Flip-disc display

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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. Bistabile anzeigeelemente.jpg
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.
DOT-LED display of a bus Irisbus Citybus 18M (made 2004), photographed while a change is scrolling across the board. Irisbus, informacni panel.jpg
DOT-LED display of a bus Irisbus Citybus 18M (made 2004), photographed while a change is scrolling across the board.
Faulty dots are a typical malfunction of flip-disc displays Horejsi nabrezi, zarizeni pro provozni informace, cas 20.06.jpg
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 destination signs in buses across North America, Europe and Australia, as well as for variable-message signs on highways. It has also been used extensively on public information displays. [1] 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 2003.

Destination sign

A destination sign or destination indicator/destination blind is a sign mounted on the front, side or rear of a public transport vehicle, such as a bus, tram/streetcar or light rail vehicle, that displays the vehicle's route number and destination, or the route's number and name on transit systems using route names. The main such sign, mounted on the front of the vehicle, usually located above the windshield, is often called the headsign, most likely from the fact that these signs are located on the front, or head, end of the vehicle. Depending on the type of the sign, it might also display intermediate points on the current route, especially if the route is particularly long and its final terminus by itself is not very helpful in determining where the vehicle is going.

North America Continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere

North America is a continent entirely within the Northern Hemisphere and almost all within the Western Hemisphere. It is also considered by some to be a northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea.

Europe Continent in the Northern Hemisphere and mostly in the Eastern Hemisphere

Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, Asia to the east, and the Mediterranean Sea to the south. It comprises the westernmost part of Eurasia.



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.

Solenoid Invention by André-Marie Ampère

A solenoid is a coil wound into a tightly packed helix. The term was invented in 1823 by André-Marie Ampère to designate a helical coil.

Electrical polarity is a term used throughout industries and fields that involve electricity. There are two types of poles: positive (+) and negative (−). This represents the electrical potential at the ends of a circuit. A battery has a positive terminal and a negative terminal. Interconnection of electrical device nearly always require correct polarity to be maintained. Correct polarity is essential for the operation of vacuum tube and semiconductor devices, many electric motors, electrochemical cells, electrical instruments, and other devices.

Magnetic field spatial distribution of vectors allowing the calculation of the magnetic force on a test particle

A magnetic field is a vector field that describes the magnetic influence of electric charges in relative motion and magnetized materials. Magnetic fields are observed in a wide range of size scales, from subatomic particles to galaxies. The effects of magnetic fields are commonly seen in permanent magnets, which pull on magnetic materials and attract or repel other magnets. Magnetic fields surround and are created by magnetized material and by moving electric charges such as those used in electromagnets. Magnetic fields exert forces on nearby moving electrical charges and torques on nearby magnets. In addition, a magnetic field that varies with location exerts a force on magnetic materials. Both the strength and direction of a magnetic field vary with location. As such, it is an example of a vector field.

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.

Reed switch

The reed switch is an electrical switch operated by an applied magnetic field. It was invented at Bell Telephone Laboratories in 1936 by Walter B. Ellwood. In its simplest and most common form, it consists of a pair of ferromagnetic flexible metal reeds contacts in a hermetically sealed glass envelope. The contacts are usually normally open, closing when a magnetic field is present, or they may be normally closed and open when a magnetic field is applied. The switch may be actuated by an electromagnetic coil, making a reed relay, or by bringing a permanent magnet near the switch. When the magnetic field is removed, the reeds in the reed switch return to their original position.

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 1/2 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.

Electric current flow of electric charge

An electric current is the rate of flow of electric charge past a point or region. An electric current is said to exist when there is a net flow of electric charge through a region. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

Electric flux number of electric field lines through a certain surface

In electromagnetism, electric flux is the measure of the electric field through a given surface, although an electric field in itself cannot flow. It is a way of describing the electric field strength at any distance from the charge causing the field.

Voltage difference in the electric potential between two points in space

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

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 need changing to be flipped. This uses less power, and may be more robust.

Diode abstract electronic component with two terminals that allows current to flow in one direction

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most commonly used type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.


The flip-disc display was developed by Kenyon Taylor at Ferranti-Packard at the request of Trans-Canada Airlines (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. [1] Ferranti-Packard and Westinghouse both bid on the project, Westinghouse using an electro-luminescent technology. Ferranti won the contract after demonstrating the system with a mock-up they built in a disused warehouse across the street from the exchange's new offices, using hand-painted dots moved by hand to show how the system would work. The dots were slowly replaced with operating modules as they became available. The $700,000 system (equivalent to $6,000,000in 2018) was beset by delays and technical problems, but once it became fully operational it was considered very reliable.

The systems were relatively expensive because of their manual construction, typically completed by women who "sewed" the displays in a fashion very similar to the construction of core memory. Worse, Ferranti signed maintenance contracts that were, by 1971, losing $12,000 a month. [1] A re-organization of the engineering and maintenance department addressed the problems, and prices started to fall. By 1977 the system had won sales with half the world's major stock exchanges.

As prices fell, they were soon found in wider roles, notably that of highway signs and information systems for public transport. In Europe and in the US, vane displays based on the same technology became popular for displaying prices at gasoline stations. In 1974 Ferranti started a project to build smaller versions for the front of buses and trains, and by 1977 revenue from these had already surpassed that from other lines of business. [1] The displays often required minor maintenance to free up "stuck" discs.

Alternative technologies

DOT-LED display at night Nocni autobus 505, celni oznaceni.jpg
DOT-LED display at night

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 a small amount of power constantly rather than each time the message changes, but are easily visible in light and darkness and, having no moving parts, require little maintenance. [2]

Some producers offer combined displays which 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 East Europe. [3] This combined technology was used for outside displays of most of new buses and trams.


See also

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  1. 1 2 3 4 Norman Ball, John Vardalas, "Ferranti-Packard", McGill Queen's Press, 1994, ISBN   0-7735-0983-6
  2. Tucker, Joanne (September 2011). "The Wireless Age for Digital Destination Signage Arrives". Metro Magazine . Retrieved 2014-11-21.
  3. BUSE s. r. o. - Technology [ permanent dead link ]