This is a guide for building a automatic color detector using LDR (light dependent resistor). This is small in size, and can be built in less than a few minutes.

Step 1: Things You Need

You need the following-

LDR (light dependent resistor).
LED- The color of the LED you need depends upon the color you need to detect. Eg., if you need to detect red color, then you need red LED.
9V Battery
small lithim 3v battery for the led.
22k resistor
small vibrator(just to alert you)
A switch
tape

Step 2:

Take the ldr and cover it with small amount of insulating tape as shown in the image. this is to protect the ldr from other lights.

Attach the led and ldr to the opposite sides of the battery. connect the led to the small lithium battery, while connect the ldr to the 9v battery, vibrator, resistor and a switch in series. The led and the ldr must be kept almost parallel, and slighty inclined to each other. cover the whole body except the led and the ldr can be covered with tape.

note: the led circuit and the ldr circuit are different.

For detecting white color, use a white led, and so on. hold the apparatus a few cm away from the surface of which you need to find the color. You can also use it in robot.

how it works-

if you want to detect white color, then use a white led. when the light from the led touches a white surface, then the light bounces and reaches the ldr as white light. The ldr responds to white light the most; therefore the vibrator gets switched on, alerting you. when it hits a surface, of different colour then an amount of light will be absorbed and a light of different wavelength reaches the ldr. In the latter, the vibrator doesn't get switched on. you can use red led for detecting red color and so on.