One of the more impressive presentations of a gadget at a recent Maker Fair was a floor-mounted four-hoop device with LEDs along each hoop. The hoops were circular in shape as though they were the longitude lines on a globe. When the contraption was rotated and the LEDs driven under computer-based control, a 3D display appeared. The resolution was visibly low but at inventor shows, it is more the idea that counts than its immediate refinement. The idea was simple and perspicuous: you could quickly see how it worked.
This article presents another idea, that of a perspicuous (to engineers of ordinary perspicacity) desktop-sized LED electromechanical display like that of an oscilloscope display, avoiding custom mechanics and using items found in the usual household electronics shop and kitchen. Our starting reference is the well-established oscilloscope display. It is two-dimensional and oscilloscope makers have conventionally settled upon a format of 8 vertical by 10 horizontal major divisions with 5 (Tektronix) or 4 (generic H-P) minor divisions, or divs. Then the aspect ratio of oscilloscope CRT screens is H/V = 10/8 = 1.25 and its inverse is 8/10 or 0.8. Some artists, including Leonardo da Vinci in painting Mona Lisa, have thought that the golden ratio was the optimum aspect ratio:(1+
1/0.618. For NTSC television it is 1/0.75. The trace resolution of some Tek oscilloscopes (those with shorter and hence lower-performance CRTs) is about a half a minor div, resulting in a resolution of about 80 lines vertical and 100 horizontal. This resolution will be used as a basis for assessing alternative displays.
A first thought at a simple perspicuous electromechanical display is a matrix of LEDs. Small LEDs are cheap about $0.04 US in any quantity and can populate a circuit board to form a matrix with a resolution comparable to a CRT with 80 × 100, or 8000 LEDs. That seems excessive, and a display of half the resolution per axis, or 2000 LEDs, is still an arduous task to assemble. The LEDs alone would cost about $80 US.
A variation on this theme is to use instead 5 × 7 LED dot matrix displays. They are more costly per pixel because of the assembly and encapsulation. A matrix of them could be formed by rotating them horizontally to better fit the aspect ratio and stacking them 10 high (50 pixels vertical) by 10 wide (70 horizontal pixels), or 100 LED matrices. At $1.50 each surplus, the cost would be around $150 US a formidable amount resulting in a rather large display and a large (and costly) circuit-board.
The LED solution does not appear to meet the design constraints for a low-cost display. Indeed, Samsung and others are now selling large LED displays, and at large prices. Dot-matrix LCDs are an alternative, and for cost and size feasibility, graphic LCD displays are a good solution. However, they require a display driver and have high levels of digital integration that for some might violate the perspicuity criterion. I leave LCDs as the default possibility, one which has been chosen for most DSO products nowadays. It is not a bad choice for a low-cost display but is very different from a CRT.