06-04-2012, 11:56 AM
MIDNIGHT security light
MidnightSecuritylightbyShinigami.pdf (Size: 216.8 KB / Downloads: 46)
Most thefts happen after
midnight hours when people
enter the second phase
of sleep called ‘paradoxical’ sleep.
Here is an energy-saving circuit that
causes the thieves to abort the theft
attempt by lighting up the possible
sites of intrusion (such as kitchen or
backyard of your house) at around
It automatically resets in the
morning.
The circuit is fully automatic and
uses a CMOS IC CD 4060 to get the
desired time delay. Light-dependent
resistor LDR1 controls reset pin 12 of
IC1 for its automatic action.
During day time, the low resistance
of LDR1 makes pin 12 of IC1
‘high,’ so it doesn’t oscillate. After
sunset, the high resistance of LDR1
makes pin 12 of IC1 ‘low’ and it starts
oscillating, which is indicated by the
flashing of LED2 connected to pin 7
of IC1. The values of oscillator components
(resistors R1 and R2 and capacitor
C4) are chosen such that output
pin 3 of IC1 goes ‘high’ after seven
hours, i.e., around 1 am. This high
output drives triac 1 (BT136) through
LED1 and R3.
Bulb L1 connected between the
phase line and M2 terminal of triac 1
turns on when the gate of triac 1 gets
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 act as power
reserves, so IC1 keeps oscillating even
if there is power interruption for a few
seconds. Capacitor C2 keeps trigger
pin 12 of IC1 high during day time,
so slight changes in light intensity
don’t affect the circuit. Using preset
VR1 you can adjust the sensitivity of
LDR1.