Dark Sensor Experiment using LDR, BC547 and LED

Hello friends,
In this blog I am sharing the information on concept and working of a LDR based Transistor switch. This circuit has a wide application in various dark sensing systems.

Concept and working of the circuit :

The working of the circuit totally depends upon the LDR. Light Dependent Resistor is a special kind of resistor whose resistance changes by differing the amount of light falling on LDR. In this circuit I am using a LDR whose resistance decreases in light condition and increases in dark condition. Using this property we are creating a voltage divider circuit using a LDR and a normal resistor by connecting both in series.
So when amount of light falling changes on LDR it creates different output voltages. We know that any transistor requires 0.7v(For Si) on its base to turn on and start conducting. So we are creating a voltage divider circuit such that in normal (light condition) condition the output of voltage divider is less than 0.5v and in dark condition it is above 0.5v so that we can connect a transistor in the circuit and use it as a switch for driving other component or circuit. In doing so we are using the property of LDR.
For more details see the circuit below:

COMPONENTS REQUIRED :
* Resistor = 1M and 330R 1/4watts
* Transistor (NPN) = BC547 or 2SC8050  or 2N2222 any general purpose npn  transistor will work. Please make sure about pin configuration.
* LDR
* LED
* 9v battery

Watch Video Tutorial :