AUTOMATIC LIGHTNING

AUTOMATIC LIGHTNING

It is a circuit which works as an automatic switch. It gets “On” & “OFF” automatically according to our desire. It works on the fact that its resistance decreases with increase in incident light i.e. daytime. And its resistance increases with the decrease in incident light i.e. night time.

 CIRCUIT DIAGRAM OF LDR

At day time

I1= 230/ (30×103+100)

I1= 7.395 mA

Voltage across 30kΩ

V (30kΩ) = 30×103×7.395×10-3

V= 221.85V

VL=230-221.85V

VL= 8.15V

The Voltage across the capacitor is 8.15V. The capacitor cannot discharge.

(i) There won’t be any pulse to the triac.

(ii)The triac will be in open circuit state no current will flow through the load .The light is ‘OFF’.

At night

I2= 230/(30×103+1×103+100×103)

I2= 1.75mA

Voltage across 30KΩ

Substitute the value of eqn.(ii) in above eqn.

We get 

V (30KΩ) = I2R

V (30KΩ) =30×103×1.75×10-5

V (30K Ω) = 52.5V

+VC=230-52.5

VC= 177.5V

The voltage across the capacitor is 177.5v .The capacitor will charge and discharge .

(i) There will be pulse through diac and hence gate pulse to the triac.

(ii) The triac will be in short circuit the lead. The light is “ON”.

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