8/2/2019 Midnight Security Light Feb 091

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circuitideas

e l ec t ron i c s fo r you Feb rua ry 200 9 95w w w . e F y m a g . c o m

Most thefts happen aftermidnight hours when peo-ple enter the second phase

of sleep called paradoxical sleep.Here is an energy-saving circuit thatcauses the thieves to abort the theftattempt by lighting up the possiblesites of intrusion (such as kitchen orbackyard of your house) at around1:00 am. It automatically resets in themorning.

The circuit is fully automatic anduses a CMOS IC CD 4060 to get thedesired time delay. Light-dependentresistor LDR1 controls reset pin 12 ofIC1 for its automatic action.

During day time, the low resist-ance of LDR1 makes pin 12 of IC1high, so it doesnt oscillate. After

sunset, the high resistance of LDR1makes pin 12 of IC1 low and it startsoscillating, which is indicated by the

ashing of LED2 connected to pin 7of IC1. The values of oscillator compo-nents (resistors R1 and R2 and capaci-tor C4) are chosen such that outputpin 3 of IC1 goes high after sevenhours, i.e., around 1 am. This highoutput drives triac 1 (BT136) throughLED1 and R3.

Bulb L1 connected between thephase line and M2 terminal of triac 1turns on when the gate of triac 1 getsthe trigger voltage from pin 3 of IC1.It remains on until pin 12 of IC1 be-comes high again in the morning.

Capacitors C1 and C3 act as powerreserves, so IC1 keeps oscillating evenif there is power interruption for a fewseconds. Capacitor C2 keeps trigger

pin 12 of IC1 high during day time,so slight changes in light intensitydont affect the circuit. Using presetVR1 you can adjust the sensitivity ofLDR1.

Power supply to the circuit is de-rived from a step-down transformerX1 (230V AC primary to 0-9V, 300mAsecondary), recti ed by a full-waverecti er comprising diodes D1 throughD4 and ltered by capacitor C1.

Assemble the circuit on a general-purpose PCB with adequate spacingbetween the components. Sleeve theexposed leads of the components.Using switch S1 you can turn on thelamp manually. Enclose the unit in aplastic case and mount at a location

that allows adequatedaylight.

Caution. Since thecircuit uses 230V AC,many of its points are

at AC mains voltage. Itcould give you lethalshock if you are notcareful. So if you dontknow much about work-ing with line voltages,do not attempt to con-struct this circuit. EFYwill not be responsiblefor any kind of resultingloss or damage.

D. MOHAN KUMAR

MIDNIGHT secURITy lIGHT s .c . d w i v

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