On all but the simplest PCBs, wires must be printed on more than one surface of fiberglass to allow for all the required component interconnections. Each surface containing printed wires is called a layer. In a relatively simple PCB that requires only two layers, only one piece of fiberglass is required since wires can be printed on both sides. In a more complex PCB where several layers are required, individual circuit boards are manufactured separately and then laminated together to form one multi-layer circuit board. To connect wires on two or more layers, small holes called vias are drilled through the wires and fiberglass board at the point where the wires on the different layers cross. The interior surface of these holes is coated with metal so that electric current can flow through the vias. Most Digilent boards are simple four or six layer boards; some more complex computer circuit boards have more than 20 layers.
Circuits often require output devices to communicate their state to an user. Examples of electronic output devices include computer monitors, LCD alphanumeric panels (as on a calculator), small lamps or light-emitting diodes (LED), etc. Digilent boards include different output devices, but all of them include some number of individual LED, and seven-segment LED displays that can display the digits 0-9 in each digit position (each segment in the seven-segment display contains a single LED). LED are two-terminal semiconductor devices that conduct current in only one direction (from the anode to the cathode). The small LED chips are secured inside a plastic housing, and they emit light at a givenfrequency (RED, YELLOW, etc.) when a small electric current (typically 10mA to 25mA) flows through them.
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