This is the light fence schematic diagram uses LDR as the light sensing. The fundamental issue with the greater part of standard light sensors is that they require exact arrangement of light bar to quiet the circuit throughout standby mode. The circuit portrayed here is sensitive to the point that it will catch a moving individual at a separation of not many metres in sunshine or under electric lighting without unwieldy arrangement of light pillar. It requires essentially no set up, and may be basically put inside the viewable pathway of just about any light source incorporating encompassing sunlight or fluorescent electric light. The beep produced from the circuit will be noisy enough to locate the passage of an individual in the room or the secured region being protected.

The circuit utilizes a voltage comparator and a monostable clock to give the cautioning caution on distinguishing a moving individual. Ic uA741 (IC1) is utilized as a voltage comparator with two potential dividers in its upsetting and noninverting inputs. Resistors R1 and R2 give half-supply voltage of 4.5 volts to its transforming include (pin 2). LDR1 and preset VR1 structure an alternate potential divider to furnish a variable voltage include to the non-upsetting enter (stick 3).

In the event that Vr1 is fittingly balanced for the obliged light level, the yield of Ic1 will be high, which drives pnp transistor T1 out of conduction. This is because of the high potential at the base of T1. The emitter voltage of T1 will be high in this condition, which hinders Ic2 from wavering and Led1 from lighting. Ic2 is wired as a monostable clock. R6 and C2 give a preset time delay.

As an individual crosses the secured territory, his shadow will be sensed by LDR1 because of progress in the light power level and the voltage at the non-modifying include of IC1 will drop immediately. The yield of IC1 abruptly comes to be low, permitting T1 to direct. This triggers the monostable (IC2) and the caution sounds.

Build the circuit on a normal PCB and house in a plastic case. Keep LDR1 inside a dark tube to build its affectability. Alter preset VR1 until LED1 turns off at the specific light level. Keep LDR1 confronting the door of the room or the range to be ensured. Affectability of the circuit hinges on upon the correct change of VR1. Assuming that VR1 is effectively balanced, the circuit can locate a moving individual from a separation of around the range of three metres.

Good luck on your light fence project

The transistor act like as a switch, when the “switch” in on condition then the relay will be activated.

The potensiometer adjust the trigger ‘on’ level. The diode in the circuit diagram shows to be 1N914. This is ok if you have a light-duty relay, also the 1N914 is a signal diode so actually does not qualify. Use a 1N4001 (or better) instead. A couple of substitutes for the 2N2222 transistor are: NTE123A, ECG123A, PN100, etc.

As light falls on the surface of the photoresistor, the?photoresistor changes it’s resistance. The more the light, the less the resistance of the photoresistor, the less the resistance, the less the voltage drop across it. The less the light, the more the resistance and thus the more the voltage drop across it.

As the voltage drop increases, so does the VB of the 2N2222 transistor and therefore the ICE increases accordingly, until the time that the current is enough to actuate the relay.

The amount of light needed to actuate the relay can be changed by changing the 100K potentiometer. Basically, any change to the potentiometer will have an effect to the voltage drop of the photoresistor, as they are both members of the voltage divider described above.

The 1N4001 diode is used to eliminate any back voltage when the relay is disarmed. It is very important to have this diode because without it, the transistor may be damaged.

The similar circuit with some improvements can be found at pcbheaven, go to this page.

The circuit is based on the fundamentals of optics and digital electronics. The object whose colour is required to be detected should be placed in front of the system. The light rays reflected from the object will fall on the three convex lenses which are fixed in front of the three LDRs. The convex lenses are used to converge light rays. This helps to increase the sensitivity of LDRs. Blue, green and red glass plates (filters) are fixed in front of LDR1, LDR2 and LDR3 respectively. When reflected light rays from the object fall on the gadget, the coloured filter glass plates determine which of the LDRs would get triggered.

The circuit makes use of only “AND” gates and “NOT” gates. When a primary coloured light ray falls on the system, the glass plate corresponding to that primary colour will allow that specific light to pass through. But the other two glass plates will not allow any light to pass through. Thus only one LDR will get triggered and the gate output corresponding to that LDR will become logic 1 to indicate which colour it is. Similarly, when a secondary coloured light ray falls on the system, the two primary glass plates corres- ponding to the mixed colour will allow that light to pass through while the remaining one will not allow any light ray to pass through it. As a result two of the LDRs get triggered and the gate output corresponding to these will become logic 1 and indicate which colour it is.

When all the LDRs get triggered or remain untriggered, you will observe white and black light indications respectively. Following points may be carefully noted :

1. Potmeters VR1, VR2 and VR3 may be used to adjust the sensitivity of the LDRs.
2. Common ends of the LDRs should be connected to positive supply.
3. Use good quality light filters.

The LDR is mounted in a tube, behind a lens, and aimed at the object. The coloured glass filter should be fixed in front of the LDR as shown in the figure. Make three of that kind and fix them in a suitable case. Adjustments are critical and the gadget performance would depend upon its proper fabrication and use of correct filters as well as light conditions