LIGHT/DARK SENSOR USING SINGLE TRANSISTOR

Circuit Diagram

This is a simple circuit to sense light/darkness. The LDR(Light Dependent Resistor) used in this circuit is used as the sensor. The resistivity of the LDR varies with the brightness of the light. As the brightness of the light increases, the resistance of the LDR decreases and when the brightness of the light decreases, the resistance of LDR increases. Here the transistor acts as a switch.

The components used are given below.......

Q = BC108 or BC548 or BC547
R1 = 10k
R2 = 27k
B = 9v
LED

NoTe:
Adjust the value of R1 and R2 to get much brightness from LED............. :D
You can also use a buzzer or a normal bulb instead of a LED.