Instructables
Picture of light decting circuit
this circuit will make a alarm when the light falls on the ldr
 
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Step 1: Things needed

you need-
1] resistors 100k ohm , 56k ohm , 10k ohm
2] transistors 1 pnp and 1 npn 
3] capacitor 0.04 uf
4] ldr
5] 8 ohm speaker
6] 2 cells
7] battery holder
8] pref board

Step 2: Ldr

A photoresistor, light dependent resistor (LDR) or cadmium sulfide (CdS) cell is a resistor whose resistance decreases with increasing incident light intensity. It can also be referred to as a photoconductor.
A photoresistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.

Step 3: Circuit digram

Picture of circuit digram
solder according to the digram.

Step 4: All done

Picture of all done
you can put the circuit in a box when you open the box in a well lit room the alarm goes off!!

MROHM3 years ago
Please!! We need the values of Components Added to your Diagram..Many Thanks!!
zazenergy3 years ago
I'd love to see this in it's finished form -- perhaps a video is in order. Thanks for sharing!
robot1398 (author)  zazenergy3 years ago
the box was just an idea and i am not able to attach the the video as it says it is more than 10 mb
jensenr303 years ago
I believe you should try out the following software for your schematics:

http://www.cadsoft.de/download