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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 wire-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

LEDs or Light Emitting Diodes have become ubiquitous since they first appeared in the 1970's. For many years only infrared, red and green were available, with the first practical blue LEDs appearing only in the 1990's, and then violet and ultraviolet. LEDs are essentially no different from other semiconductor diodes except that instead of (most often) silicon, they use semiconductor materials with particular quantum mechanical properties causing the energy lost by electrons passing through them in a forward direction to be released as photons of light rather than as heat.

LEDs come in many different forms. As electronic components, the 3 or 5mm clear or coloured packages are most familiar but other shapes and sizes are available. Sometimes these contain 2 or 3 LED chips emitting different colours and with up to 4 wires, allowing different colours to be produced. Much smaller surface mount LEDs are often found on circuit boards adjacent to a lens or light pipe allowing their glow to be seen on the outside of an appliance.

In COB or Chip On Board LEDS, several or many blue LED chips are mounted directly on a circit board and covered with a phosphor which converts the blue to white light. In "filament" LED light bulbs the LED chips are mounted on a thin flexible strip, sometimess bent into a spiral for aesthetic effect.

Lasers diodes

A diode laser.

There are many types of laser but their defining property is that they produce light in which all the light waves are lined up perfectly into a single beam. Think of the single continuous tone produced by an organ pipe as opposed by the hissing sound of an escape of high presure air.

An LED can be specially constructed so that much of the light is bounces back and forth between two reflecting surfaces like sound in an organ pipe. Fed with a fairly high current, this can produce laser light instead of the disorganised light that a LED normally produces as electrons pass randomly through it.

The current through a laser diode has to be controlled within fairly precise limits. Not enough and it simply won't "lase", too much and it will be destroyed by the heat generated. Consequently a laser diode is normally housed in a metal can with a glass window allowing the heat to be carried away and the light to escape, and also containing a photodiode (see later) to monitor the light output. This allows the controlling circuitry to supply sufficient current but not too much.

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.