How to design a circuit using LED, Cds, transistor and resistor that turn lights on when dark?

Question

when its dark. the led turns on. when its bright the led turns off

Answer 1

There's just the circut you need here

http://www.kpsec.freeuk.com/trancirc.htm...

(Think CDrs, and a Cds cell behave just the same!)

The LDR and 10K resistors work as a Potential divider which control the voltage applied to the NPN transistors base terminal. When it's light the LDR has a low resistance so the potential divider's output is tiny, so the transistor is OFF. When it's dark the LDR has a high reistance so the Potential dividors output is higher than the transistors saturation voltage, then turns the transistor ON, and allows a current through the LED.