Objective

• Understand the function of a light sensing circuit

Tasks

1) Go to the following website to http://www.technologystudent.com/elec1/lgtdrk1.htm to learn about light sensor

2) Watch video on light sensor https://www.youtube.com/watch?v=DSvqmOO2EFI&feature=youtu.be

3) Read through the document below on light sensor circuit, LDR, Relay, Transistors and Preset resistors.

4) Complete the Quiz here

How it works

1. During the daytime, the light sensor LDR has very-low resistance of around a few 100Ω. Thus, electricity passes through the LDR and ground through the resistor and variable resistor.

2. This is due to the fact that the resistance offered by the LDR during daytime or when the light is illuminated on LDR is less compared to the resistance of the remaining part of the circuit. Hence it draws current away from the transistor

3. The transistor does not receive enough current to turn on, hence the transistor is off.

4. Thus, the relay coil does not get enough supply to get energized. Hence, the load is switched off during the daylight.

Similarly, during the night time

1. (when the light illuminated on LDR is very less), the LDR resistance increases to a very high value of around a few Mega ohms (approximately 20MΩ). Thus, due to very high resistance of LDR the current flow is very less or almost zero like an open circuit condition.

2. Now, very little current flows through the LDR, and hence, the current is able to turn on the transistor.

3. The relay coil gets enough supply to get energized, and hence, the load is switched on during night time or when no light is illuminated on LDR.

LDR

There are different types of light sensors available. LDR (Light Dependent Resistor) is one common type used as a light sensor in this light sensor circuit. These LDR sensors are passive and do not produce any electrical energy.

But, the resistance of the LDR changes with the change in the (light illuminated on the LDR) daylight intensity. LDR sensor is rugged in nature, hence can be used even in dirty and rough external environments. Hence, LDR is preferable compared to other light sensors as it can be used even in the outdoor lighting of homes and in the automatic street light.

During night time (when the light illuminated on LDR decreases), the LDR exhibits a very high resistance of around a few MΩ (Mega Ohms). During the daytime, (when the light is illuminated on LDR), the resistance of LDR decreases to around a few 100Ω (hundred Ohms). Hence, the resistance of LDR is inversely proportional to the light illuminated on LDR.ghts as well.

Relay

A relay plays a vital role in the light sensor circuit for activating a load e.g lamp, alarm, or motor. It acts as a form of a magnetic switch. the relay consists of a coil, this coil gets energized whenever it gets enough supply (required amount of supply depends on the type of the relay). When electricity is passed to one side of the coil, it feeds current to an electromagnet that pulls a metal switch closed and activates the second, output circuit (on the right side). The relatively small current in the input circuit thus activates the larger current in the output circuit:

1. The input circuit (blue loop) is switched off and no current flows through it until something (either a sensor or a switch closing) turns it on. The output circuit (red loop) is also switched off.

2. When a small current flows in the input circuit, it activates the electromagnet (shown here as a dark blue coil), which produces a magnetic field all around it.

3. The energized electromagnet pulls the metal bar in the output circuit toward it, closing the switch and allowing a much bigger current to flow through the output circuit

Transistor

transistors are electronic components that are commonly used as switches. It has 3 legs named:

• Collector

• Emitter

• Base

• A tiny electric current flowing through one part of a transistor (from base to emitter ) can make a much bigger current flow through another part of it ( collector to emitter).

• In other words, the small current switches on the larger one.

• Based on this, a transistor can be used as a switch where a small current going into the base can control ( switch on or off ) the current flowing from collector to emitter.

Preset Resistor

A preset resistor is a small adjustable resistor. These are useful where adjustment or configuration of a circuit needs to be made but such adjustment only occurs during building a circuit, not during normal use. Its resistance can be changed by using a screwdriver.

In a sensor circuit, it is used to adjust the resistance of the circuit so that the sensitivity of the circuit can be adjusted. ( User can control how much light is received before the transistor is turned on ).