It
is more important to save power. here is the one way to save the power by
automatic controlling of room lights. This is also an application of
OpAmp based comparator. This is based on the light intensity in the
room, here we use LDR(light dependent resistor)
who's resistance decreases with increase in room light intensity. There
is a choice that you can change the threshold level to your desired
level so that it can operate at that intensity level. It does not
require any microcontroller or any other programmable devices.
components procured:
>LDR
>OpAmp 741
>resistor 10k
>BC547 transistor
>SPDT relay 12V
>1N4001 diode
>battery and a bulb
here
i connected the LDR output to the inverting terminal and a threshold
voltage of 1.5v is connected to the non-inverting terminal. so that
whenever the light intensity increases the LDR output will decrease and
if it is less than the threshold the output of the opamp will
be +Vsat=+5v.This will trun on the BC547 so that relay will be connected
as shown here, bulb will not glow
VLDR >1.5v ; transistor ON; Light OFF; LDR sensor 
when
the room light intensity decreases LDR resistance will decrease and
voltage across the resistor goes high. when it is greater than the
threshold opamp output goes to -Vsat=-5v. then the BC547 will goes to
the cutoff region (OFF state). so the light will glow.
VLDR < 1.5v ; transistor OFF; light ON; SPDT Relay
