Introduction: Light Detector, No Microprocessors, Just Simple Electronics :)

I will show you two circuits, one turns an LED off when theres no light, and the other turns an LED on when theres no light.

for the first one you will need:
-R1(LDR) 10K
-R2 (1.2K) colour code: brown, red, red.
-R3(10 ohms) colour code: brown black black.
-T1 almost any low voltage NPN transistor will do.
-L1 LED i used a red one.
-potenciometer(optional)
-jump wires
-2 1.5 volt bateries and pack for them

for the second one you will need:
-R1 (LDR) 10K
-R4 (10K) colour code: brown, black, orange.
-R3(100 0hms) colour code: brown, black, black.
-T1 any low voltage NPN transistor
-L1 LED i used a red one.
-potenciometer(optional)
-jump wires
-2 1.5 volt bateries and pack for them

it cost me under 1€ to make this, everything was salvadged from old radios, etc...
exept the LDR, but you can easiy get one from those light switch thingies at the dollar shop, or from the internet very cheap... add some more stuff if you get it from the net though... you dont wanna pay 10 cents for the part and 8 dollars for shipping =|

Step 1: Turn LED Off When Theres No Light

the 1.2 K resistor goes in the middle pin of the transistor and to - on battery
dont forget the round part of the transistor is facing you!

DONT FORGET THE POLARITIES OF THE LED!!!

Step 2: Turn LED on Whe Theres No Light

it's basically the same circuit but the LDR now goes to - and the R4(10K resistor) goes to plus.
all you have to do is this small change and the whole purpose of the circuit changes!

dont forget the round part of the transistor is facing you!

DONT FORGET THE POLARITIES OF THE LED!!!

if you have any doubts/problems plz tell me.

Step 3: Apendix

Now i'm going to try to explain what some parts do.
on the first circuit the 1.2K resistor controls the sensitivity, go ahead change it! if it's 10K you will need total darkness to turn the LED off, so change it betwen 1K and 10K less than 1K will be way sensitive and wont work, and over 10K you will need something darker than total darkness, wich i dont think exists, maybe outer space :P

the second circuit the 10K resistor also controls the sensitivity, if you put 150K you will need total darkness to turn the LED on, so go ahed change it aswell! but only betwen 1K and 150K.

If it doesnt work:
check the polarities of the LED, and see if the transistor is the right way in, if the LED doesnt go on, on the first circuit, dont be sad, that circuit is tuned so it works in my room, just grab a potenciometer and tune it the way you want to, then with a multimeter read the resistance of the pot, and put the closest resistor you can find to that resuslt, thats how i got 1.2K and 10K

best of luck!

Now a video of what you should expect: