primary to 0-9V, 300mA secondary), rectified by a full-wave rectifier comprising diodes D1 through D4, and filtered by capacitor C1. Assembly of the circuit should take place on a general-purpose printed circuit board with ample spacing between the components. The exposed leads of the components should be protected by sleeving. A switch S1 enables manual operation of the lamp. The unit should be enclosed in a plastic casing and installed in a location that provides adequate daylight.
Circuit diagram:
Parts:
- P1 = 100K
- R1 = 120K
- R2 = 1M
- R3 = 100R
- R4 = 100R
- C1 = 1000uF-25V
- C2 = 100uF-25v
- C3 = 100nF-63V
- C4 = 1uF-25V
- D1 = 1N4001
- D2 = 1N4001
- D3 = 1N4001
- D4 = 1N4001
- D5 = Red LED
- D6 = Green LED
- IC = CD4060
- TR = BT136
- T1 = 9v 300mA Transformer
- L1 = 230V-60W Bulb
- SW = On/Off Switch
Caution:
Because the circuit utilizes 230V AC, numerous points are at alternating current mains voltage. This could potentially cause a lethal shock if adequate precautions are not taken. Therefore, if you lack considerable expertise in working with high-voltage circuits, do not attempt to construct this circuit. We assume no liability for any resulting loss or damage.