Light sensors basics

1. LIGHT SENSORS
LDR

2.  A device that detects the intensity of the light
incident on it is a light sensor.
 Different types of light sensors are Light Dependent
Resistors, Photo Diodes, Photo Transistors
 These are also called as photo resistor.
Introduction

3.  A light Dependent Resistor that is LDR or a photo
resistor is a device whose resistivity is the function of
a incident electro magnetic radiation or simply light
 That is why they are light sensitive devices.
 These are also Called as photo conductors, photo
conductive Cells or photo cells
Introduction

4.  Made of Semiconductor materials having high
resistance.
 There are different symbols used to represents the
LDR .
Introduction

5.  Commonly used Symbol is
 Arrow indicates light falling
on it
SYMBOL

6.  LDR works on the principle of photo conductivity.
 Photo conductivity is an optical phenomenon in which
material conductivity is reduced
when the light is absorbed by the material.
Principle

7.  When light falls, or when the photons fall on the
device the electrons on the valence band of the
semiconductor material are exited to the conduction
band
Principle
Conduction Band
Valence Band

8.  These photons in the incident light should have
energy greater than the band gap of the
semiconductor material to make the electrons jumps
from valence band to conduction band
 Hence when light having enough energy strikes on
the device more and more electrons are exited to the
conduction band which results large no of carriers.
Principle

9. Characteristics of LDR
a. When Light falls on the LDR its resistance decreases.
b. And the resistance is increases during the dark and this is
called as dark resistance.
Resistance
Light intensity
Resistance Decreasing with light intensity
Dark Light

10.  It can be high as 10^12 ohm.
 When light falls on the LDR its resistance decreases
drastically
 If constant voltage applied to it and intensity of the
light is increases the current starts increasing.
Characteristics of LDR

11.  LDR are less sensitive than photo diodes and photo
transistors.
 LDR and a photo diodes are not the same
 Photo diode is a pn-junction semiconductor device
that converts light to electricity.
Characteristics of LDR

12.  Where as photo cell is a passive device there is no PN-
junction in LDR. It will not convert light to electricity.
 LDR only changes the conductivity of the path for
electricity depending on the light falls on it.
 LDR are non Linear Devices. There sensitivity varies
with the wavelength of light incident on them.
Characteristics of LDR

13.  Based on the materials used they are classified as
a. Intrinsic photo resistors
b. Extrinsic photo resistors
Different types of LDR

14.  These are made of pure semi conductor materials
 Such as Silicon or germanium
 Electrons are get exited from valence band to
conduction band when light fall on it and no of charge
carriers increases.
Intrinsic photo resistors

15.  These are semiconductor materials doped with
impurities which are the dopants
 These dopants creates new energy band above the
valence band which are fill with the electrons hence
this reduces the band gap and less energy is required
in exiting electrons
 These are used for long wave lengths.
Extrinsic photo resistors

16. Construction of LDR

17.  Structure Consists of light sensitive material which is
deposited on an insulating substrate such as ceramic
 Material deposited on it as zigzag pattern in order to
obtain the desired resistance and power rating
 Zigzag area seperates the metal deposited Area into
to 2 regions
Construction of LDR

18.  Then the ohmic contacts are made on either sides of
area.
 The resistance of these contacts should be low as
possible to make sure that the resistance mainly
changes due to the effect of light only.
 Normally used materials are cadmium sulphide,
cadmium selenide ,induim antimonide .
Construction of LDR

19.  Quick Response
 Reliable Performance
 Good Characteristic of Spectrum
Features

20.  High sensitivity
 Easy employment
 Low cost
 No union potential
 High light-dark resistance ration
 LDR based light sensors are available in different
shapes and sizes
Advantages

21.  LDRs are highly inaccurate with high response
time(about 10s or 100s of milliseconds)
 Slow response in stable material
 Narrow Spectral response
 Hysteresis effect
 Low temperature stability for the fastest material
Disadvantages

22.  Photo switch
 Photoelectric Control
 Auto Flash for Camera
 Electronic Toys, Industrial Control
Applications of LDR