Light Dark Sensor

This is a light dark sensor, it uses BJT (bipolar junction transistor), LDR (Light dependent resistor), LED (Light emitting diode), and a resistor for biasing BJT, it uses 6V but you can use 9V too because i used a 9V battery.
care should be taken in determining base,emitter and collector for BJT.

In its working, at night resistance of LDR increases and BJT starts conducting but during day its resistance is almost negligible and BJT is in non-conducting state.