Automatic Room light Controller using OpAmp:

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