Photosensors or photodetectors are sensors of light or other electromagnetic energy. There are several varieties:

Most optical detectors are quantum devices in which an individual photon produces a discrete effect.

• Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
• Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity
• Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated
• Photodiodes which can operate in photovoltaic mode or photoconductive mode
• Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
• Phototubes containing a photocathode which emits electrons when illuminated and in general behaves as a photoresistor.

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