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 


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 

3.low cost

4.good con­trast


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 

Opto Coupler

Opto cou­pler

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 

Read more High Ten­sion Cable and LT Cables


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