can u give me the circuit of automatic night light & its working principle?

Answer 1

Here ya go....

http://www.paulgardiner.dsl.pipex.com/EiSS/eiss.html?nightlight.html

Answer 2

It uses a LDR (Light dependant resistor) to detect when there is no more light (bed time!) and activates the light. How it does I don't know because this is the definition of an LDR

'A photoresistor is an electronic component whose resistance decreases with increasing incident light intensity. It can also be referred to as a light-dependent resistor (LDR), or photoconductor.'

Answer 3

a current generating photocell or a resistance changing photo resistor produces a small voltage which is applied to the gate of a triac, scr, or opto-isolator to block the flow of current to the light. When the input blocking voltage falls low enough, the device blocking the current allows it to pass, lighting the light.