The LDR is a semiconductor device whose resistance value changes when exposed with light energy. It is also called photo conductive cell, photo resistor or Light Dependent resistor
It consists of a thin layer of cadmium sulphide containing a small amount of silver,antimony or iridium, deposited on an insulating substrate. Ohmic contacts are made at the opposite ends of the CDS layer. The value of resistance is about 2mega ohm at absolute darkness. In strong incident light it’s resistance is about 10 ohm only
Principal of working
The LDR operates with a principal of photo conductive effect. The conductivity of certain material is varied when they are exposed to light. This effect is called photo conductive effect
when light energy is exposed to a photo conductor,the co-valent bonds are broken. This effect produces more charge carriers
Electrons in the conduction band and holes in the valence band. The amount of carriers generation depends upon the intensity of light. The large amount of light generates more number of carriers and reduces the value of resistance of the photo conductor. Similarly in lower amount of light intensity,the resistance value of the photo conductor is high
The characteristics curve of resistance versus illumination for a typical LDR
Zero incident of light the resistance of the LDR is very high. Hence a very small amount of current flows through the LDR. According to the increasing of the incident of light the resistance of the LDR also decreases
Uses Of LDR
1.Used to measure the intensity of light
2.Used in object counters
3.Used as ON-OFF switch
4.Used in burglar alarms
LIGHT EMITTING DIODE (LED)
LED is a specially made forward biased PN junction diode which emits light when current flows through it
In this diode a P‑type semiconductor material is deposited on the N‑type substrate layer by using diffusion method. Metal contacts are made at the outer edge of the P‑layer. A cathode connection is also formed by coating a metal film at the bottom of the N‑substrate. The metal film reflects more light to its surface
Principal of working
When a forward bias supply is applied to the diode, the electrons and holes moves towards the junction and recombination takes place. After recombination the electrons lying in the conduction band of N region move towards the holes lying in the valence band of P — region. The energy difference between valence band and conduction band is radiated in the form of light.
Uses Of LED
1.Used in ON and OFF indicators
2.Used programmable advertisement boards
3.Used in optical switching applications
4.Used in optical communications
5.Used in 7 segment and 14 segment displays
Liquid Crystal Display (LCD)
Liquid crystal display will not emit light or generate light but it will alter the externally available illumination. An organic compound material is used in LCD. it is a low power display. It’s life time upon the degradation of chemical used
There are two types of LCD
1.Dynamic scattering type LCD
2.Field effect type
1.Dynamic Scattering type LCD
A liquid crystal is an organic material. It will flow like a liquid. It’s molecular structure is normally associated with solids. The display consists of two glass plates coated with indium oxide on the inner side. Numatic liquid crystal is placed between the two glass plates.
The indium oxide conducting surface is transparent. When no voltage is applied to the crystal to the molecules in the crystal are aligned in the direction of field. Hence the incident light will simply pass through and the liquid crystal structure will appear clear
When sufficient voltage is applied across the conducting surfaces the molecular arrangement is disturbed with the result that the region will be established with different indices of refraction. The indident light is therefore reflected in different directions at the interface between the two glass plates. This scattered light appears as a frosted glass that appears as bright.
Field Effect Type
The LCD consists of a thin layer of liquid crystal fluid sandwiched between two glass plates. The plates are coated with transparent conductive film in the form of desired alphanumeric image.
Polarizers are placed on the top and bottom glass surfaces. A reflecting surface is placed below the bottom polarizer. When no voltage is applied the LCD material twists the light passing through it now cell appears bright. When a sufficient voltage is applied no twisting of light takes place now the cell appears dull
1.consume very low power
2.small in size and weight
1.Used in solid state video games
2.Used in image sensing circuits
3.used in instrument displays
4.Used in Laptop, computer
5.used in packet calculators
Electromechanical relays are relatively large and expensive. It can cause unwanted voltage spikes and noise due to coil windings and closing and opening of contact points
Opto isolator is an useful alternative for electromechanical relays. It is very much used in digital circuits.
Opto coupler is a solid state component it encloses light emitter light path and light detector. The characteristics of the opto coupler cannot be changed externally. It provides electrical isolation between circuits it is also called opto isolator.
The opto coupler consists of an infrared LED and a photo detector. A PIN photo diode darlington pair photo transistor or photo SCR can be used as a photo detector. The opto coupler permits one way transfer of electrical signals from the LED to the photo detector
There is no electrical connection between the input and output circuit. A transparent insulating cap is provided in between the emitter and the detector used to permit passage of light
The light emitter converts the input signal to light signal. The intensity of light depends upon the magnitude of the input signal. The light signal then passes to the detector through transparent insulator. Then the photo detector converts the light signal into Electrical signal. So the opto coupler allows signal transfer without any coupling wires of capacitors or transformers.
Uses of Opto Coupler
1.It is used for interfacing high and low voltage systems
2.It is used to couple digital analog signals
3.It is used for interfacing different types of logic circuits