Numerical indicators, Infra-red remote controllers and Fibre optic communication.

Numerical indicators

If seven light emitters are arranged in the layout shown, any number in the range 0--9 and any letter from A to F can be displayed. This is known as a seven-segment display. The spectacular increase in calculator sales has made seven-segment display manufacture a major industry, and LED and LCD arrays are available in a vast range of sizes.

Decoders are available to decode directly from binary to the seven segments without the need for a driver stage.

With multidigit LED displays, the current requirements can be quite large. To reduce this, it is common to multiplex the display, illuminating each digit in turn. A common multiplexer scheme is shown. LCD displays also use multiplexing to reduce the number of connecting wires.

Four binary digits are applied in parallel to the multiplexer (MUX). This selects each display in tum and presents it to the binary-to­ seven-segment decoder. All corresponding cathodes in each decade are driven together, but the MUX only applies positive supply to the common anode on the display, whose data is coming from the MUX.

Each display is lit in tum, although this is not apparent to the eye because of the high clock frequency (typically 15kHz). Current consumption is reduced considerably at the expense of only a slight reduction in brilliance.

The seven-segment display only allows numerals to be displayed, along with some letters (A, C, E, F for example). Full alphanumeric displays can be obtained with the 16-segment display of (a) or the dot matrix displays of (b) and (c).

Infra-red remote controllers

Most televisions and video recorders are now equipped with remote controllers that allow the viewer to change channel, adjust volume and similar functions. These are based around the principle shown in (a). The keypad input is decoded to give a 5-bit code (allowing 32 options) which is transmitted by several LEOs operating in the infra­ red region of the spectrum.

The modulated infra-red signal is received by a photocell at the main equipment, where the 5-bit code is decoded to produce the desired effect. Receiver integrated circuits can have digital outputs (for channel changing or sound muting) and analog outputs which are ramped up and down remotely (for volume and similar fine adjustments).

The 5-bit code is a form of pulse position modulation (PPM) and is shown in (b). There are three distinct signals, I, 0 and S, the last one signifying that all five bits have been sent and serving as a delimiter. These three signals have a fixed interval relationship of 2:3:6 (typically 18, 27 and 54 ms). The coded signal is sent continuously as long as the keypad button is pressed.

The receiver uses a counter running at a fixed high frequency to time the interval between successive pulses, thereby identifying if a pulse is a I, 0 or S. If it is an S the receiver has the complete 5-bit code. As a precaution this is checked against the previous code received, and only if the two are the same is action taken. In this way a high degree of security is obtained against multi-path reflections and spurious signals.

Although the technique was developed for the control of domestic equipment it is sufficiently reliable for short range remote control in many other applications.

Fibre optic communication

When a light beam passes from a less dense medium (such as air) to a more dense medium (such as glass) it is bent towards the vertical, as shown in (a). This effect is known as refraction. Light passing to a less dense medium (e.g. from glass to air) is also bent, as in (b), but as the angle increases total reflection takes place beyond a certain critical angle. For a glass-to-air transition the critical angle is about 40 degrees.

In (c), light is entering a glass rod. As the beam passes down the rod it strikes the edges, but because the angle of incidence is larger than the critical angle, internal reflection occurs and the light beam is conveyed without loss (although there is some attenuation caused by scattering off inevitable flaws in the glass). This is the simple basis of data transmission by light signals. All that is now required is a modulated light source, a transparent conductor arranged to provide internal reflection and a light-sensitive receiver.

In practice, very small optical fibres (of glass or polymer material) are used in place of the glass rod. This gives a flexible 'cable' and lower losses than the simple arrangement of (c). The technique is called fibre optic transmission. Two types of fibre are commonly used: step index, which operates as in (c) with reflection taking place at a boundary; and graded index, where density varies in a uniform manner across the fibre, giving a gentler reflection, as in (d). Graded index fibre has a lower transmission loss but is more expensive to manufacture.

Light-emitting diodes (LEDs) are usually employed as transmit­ ters, although low powered lasers are often employed on long distance links. Photo avalanche diodes are commonly used as receivers. Digital encoding and transmission techniques are used. Elements of a typical system are shown in (e).

There's a number of advantages in using fibre optic transmission. In theory very large bandwidths are available; up to 10,000 times higher than the highest achievable radio frequency. Fibre optic cables are physically much smaller than conventional low loss coaxial cables. There is no electromagnetic interference from the fibres, and the signal is unaffected by external interference. Finally, if a fibre cable is damaged or broken there is no risk of fire or sparking. This latter characteristic makes fibre optic cable particu­ larly attractive for data transmission through hazardous areas in petrochemical plants and similar sites.

Losses in fibres occur from internal scattering off flaws and at bends where the angle of incidence can decrease. Minimum bending radii are determined by losses rather than physical strength. These losses are length related, and are typically 4 dB/km. Coupling losses, typically 2 dB per connection, also occur at the transmitter and receiver or where cables are jointed. In transmission systems over long distances repeaters are used.