Fullscreen HTML5Med Res (??MB)HTML4

SILICON CHIP Publisher & Editor-In-Chief Leo Simpson, B.Bus. Editor Greg Swain, B.Sc.(Hons.) Technical Staff John Clarke, B.E.(Elec.) Robert Flynn Advertising Manager Paul Buchtmann Regular Contributors Neville Williams, FIREE, VK2XV Bryan Maher, M.E. B.Sc. Jim Yalden, VK2YGY Garry Cratt, VK2YBX Jim Lawler, MTETIA John Hill David Whitby Photography Bob Donaldson Editorial Advisory Panel Philip Watson, MIREE, VK2ZPW Norman Marks Steve Payor, B.Sc., B.E. SILICON CHIP is published 1 2 times a year by Silicon Chip Publications Pty Ltd. All material copyright (c). No part of the contents of this publication may be reproduced without prior written consent of the publisher. Kitset suppliers may not photostat articles without written permission of the publisher. Typesetting/makeup: Magazine Printers Pty Ltd, Waterloo, NSW 2017. Printing: Masterprint Pty Ltd, Dubbo, NSW 2830. Distribution: Network Distribution Company. Subscription rate: currently $42 per year (12 issues) inside Australia. For overseas rates, refer to the subscription page in this issue. Liability: Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. Address all mail to: Silicon Chip Publications Pty Ltd, PO Box 139, Collaroy Beach, NSW 2097. Phone (02) 982 3935. ISSN 1030-2662 * Recommended and maximum Australian price only. 2 SILICON CHIP PUBLISHER'S LE'l-l'ER Deceiving the eye of the beholder This month we have a number of interesting constructional projects in the magazine but the one which is most appealing to us is the LED Message Board. We feel that this will be very popular among computer users because it is one of the few build-it-yourself computer peripherals that is really useful. Apart from its ability to be controlled by a personal computer, the LED Message Board is intriguing because of its wide variety of visual displays: scrolling up and down, wiping up and down and from side to side, flashing words and so on. It is all made possible by clever programming of the Message Board's ZBOA microprocessor but there is another interesting aspect - the use of multiplexed drive to the LEDs, all 672 of them. Multiplexed displays make use of the well-known persistence of vision of the human eye. If it were not for this persistence, movie films, TV screens and fluorescent lamps would flicker badly and virtually every digital display would be unreadable as the digits flashed individually on and off. Multiplexing digital displays achieves two economies. First, it reduces the power needed to produce a perceived brightness level in the display and secondly, it greatly reduces the number of connecting wires. In a typical 4-digit 7-segment display, a minimum of 11 drive wires is required if multiplexing is used, compared with 32 if each digit is driven separately. That is a big reduction but consider the Message Board: with 672 LEDs it would need a phenomenal number of drive wires and it would be difficult to get the variety of visual effects. With multiplexing and a clever serial to parallel data conversion it only needs a minimum of 8 signal drive wires. But perhaps the most intriguing aspect of the Message Board is not all the clever driving circuitry but the way in which the observer must "cooperate" with it in order to see the moving messages. Your eyes must follow the words as they move from right to left or from left to right. If you stare fixedly at one of these displays, you will only see a jumble of lights. So as well as exploiting the persistence of vision, message boards also exploit the natural tendency of our eyes to constantly scan our field of vision and to follow moving objects. Not only are our eyes being deceived but they are cooperating in the process. Leo Simpson