physics investigatory project- LDR. IATION ‘N LDR CONCLUSION Based on the obsewation above, it eam be cometuded that the LDR based. INTRODUCTION • A light dependant resistor(LDR) or a photoresistor or photocell . Physics Investigatory Project on LDR as Variable Resistor. Project Report Submitted On Light Dependence Resistance  Submitted By: A. Varun Class â XII Kendriya Vidyalaya no. 2 Pondicherry.

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Light dependent resistor(LDR) – physics investigatory project

I came to know about many new things. I would like to thank my parents who helped me in gathering diverse information, collecting data and guiding me from time to time in making this project, despite their busy schedule.

They gave me different ideas in making this project unique. This project could have been done using a photo-voltaic cell in place of LDR but the circuit would have become complicated in the case of photocell. Introduction A photoresistor or light-dependent resistor LDR projfct a light- controlled variable resistor.

The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits Photoconductivity. A photoresistor can be applied in light-sensitive detector circuits, and light and dark activated switching circuits.

Physics Investigatory Project On LDR | Richard Parker –

These resistors use pure semiconductors like silicon or germanium. In other words, the conductivity goes up. Distinction needs to be made here between photocells and LDRs. In a photocell, when it is excited by light photonselectricity is generated. Unlike photocells, LDRs, do not generate electricity but only change their conductivity. Theory Alight dependent resistor works on the principle of photo conductivity.

Modern light dependent resistors are made of materials such as lead sulphide, lead selenide, indium antimonide and most commonly cadmium sulphide CdS and cadmium selenide. These photons in the incident light should have energy greater than the band gap of the semiconductor material to make the electrons jump from the valence band to the conduction band.

Hence when light having enough energy strikes on the device, more and more electrons are excited to the conduction band which results in large number of charge carriers.

The result of this process is more and more current starts flowing through the device when the circuit is closed and hence it is said that the resistance of the device has been decreased. This is the most common working principle of LDR. The resistance of the LDR falls rapidly with the increasing intensity of the incident light. The converse is also true when light intensity is reduced or cut off. Light source white, ultra-violet and laser 2. An opening was cut on the box to present a door.


Behind the door, a light source is placed and in the line of vision of the light source, an LDR is also placed. Light BUZZER As seen in the above diagram, a light source is positioned behind the door on the right hand side and an LDR is placed in the same line of vision at the opposite end of the door.

The transistor circuit described later is connected to a relay which is in turn connected to an external buzzer. The relay is a special type of switch which is driven by its magnetic coil. The relay has two positions. In de- energized condition, position-1 will be active and in energized condition, position-2 will be active. When the circuit is powered, the relay goes to position-1 to which the buzzer is connected.

Thus, the buzzer will start ringing. But when the light source is switched on, the resistance of LDR falls and this drives the transistor switching circuit and the relay gets energized.

This puts the relay switch in position-2, which cuts off the buzzer. In this condition, whenever the light beam is intercepted by an opaque object, the LDR stops receiving the incident light and its resistance becomes high.

This de-energizes the relay and puts it to position-1 and the buzzer starts ringing. Thus, to summarize, we can say projwct the as long as the light source is on and the LDR is illuminated, the buzzer will not ring. But the moment, the light source is interrupted, investigatiry buzzer will start ringing. We can Imagine the interruption of the light beam to be caused by an intruder and hence this arrangement can automatically detect any intruder by turning on the buzzer.

As such we can connect many devices to the relay such as sirens, investigatorry lights or even an automatic dialer to the nearest police station.

This intruder alarm can be easily set up in houses, banks, schools etc.

Light dependent resistor(LDR) – physics investigatory project

The same circuit can be rewired to reverse the effect of light. Circuit Diagram Now, let us understand how the switching takes place when the LDR is illuminated by light.

For the automatic switching, we have used the following circuit consisting of Transistors.

One of the most common uses for transistors in an electronic circuit is as simple switches. In short, a transistor conducts current across the collector- emitter path only when sufficient voltage is applied to its base. When no sufficient base voltage is present, the switch is off. When sufficient base voltage is present, the switch is on. The circuit uses two transistors, connected in series.

Thus, when there is no light on the LDR, the transistor remains un-fired. But when light falls on the LDR, the LDR starts conducting and the net base voltage of transistor T1 crosses the threshold voltage, causing T1 to fire.


As a consequence, the collector current of T1 drives transistor T2 and T2 begins to fire. The collector current of T2 is now large enough to energize the relay.

The relay gets energized and changes the contact position from 1 to 2. When light is cut off, the base voltage of T1 falls back to less than the threshold voltage of its base —emitter junction and therefore, the relay gets de-energized. As stated earlier, the relay is connected to a buzzer through a battery.

So, whenever, light is interrupted, the relay goes to position-1 where, the buzzer is connected and the buzzer starts ringing. The ringing buzzer wards off the intruder and also alerts the neighbors about the intrusion. In addition to the buzzer, one can also connect a high intensity flash light to warn the people in the neighborhood about the intrusion. Needless to say, one has also to install a stealth switch that is known only to the owner so that the owner can disable it for his own entry.

Table of observations S. The same observations were made as above. This extends the scope of the project to the invisible light sources such as ultraviolet and infrared.

The advantage is that; we can dupe the intruder by using invisible light as the light rays will not be visible even in the night. For this, we have to simply replace the buzzer with a digital counter.

A typical digital counter is shown in the accompanying picture. Conclusion Based on the observation above, it can be concluded that the LDR based electronic switch can be successfully deployed for the following applications: Anti-theft alarm for Banks and offices. People Counter at Auditoriums 4.

Vehicle counter at traffic check posts. Although we have used a low intensity laser, this can be harmful if aimed accidently at the eyes or projecct the reflected beam is seen through naked eye. However, prolonged exposure should be avoided which can damage the skin and eyes. This device emits high frequency audio sounds that can damage the ears when put close to the ears or heard for prolonged period.

Should a need arise to destroy the project; care should be taken to dispose off the LDR properly as it contains harmful chemicals that can damage the environment. Remember me on this computer. Enter the email address you signed up with and we’ll email you a reset link.

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