The circuit diagram for the Line Follow Sensor board is shown below:
The board is connected directly to the Light IO board via PL4.
This circuit is very simple. +5V is applied via PL4 pin 4 to the circuit, and 0V is connected to pin 1 of PL4. When power has been applied, which is controlled by the processor1 board, Infra-Red LEDs D1 and D2 will emit Infra-Red light. If you look at the board, these LEDs are the two clear ones. D3 and D4 are two photodiodes, which will detect infra-red. If the sensor board goes over a reflective surface, the infra-red transmitted by D1 and D2 will be reflected back to D3 and D4. This will cause D3 and D4 to conduct, and and cause pins 2 and 3 of PL4 to go low. If the sensor does not pass over a reflective surface, such as a black line, then no infra-red will be reflected back, and pins 2 and 3 of PL4 will remain high.
Resistor R1 limits the current passing through D1 and D2. R2 limits the current passing through D3, and similarly R3 for D4.
The diodes are paired up for left and right. This is so that the sensor will be able to detect the edges of a thick line. D3 and D4 are the two dark LEDs. The reason that they are dark is because the plastic encapsulation acts as an optical filter. The filter is tuned for infra-red, so that visible light will not affect the operation of the device.