thumb|right|Flip-disc display elements (close up). The disc rotates on the shaft that is carried in the two triangular posts. The magnet that powers the rotation can be seen embedded in the disc. Under the disc is the driving solenoid; when powered, a field is induced into the two posts, flipping the discs. Rotation stops when the disc hits the post.
thumb|DOT-LED display of a bus Irisbus Citybus 18M (made 2004), photographed while a change is scrolling across the board
thumb|[[Defective pixel|Faulty dots are a typical malfunction of flip-disc displays.]]
The flip-disc display (or flip-dot display) is an electromechanical dot matrix display technology used for large outdoor signs, normally those that will be exposed to direct sunlight. Flip-disc technology has been used for external destination signs on buses and trains across North America, Europe, Australia and Asia, as well as for variable-message signs on highways. It has also been used extensively on public information displays. A few game shows have also used flip-disc displays, including Canadian shows like Just Like Mom, The Joke's on Us and Uh Oh!, but most notably the American game show Family Feud from 1976 to 1995, and its British version Family Fortunes from 1980 to 2002. The Polish version of Family Feud, Familiada, still uses this board, which was bought from the Swedish version of the show.
Design
The flip-disc display consists of a grid of small metal discs that are black on one side and a bright color on the other (typically white or day-glo yellow), set into a black background. With power applied, the disc flips to show the other side. Once flipped, the discs will remain in position without power.
The disc is attached to an axle which also carries a small permanent magnet. Positioned close to the magnet is a solenoid. By pulsing the solenoid coil with the appropriate electrical polarity, the permanent magnet on the axle will align itself with the magnetic field, also turning the disc. Another style uses a magnet embedded in the disc itself, with separate solenoids arranged at the ends or side to flip it.
A computerized driver system reads data, typically characters, and flips the appropriate discs to produce the desired display. Some displays use the other end of the solenoid to actuate a reed switch, which controls an LED array behind the disc, resulting in a display that is visible at night but requires no extra drive electronics.
Various driving schemes are in use. Their basic purpose is to reduce the amount of wiring and electronics needed to drive the solenoids. All common methods connect the solenoids in some sort of matrix. One driving method is similar to that of core memory: the solenoids are connected in a simple matrix. Those solenoids at the crossing point of two powered wires are driven with enough current to flip their discs; those powered on only the vertical or horizontal line see only half of the required force (as flux is proportional to current, which in turn is proportional to the voltage). Those on unpowered lines also do not flip.
Typically, the driving scheme 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.
Other driving schemes use diodes to isolate non-driven solenoids, which allows only the discs whose state needs changing to be flipped. This uses less power and may be more robust.
History
The flip-disc display was developed by Kenyon Taylor at Ferranti-Packard at the request of Trans-Canada Air Lines (today's Air Canada). By the time the system had been patented in 1961, TCA had already lost interest and Ferranti's management didn't consider the project very interesting.
The first big opportunity for this system came in 1961 when the Montreal Stock Exchange decided to modernize its method of displaying trading information.
Some producers offer combined displays that use flip-dot and LED technologies together (every dot-disc has its own LED) and thereby they combine their advantages. For example, the Czech company BUSE from Blansko supplies self-patented DOT-LED displays (only DOT and only LED as well) in Central and Eastern Europe. This combined technology was used for outside displays of most of new buses and trams.
Application
<gallery mode="packed" heights="120px">
File:Tatra KT8D5.N2P, čelní orientace, 9 Spojovací.jpg|Destination and line number signs on public transport vehicles (tram in Prague, Czech Republic)
File:Esko S7 Praha hlavní nádraží – Řevnice.jpg|Destination and line number signs on public transport vehicles (a suburban train in Prague, Czech Republic)
File:Elbfähre Bad Schandau, Tafel.jpg|Departure boards at public transport stations and terminals (Elbe ferry Bad Schandau, Germany)
File:Děčín, autobusové nádraží, displej odjezdů.jpg|Departure boards at public transport stations and terminals (bus station in Děčín, Czech Republic)
File:Podbabská, zastávka Podbaba, displeje.jpg|Line signs at public transport stops (Prague, Czech Republic)
File:Proměnná tabule Radlická.jpg|Road transport information (Prague, Czech Republic)
File:2002 D4500 2905.jpg|Destination sign on a MCI D4500 operating for the Metropolitan Transportation Authority (New York City, United States)
File:Richard_Dawson_Family_Feud_1976.JPG|Ferranti-Packard display board used on the original version of Family Feud
</gallery>
See also
- Digital micromirror device
- History of display technology
- Vane display, a display using a similar mechanism but configured as a 7-segment display