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.

Battery operated torches and bicycle lights once exclusively used small low voltage filament bulbs with a screw base and somewhat larger 12V versions were used in cars. You may still sometimes come across small low voltage filament bulbs, either with a screw base, or wire-ended and only a few millimeters in diameter illuminating the dial in an older radio or alarm clock.

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, followed by 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, sometimes bent into a helix or other shape 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 bounces back and forth between two reflecting surfaces like sound in an organ pipe. Fed with a sufficiently large 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.

Light detectors

Photo-diodes and photo-transistors

A diode only conducts in one direction, but in the reverse direction, if you shine a light on it the light will release electrons, enabling a small current to flow. Most diodes come in an opaque housing, or else the light-sensitive region is so small that the effect is minimal, but a photodiode by contrast is constructed so as to maximise the effect.

A junction transistor consists essentially of two diodes in close proximity such that a small current passing through one allows a much larger current to flow through the other. Exposed to light, this is equivalent to a photodiode connected to a transistor to amplify the photodiode's current.

Two transistors can be connected in a "Darlington pair" in which the second transistor further amplifies the current from the first. If the first is a photo-transistor the result is a darlington photo-transistor with still greater sensitivity.

Light-dependant resistors (LDRs)

A Light Dependant Resistor, or LDR.

A Light Dependant or Photo Resistor uses a semiconductor such as cadmium sulphide which, in the dark, has few electrons free to carry a current and so has a high resistance. If you shine a light on it, more electrons are released and so its resistance is reduced. Unlike a photodiode, it's a pure resistance, conducting equally in either direction.

The use of cadmium is now restricted on account of its toxicity and so LDRs are most likely to be found in older equipment. They were frequently used in photographic light meters, to control street lights, night lights and the brightness of a digital clock with a LED-based 7-segment display. They have also been used in musical equipment. Some kind of vane actuated by a foot pedal controls the amount of light from a lamp falling on an LDR, which in turn controls the volume or other characteristic of the music. This avoids any crackling that a potentiometer used for this pupose might produce after heavy use.

PIR detectors

A PIR detector

Passive Infra-Red or PIR detectors work on a principle related to the piezoelectric effect, in which a strain applied to certain types of crystal slightly displaces the positive and negative charges within it, creating a net voltage across opposite faces. In the pyroelectric effect, this strain is produced by the very slight warming produced by infrared (heat) radiation. In a PIR detector, a number of plastic lenses formed i a single piece of transparent plastic are placed in front of a pyroelectric sensor causing multiple images of a warm body (a person, an animal or a vehicle) to move across the sensor as it moves across the field of view. In fact there may be two sensors with the detector designed to detect differences between their outputs, reducing false detections which might be caused by environmental or electrical effects affecting both equally.

Optical sensors

Optical sensors are often used to detect a mechanical condition, such as as a lid being closed, a finger guard in place, a moving part being at the limit of its travel or the motion of a wheel. This can be done with a LED and a light detector such as a photodiode, phototransistor or photo Darlington, but for convenience, the two are often packaged together. Very often an infrared LED is used since photodiodes and their cousins have good senstivity in the infrared.

There are two types. In the reflective type the LED and light detector face the same way in order to detect a reflective surface in front of them. In a transmissive one, they face each other across a gap. These are very common in older mechanical mice, in which a wheel with slots in it is placed in the gap in order to sense its rotation as the mouse moves.

There are other types of optical sensor for more specialised purposes, for example three light sensors covered by three different coloured filters can be used to determine a colour, or the colour temperature of alight source or a scene.

Opto isolators

Sometimes it's necessary to electrically isolate one part of a circuit from another. This is easily done, as in an optical sensor, with a LED and a photo detector mounted in a single package. As with an optical sensor, the photo detector can be a phototransistor or photo darlington, or if the intention is to isolate a purely digital (on/off) signal, there may be additional circuitry included to further condition the output.

