The flip-disc display is an electromechanical dot matrix display

Dot matrix display

A dot matrix display is a display device used to display information on machines, clocks, railway departure indicators and many other devices requiring a simple display device of limited resolution...

 technology used for large outdoor signs, normally those that will be exposed to direct sunlight. Flip-disc technology has been used on buses across North America

North America

North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

. It has also been used extensively on public information displays. A few game shows have also used flip-disc displays, most notably Family Feud

Family Feud

Family Feud is an American television game show created by Mark Goodson and Bill Todman. Two families compete against each other in a contest to name the most popular responses to a survey question posed to 100 people...

from 1976-1995. (Eggcrate displays were used from 1999-2006, and LCD screens since 2002.)

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

Solenoid

A solenoid is a coil wound into a tightly packed helix. In physics, the term solenoid refers to a long, thin loop of wire, often wrapped around a metallic core, which produces a magnetic field when an electric current is passed through it. Solenoids are important because they can create...

. By pulsing the solenoid coil with the appropriate electrical polarity

Electrical polarity

Electrical polarity is present in every electrical circuit. Electrons flow from the negative pole to the positive pole. In a direct current circuit, one pole is always negative, the other pole is always positive and the electrons flow in one direction only...

, the permanent magnet on the axle will align itself with the magnetic field

Magnetic field

A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...

, 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

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 W. B. Ellwood. It consists of a pair of contacts on ferrous metal reeds in a hermetically sealed glass envelope...

, which controls an LED

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

 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

Electric current

Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...

 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

Voltage

Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...

). 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 diode

Diode

In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...

s 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.

History

The flip-disc display was developed by Kenyon Taylor

Kenyon Taylor

Kenyon Taylor was a British electrical engineer and inventor. Taylor was part of the original Ferranti Canada, working on the Royal Canadian Navy's DATAR system in the late forties and early fifties assisting in developing the world's first trackball. In the seventies, he developed the flip-disc...

 at Ferranti-Packard

Ferranti-Packard

Ferranti-Packard Ltd. was the Canadian division of Ferranti's global manufacturing empire, formed by the 1958 merger of Ferranti Electric and Packard Electric...

 at the request of Trans-Canada Airlines (today's Air Canada

Air Canada

Air Canada is the flag carrier and largest airline of Canada. The airline, founded in 1936, provides scheduled and charter air transport for passengers and cargo to 178 destinations worldwide. It is the world's tenth largest passenger airline by number of destinations, and the airline is a...

). 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. Ferranti-Packard and Westinghouse

Westinghouse Electric (1886)

Westinghouse Electric was an American manufacturing company. It was founded in 1886 as Westinghouse Electric Company and later renamed Westinghouse Electric Corporation by George Westinghouse. The company purchased CBS in 1995 and became CBS Corporation in 1997...

 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 (US$4,964,772 in 2009 dollars) 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. 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 they 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. The displays often required minor maintenance to free up "stuck" discs.

Alternative technologies

Flip-disc systems are still widespread but are not often found in new installations. Their place has been filled by LED

LEd

LEd is a TeX/LaTeX editing software working under Microsoft Windows. It is a freeware product....

-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.

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

Blansko

Blansko is a town in Blansko District in the South Moravian Region of the Czech Republic. Blansko is the biggest town in Blansko District.Blansko was mentioned for the first time as a castle in 1141, while a town began to develop near it by 1277...

supplies self-patented DOT-LED displays (only DOT and only LED as well) in Central and East Europe. This combined technology was used for outside displays of most of new buses and trams.