LIGHT SENSOR

What is a light sensor

A light sensor, as its name suggests, is a device that is used to detect light. There are many different types of light sensors, each of which works in a slightly different way. A photocell or photoresistor, for example, is a small sensor that changes its resistance when light shines on it; they are used in many consumer products to determine the intensity of light. A charged coupled device (CCD) transports electrically charged signals, and is used as a light sensor in digital cameras and night-vision devices. Photomultipliers detect light and multiply it.

Devices that include these sensors have many uses in scientific applications, but are also found in items that people encounter each day. A simple light sensor may be part of a security or safety device, such as a burglar alarm or garage door opener. These types of devices often work by shining a beam of light from one sensor to another; if the light is interrupted, an alarm sounds or the garage door won't close.

Many modern electronics, such as computers, wireless phones, and televisions, use ambient light sensors to automatically control the brightness of a screen, especially in low-light or high-light situations. They can detect how much light is in a room and raise or lower the brightness to a more comfortable level for the user. Light sensors also may be used to automatically turn on lights inside or outside a home or business at dark.

project

Today  we are going to look at simple circuit that can basically turn ON or OFF a bulb using  simple combination of few active and passive component hence there is need to understand the  how  LDR works

What is a Light Dependent Resistor or a Photo Resistor?

A Light Dependent Resistor (LDR) or a photo resistor is a device whose resistivity is a function of the incident electromagnetic radiation. Hence, they are light sensitive devices. They are also called as photo conductors, photo conductive cells or simply photocells.

However  light dependent resistor works on the principle of photo conductivity.

Photo conductivity is an optical phenomenon in which the materials conductivity is increased when light is absorbed by the material.

Circuit diagram

This an electronic circuit built with LDR, lamp, battery, transistor & resistors shown in the below circuit diagram.

The required DC voltage of the LDR circuit is supplied from a bridge rectifier circuit or a battery.

if one is using a rectifier it changes the AC supply into a DC. The bridge rectifier circuit uses a step-down transformer to step-down the voltage from 230v into 12v  . The diodes are connected in the form of a bridge used to alter the AC voltage into DC. The voltage regulator is used to change the 12v DC-6v DC, and then, this DC voltage is supplied to the entire circuit. A 230v AC supply for both the bridge rectifier and the load is to be kept continuously for continuous operation of the light sensor circuit.

In the morning time, this sensor has a low resistance around 100Ω. Thus, the power supply flows through the LDR & ground through the variable resistor and resistor as shown in the above light sensor circuit. This is due to the resistance offered by the light dependent resistor in the daytime or when the light falls on the LDR, then it is less compared to the resistance of the remaining part of the sensor circuit . We are alert of the principle of current, that the flow of current always flow in the path of low resistance.

Therefore, the lamp does not get sufficient supplies to get strengthened. Hence, the light is switched off in the daylight. In the same way, during the night time, the resistance of the LDR increases to a high value (20MΩ). Thus, due to the high resistance of the resistor, the flow of current is low or almost zero. Now, the flow of current through the low-resistance lane such that it increases the base voltage of transistor to reach more than 1.4v. As the  transistor is triggered, lamp acquires enough supply to get brighten up .

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