Opto isolators commonly come as a 4 or 6 pin dual-inline package just like a simple IC.

Opto isolators are extremely common in switch mode power supplies. Isolation of the output power from the mains input is achieved with a transformer but feedback is required from the output to the input in order to regulate the power so as to produce a constant output voltage. If you look inside such a power supply you will find the transformer and an opto isolator both straddling a very clear demarcation on the board (often a physical slot) between all the circuitry directly fed from the mains and all the circuity feeding the output.

Musical instruments with a MIDI interface (allowing them to be connected to MIDI synthesisers, sequencers and other instruments) always contain an opto isolator on the MIDI input, as required by the MIDI interface specification. This eliminates the risk of ground loops - spurious currents flowing between different pieces of equipment, each with its own mains power supply.

Image sensors

An image sesor on a flexible circuit board.

Until semiconductor image sensors started to appear in the 1990's, all TV and video cameras used vacuum tubes to capture images. Like an old fashioned CRT (used in TVs before flat screens became available), these used a scanning electron beam to read out the brightness of the image at each point. There were various different types of video camera tube but you are never likely to see any of them today outside a museum.

All image sensors today consist esentially of a large array of photodiodes fabricated on a single silicon chip. A colour image sensor will have separate photodiodes for each of the three primary colours. There are two types:

In CMOS sensors the pixels are read out by row and column in the same was as the pixels are addressed in a flat screen display. These are the cheaper sort and use the least power and hence are used in the majority of still cameras and webcams.
CCD or Charge Coupled Devices read out the data by passing a charge from each pixel along its row (or column) to the end, like a bucket brigade. This requires less circuitry per pixel for the readout and hence allows more area for collecting light, and so is used in high end cameras.

Displays

Whatever technology they use, displays can be made in several different formats of increasing complexity.

7-segment displays are very familiar as calculator displays. Each digit is formed by a unique combination of the 7 segments. 9 segments have sometimes been used.
14- and 16-segment displays allow alphanumeric data to be represented in a similar way. These and 7 segment displays may additionally include a decimal point.
Dot matrtix displays represent data graphically, ranging from simple devices representing a single line of alphanumeric characters by an 8x5 matrix of dots per character, up to high-end TV and computer screens with many millions of pixels.

LCD

LCDs (or liquid crystal displays) depend on polarisation of light. Between two polarising filters, a special liquid rotates the plan of polarisation of the light passing through if subject to an electric field.Hence light can pass though only if the plane of polarisation is aligned with both filters.

Very often, as in a calculator display, the LCD is configured to be view from the front. With a singe polarising filter and a reflective layer at the back, a segment appears dark if the polarisation of light is twisted and hence unable to pass through the filter on its way out again. Other LCD displays have a back-light to allow them to be read in poor light. Laptop and LCD TV screens have a large bck-light, and LCD projectors ocus the light from a bulb through the LCD. Colour displays have have separate LCD pixels for the three colours, each with a coloured filter.

LCDs are activated by an electric field and don't rerquire any current to flow hrough them. As a result, simple CD displays are well suited to low power battery devices such as calculators ad digotal clocks and watches.

Plasma

Construction o a plasma display.

Plasma displays are largely obsolete, having been overtaken by LCD and OLED displays as these have improved, largely eliminating their advantage of high contrast.

Plasma displays were hthe first technology enabling large screen TVs to be produced. Each pixel consisted of a tiny gas-filled cell which could be ionised by a high voltage. This caused a tiny quantity of mercury vapour to emit ultraviolet light which was converted to red, green of blue light by a phosphor.

OLED

Normal LEDs use a crystaline semiconductor material but certain organic materials can be made to emit light in the same way, albeit with lower efficiency. However, the principle of operation is not quite the same.

OLED displays consist of a matrix of pixels. They range from very small devices with just a few hundred pixels, oftem blue-ish white, up to large format TV screens. Unlike LCD sisplays, they need no back light and so cab be made thinner annd produce higher contrast levels.