A keyboard circuit board is a sheet or panel in a computer keyboard that is designed to sense when a key is pressed, determine what key was pressed, and send the information to a computer or other devices to which the keyboard is attached. There are two main parts that comprise a keyboard circuit board, with the largest and most prominent section being the area under the physical keys that can be pressed. Although there are different mechanisms that can be used, this area generally uses a pattern of circuits that are either completed or broken when a key is depressed, telling the keyboard controller the location of the pressed key. Also on the keyboard circuit board is a microprocessor known as a keyboard controller, some read-only memory (ROM) that is used to store information about what key locations equate to what characters, and some type of hardware interface for sending the key data to another device, such as a universal serial bus (USB) or serial port connector. The actual material from which a keyboard circuit board is made can be metal, epoxy or thin sheets of plastic on which the necessary circuits are printed.
There are two primary methods that can be used on a keyboard circuit board to capture key presses. The first method involves placing a small conductive plate on the bottom of the physical keys. On the circuit board beneath the keys, there is a pattern of incomplete circuit pathways. When a key is depressed, it completes the circuit on the board, sending a signal to the controller informing it of the location of the key that was pressed. The controller then can use a table stored in the keyboard’s ROM to determine what character is at that location.
A second method that also is frequently found in keyboards involves a keyboard circuit board that is covered in a pattern of active, completed circuits. Sometimes called a capacitive keyboard, an electrical signal is constantly running through this type of keyboard circuit board. On the bottom of each key is a small plate that, when the key is pressed, gets close enough to the array of circuits that it changes the charge passing through the circuit underneath. This change in current is detected by the keyboard controller to determine which key was pressed.
The material from which a keyboard circuit board is actually made can vary. Older models can be made from metal, while other types can be made from layers of thin sheets of plastic with circuits printed on their surface; these often are called printed circuits. In addition to the area of the circuit board that is used to detect key presses, the board also usually contains a microchip — known as the keyboard controller — that functions as a very basic processor and usually contains some ROM and a digital comparator for basic computations. A circuit board also has some type of output interface attached, such as a serial connector or wireless transmitter, so the information can be relayed to the device with which it is being used.