The circuit is light seeking; it will follow a light source around a darkened room or any kind of  area. The circuits detects source of light and controls left and right wheel of the robot accordingly. The circuit does not control the speed of wheel but turns the robot by switching on and off the motor. Since the wheel motor is controlled by relay, any kind of DC motor can be used in the system (Motor supply voltage is chosen according to motor voltage rating).

A pair of photocell determines the direction in which the robot will move. Each photocell is connected to an op amp configured as a comparator. When sufficient light falls on photocell R2. the voltage at the inverting input (pin 6) of lCl-a will fall below the voltage at the non-inverting input (pin 5), so the output of the comparator will go high, and transistors Ql and Q2 will turn on. That will enable relays RYI and RY2, and thereby provide power for the right motor. The robot will then turn left. Likewise, when light falling on R3lowers its resistance, Q2 and Q3 will turn on, the left motor will energize, and the robot will turn right.

Note: All ground should be made common.
 

   Schematic

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