0 Automatic Night Light

This circuit is one of the simplest to build considering the fact that it requires no microprocessors at all! It dims in the light and brightens in the dark.

Parts required :

•  D1—1N400X diode
•  P1—100 k ohm potentiometer
•  R1—22 k ohm
•  R2—470 ohm
•  LDR—1 M- ohm dark
•  Q1—NPN transistor 3904
•  LEDs—5 mm round

Procedure :

• Connect the parts together according to the schematic on a breadboard.
• Set the circuit so that the LDR (Light Dependent Resistor) recieves a fairly good amount of light. 
• Now turn the potentiometer so that the LEDs are barely off, take the circuit under a dim light and turn the pot again so that the LEDs are barely off.
• Any further reduction in the light will turn your LEDs on.

Working :

The current from V+ has two paths to go to the lower potential. Naturally it will chose the easiest path, or the path of least resistance. When the circuit is under bright light, the LDR will have very low resistance, so all the current will pass through the LDR thereby preventing the LEDs from lighting up.(They have no current). As the light begins to dim, the resistance of the LDR keeps rising (Just like you getting angrier when you get no food!). And as the resistance rises the current must find an easier path. Just at the moment when the resistance through both the paths become equal, the current begins travelling through the transistor thereby lighting the LEDs (much to your joy)!

Your Automated Night Light is ready!