A light fence circuit diagram with LDR

This is a schematic diagram of a grating using LDR as a light sensor. The basic problem with most standard light sensors is that they require precise placement of the light strip to keep the circuitry quiet throughout standby mode. The circuit depicted here is sensitive enough to capture moving individuals in sunlight or electric light at distances of just a few meters, without the need for bulky lamppost arrangements. It requires essentially no setup and can be placed essentially within the viewing path of almost any light source that includes sunlight or fluorescent lighting. The circuit produces a beep sound sufficient to determine the passage of persons within a room or a protected safe area.

This circuit uses a voltage comparator and a monostable clock to give a warning when distinguishing moving individuals. IcuA741 (IC1) is used as a voltage comparator with two voltage dividers on its inverting and non-inverting inputs. Resistors R1 and R2 provide half the supply voltage of 4.5 volts to its switching component (pin 2). LDR1 and preset VR1 form a backup voltage divider to provide a variable voltage included to the non-bumping input (stick 3).

If Vr1 is properly balanced with the forced light level, the yield of Ic1 will be high, which drives pnp transistor T1 non-conducting. This is because of the high potential at the bottom of T1. In this case, the emitter voltage of T1 will be very high, which will prevent Ic2 from swinging and Led1 from emitting light. IC2 is wired as a monostable clock. R6 and C2 give the preset time delay.

When a person crosses the safe area, LDR1 will sense his shadow and the voltage at the non-modified containment of IC1 will immediately drop due to the advancement of optical power level. IC1's output suddenly goes low, allowing T1 to take command. This triggers the monostable (IC2) and sounds a warning sound.

Build the circuit on a regular PCB and place it in a plastic enclosure. Place the LDR1 inside a dark tube to enhance its impact. Change preset VR1 until LED1 turns off at a specific brightness level. Keep LDR1 facing the door of the room or the range to be secured. The effect of the circuit depends on the correct change of VR1. Assuming VR1 is effectively balanced, the circuit can locate moving individuals within a distance of approximately three meters.