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 LED (LIGHT EMITTING DIODE)
A light-emitting diode (LED) is a two-lead semiconductor light source. It is a p–n
junction diode that emits light when activated. When a suitable current is applied
to the leads,
Electrons are able to recombine with electron holes within the device, releasing
energy in the form of photons. This effect is called electroluminescence, and the
color of the light (corresponding to the energy of the photon) is determined by the
energy band gap of the semiconductor. LEDs are typically small (less than 1
mm2) and integrated optical components may be used to shape the radiation
pattern.
 NPN TRANSISTOR(BC548)
The BC548 is a part of a family of NPN and PNP epitaxial silicon transistors that
originated with the metal-cased BC108 family of transistors. This series,
introduced in 1966 by Philips, became the most used transistors in Australia and
was taken up by many European manufacturers. The BC548 is the modern
plastic-packaged BC108 the BC548 article at the Radiomuseum website
describes the BC548 as a successor to the BC238 and differing from the BC108
in only the shape of the package. Datasheets for the BC548 give specifications
that are identical to, or exceed, those of the BC108, BC148 and BC238
predecessors. Thus the BC548 (or BC546 to 550) is a valid substitute in any
circuit designed for the older BC108 (or BC148), which includes many Mullard and
Philips published designs. citation needed
As of 2016, the BC548 is principally produced by ON Semiconductor and Fairchild
Semiconductor.
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