Wiki

This page covers components which emit or detect light, or change their appearance under the control of an electrical input: how to recognise them and understand how thy might fail.

Summary

Of all our senses, sight is our richest source of information about the world around us. Naturally, then, this is often the best way in which electronic devices can communicate with us. This can be using simple indicator lights through to highly complex large format diplay screens, and can embody several different technologies. Conversly, our gadgets often need to detect light, whether just to sense when a beam has been interrupted for example by closing a lid, or in order to image a scene as in a camera. Again, several principles are used across different devices.

In total, this amounts to a fair number of different types of component, but it's generally easy to identify them, to understand what they're supposed to be doing, and whether they are indeed doing it.

Emitters of light

Filament bulbs

Filament bulbs consist of a very fine coiled tungsten wire which glows white hot when a current is passed through it. A glass envelope containing an inert gas protects it from very quickly burning out. Filament bulbs are very inefficient and so can no longer be sold for domestic lighting.

Quartz halogen bulbs are similar but have a fused silica envelope which contains a small quantity of a halogen gas. This combines with the tungsten as it gradually evaporates from the filament, and decomposes when it comes into contact with the hot filament, depositing the tungsten back on it. As a result, the filament can be run significantly hotter without shortening its life, increasing its efficiency and producing a whiter light. Even so, the efficiency is not great. Often, the silica envelope is itself enclosed in a glass bulb as it can become very hot.

You will sometimes come across small low voltage filament bulbs, either with a screw base, or wore-ended and only a few millimeters in diameter.

Neon bulbs

Before LEDs became cheap and common, a neon bulb would very often be used as a power-on indicator. A common form consists of two electrodes sealed inside a 5mm glass tube containing a trace of neon gas. When lit, it gives a gently orange glow. Other clours are sometimes seen, in particular green. These either use a different gas which naturally glows green, or a gas producing blue and ultraviolate with a phosphor coating inside the glass which glows with the required colour.

Neon bulbs may require over 100V in order to light, but then operate at a voltage of around 30% less. With the added fact that they work equally with AC or DC, they are very easy to operate from the mains. All you need is a 1MΩ resistor in series to limit the current. On no account must this be omitted!

LEDs

Discrete, COB, IR/UV

Photo-sensors

Light level, Transmissive, reflective, colour temerature

Displays

7 segment, dot-matrix

And now ...

... you might like to continue by reading about Passive components.