Step 1: How Does It Work?

The black color doesn't reflect the light but white does.The line is black on white background.When the line is straight the both sensors are above the white background and it reflects the light from the LED to the photo-transistor.In this case the both motors are turning.But when the robot is on the turn one of the sensors is above the black line ,the light is not reflected and the motor on this side stops.When the one motor is stopped and the other is turning ,the robot goes to the side of the stopped motor.

Step 2: Materials

What you'll need:

Materials:

CD/DVD disc
Wires
Power Switch
Motor Driver L293D
Battery Hold
Batteries
Resistors
LED
Photo-transistors
Motors
Bolt and nut

Tools

Scissors
Pliers
Knife
Ruler
Pen
Glue

Step 3: Electronics

Step 4: Sensors

The sensors are made from photo-transistor and LED and also two resistors 220R.

Step 5: Body

Cut one square or triangle or circle from the CD/DVD or don't cut anything if you wish :D
On the obverse of the CD there are the motor driver and some of the wires ,battery hold and the power switch. I drilled holes in the CD for the wires ,the power switch and the bolt.
In fact first I put the battery hold on the underside but after that I put 3 batteries and because there was no place for them I put them above everything.But I haven't photo of the final version.
The bolt with the LED on it is for 3rd leg or something like that.
Also the nut is glued to the body and the bolt gets into it.
Motors are on 90 degrees from each other.