Circuits often require inputs that come directly from users (as opposed to inputs that come from other devices). User-input devices can take many forms, among them keyboards (as on a PC), buttons (as on a calculator or telephone), rotary dials, switches and levers, etc. Digilent boards include several input devices, typically including push buttons and slide-switches. Since digital circuits operate with two voltage levels (LHV or Vdd, and LLV or GND), input devices like buttons and switches should be able to produce both of these voltages based on some user action.
Circuit components are manufactured with exposed metal pins (or leads) that are used to fasten them to the PCB both mechanically (so they will not fall off) and electrically (so current can pass between them). The soldering process, which provides a strong mechanical bond and a very good electrical connection, is used to fasten components to the PCB. During soldering, component leads are inserted through the holes in the PCB, and then the component leads and the through-hole plating metal are heated to above the melting point of the solder (about 500 to 700 degrees F). Solder (a metallic compound) is then melted and allowed to flow in and around the component lead and pad. The solder quickly cools to form a strong bond between the component and the PCB. The process of associating components with reference designators, loading them into their respective holes, and then soldering them in place comprises the PCB assembly process.
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