diycircuits4u

#) The bulb is Off during day time and it turns on automatically during night

#) The unique property of LDR (Light Depented Resistor) is utilized here

#) LDR is a variable resistor. It has very low resistance in the presence of light

and very high resistance in the absence of light

#) In this circuit, we are created a potential devider with 1 arem is ordinary

resistor and other arm is an LDR

#) As we know that voltage drop across a resistor increases, when resistance

increases

#) Here the drop across the LDR varies as light intensity changes

#) That is voltage drop across the LDS is minimum in the Light and maximum

in the absence of light

#) The drop across the LDR is connected to the Base of the BC 187 transistor

#) In the dark, the drop is very high (> 0.6V) and it is sufficient to turn on the

transistor

#) When 1st transistor is On next one will also turns on then the relay coil

energizes and the bulb will glow

#) In the Light, the drop is very small (< 0.6V), then the transistor remain in

off state