In many respects, an LCD module is an ideal display; easy to use, available in a wide choice of sizes and capacity and supported in MBasic with one easy-to-use function, LCDWrite. But, as good as LCD modules are, there are times when they just won't do. Chapter 7 looks at an earlier but still common technology, seven-segment LED displays. (Indeed, much of what we learn in this chapter may be directly applied to an even older technology, the Nixie tube.) Seven-segment displays have been built with other technologies, including incandescent filament and vacuum fluorescent, but these are rarely used today.

LED displays are available in several formats, including dot matrix (5x7, 8x8 and other pixel arrangements) and alphanumeric multisegment (16-segment, for example). We're going to work with a simpler device, though, the humble 7-segment numeric display, intended to display the digits 0 9. (It's possible, after a fashion, to display the letters "a","b","c","d","E" and "F" on a seven-segment display, should you desire.) The particular device used in this chapter is an Agilent Technologies HDSP-5721, two digit, common anode yellow LED display, with a 0.56" character height. Any common anode display may be substituted for the HDSP-5721 with, at most, minor changes to the circuitry. (The main difference will be the value of current limiting resistors.) We'll also use a variant seven-segment display in Chapter 11.