CIRCUIT

IDEAS

8 4 • J U N E 2 0 0 7 • E L E C T R O N I C S F O R Y O U W W W . E F Y M A G . C O M

� D. MOHAN KUMAR

LIGHT FENCE S.C. DWIVE

DI

T he basic problem with most ofstandard light sensors is thatthey require precise alignment

of light beam to mute the circuit dur-

ing standby mode. The circuit de-scribed here is so sensitive that it willdetect a moving person at a distanceof few metres in daylight or underelectric lighting without cumbersomealignment of light beam. It requires vir-tually no set up, and may be simplyplaced within the line-of-sight of al-most any light source including ambi-

ent day light or fluorescent electriclight. The beep generated from the cir-cuit will be loud enough to detect theentry of a person in the room or theprotected area being guarded.

The circuit uses a voltage compara-tor and a monostable timer to give thewarning alarm on detecting a movingperson. IC µA741 (IC1) is used as avoltage comparator with two poten-tial dividers in its inverting and non-inverting inputs. Resistors R1 and R2provide half-supply voltage of 4.5volts to its inverting input (pin 2).

LDR1 and preset VR1 form anotherpotential divider to provide a variablevoltage input to the non-inverting in-put (pin 3).

If VR1 is properly adjusted for therequired light level, the output of IC1will be high, which drives pnp tran-sistor T1 out of conduction. This is dueto the high potential at the base of T1.The emitter voltage of T1 will be high

in this condition, which inhibitsIC2 from oscillation and LED1from lighting. IC2 is wired as amonostable timer. R6 and C2 pro-vide a preset time delay.

As a person crosses the pro-tected area, his shadow will besensed by LDR1 due to changein the light intensity leveland the voltage at the non-invert-ing input of IC1 will drop mo-mentarily. The output of IC1 sud-denly becomes low, allowing T1to conduct. This triggers themonostable (IC2) and the alarmsounds.

Assemble the circuit on acommon PCB and house in aplastic case. Keep LDR1 inside ablack tube to increase its sensi-tivity. Adjust preset VR1

until LED1 turns off at the particularlight level. Keep LDR1 facingthe entrance of the room or the areato be protected. Sensitivity of the cir-cuit depends on the proper adjust-ment of VR1. If VR1 is correctly ad-justed, the circuit can detect a movingperson from a distance of about threemetres. �