At minimum our LFR design require the following components.
A sensor to detect the line on the surface.(It can be a simple IR Reflectance Sensor or an high end camera!). We will use a simple IR based sensor, it is cheap and easy to build and use. This article describe how to make a line sensor array.
A microprocessor to run the code that takes inputs from the sensor and control the motion of robot. We will use a cheap single chip computer called an microcontroller (MCU). A popular family of microcontroller is AVR series from Atmel. A member of the popular family is ATmega8. ATmega8 is chosen because it has just the required amount of resources and is very low cost.
The MCU cannot drive the motors (used to actually make the robot run) directly, so a motor driver is used. A motor driver in our case is a simple 16pin chip called L293D. It can drive 2 DC motors. We use a technique called PWM to vary the speed of motor digitally from the MCU. The drive mechanism is a differential drive in which we do not require to turn the front wheel for changing direction of heading. Only two rear powered wheel is enough for motion and turning. This is different from the way cars and bikes are turned, in these the front wheel is turned to turn the vehicle. More information on differential drive is give in this article.
Wheels that are connected to motors shaft the motor.
Battery to power the whole thing.
A chassis to hold everything.