Flip-disc display
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The flip-disc display (or flip-dot display) is a display technology used for large outdoor signs, normally those that will be exposed to direct sunlight. Flip-disc displays became common in the 1980s and 90s, but have since been superceeded by high-intensity LED matrix displays.[citation needed]
The flip-disc display consists of a grid of small metal discs painted black on one side and a bright color on the other (typically white or day-glo yellow), set into a black background. In response to a microcontroller, the disc can be flipped to show either the colored or black side, using the colored side to form characters.
The basic operation is similar to that of core memory; a grid of X and Y wires carrying current generates a magnetic field where they cross, interacting with a small magnet on the disk's hinge. Only those discs at the crossing point of two powered wires will see enough force to flip over, those on the powered X or Y line only will see only 1/2 the required force. Once flipped, the disks stay in that state and no further power is needed to maintain it. Typically the display works its way from top to bottom, powering each horizontal line "on" and then powering the needed vertical lines to set up that row. The whole process takes a few seconds, during which time the sound of the discs being flipped over is quite distinctive.
The flip-disc display was developed by Kenyon Taylor at Ferranti-Packard in the 1970s. The displays were initially very expensive[citation needed], and their first applications were in stock markets around the world. As their price fell, they were soon found in wider roles, notably that of highway signs and information systems for public transport. The displays often required minor maintenance to free up "stuck" discs.
Flip-disc systems are still widespread but are not often found in new installations. Their place has been filled by LED-based products, which use power constantly but are easily visible in light and darkness and, having no moving parts, require little maintenance.[citation needed]
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