A dark sensor is a device which senses the presence of darkness with the help of the LDR.
Whenever light is focused on the LDR the LED will not glow and when the LED is kept in a dark room without lights the LED will glow.
It can also be called an Automatic street light circuit.
Step 1: Components Required
LDR(light dependent resistor)
LED(light emitting diode)
100k ohm resistor
9v battery with battery clip and some wires.
Step 2: Steps of Construction
Insert BC547 transistor on breadboard as shown.
1. Connect the LDR to the'base' and 'emitter' of the BC547 transistor.
2. Then connect one end if the 100 k ohm resistor from the 'base' of the BC547 transistor and the other end of the 100k resistor to anode(+) side of the L.E.D.
3.The cathode (-) side of the LED will get connected to the 'collector' of the BC547 transistor.
Join the wires as shown .
One end of wire to end of emitter and the other wire connect to end of the resistor.
The positive side of the battery will get connected to the 100k ohm resistor and anode(+) of the led while the negative side of the battery will get connected to the 'emitter'of the BC547 transistor and the other end of the LDR.
When there is light the LED will not glow but as the circuit is kept in dark the LED glows.