Flip-disc display

matrix displays.Fact|date=November 2007

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. The disc can be flipped to show either the colored or black side, using the colored side to form characters. Once flipped, the disks stay in that state and no further power is needed to maintain it.

The disk is attached to an axle which also carries a small permanent magnet. The magnet is close to a solenoid with a magnetic core with a high remanence. By pulsing the winding of the solenoid with the appropriate current polarity the core can be magnetised with either polarity of magnetic field, which causes the permanent magnet on the axle to align itself with the field, also turning the disk.

Some displays use the other end of the solenoid to actuate a reed switch, which is used to control a LED array behind the disk, resulting in a display that is visible at night that requires to extra drive electronics.

Various driving methods are in use, the basic purpose of them is to reduce the amount of wiring and electronics necessary to drive the solenoids. All common methods connect the solenoids in some sort of matrix. 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.

One driving method is similar to that of core memory; the solenoids are connected in a simple matrix. Only those solenoids at the crossing point of two powered wires are driven with enough current to flip their disks over, those on the powered X or Y line only will see only 1/4 of the required force (as flux is porportional to current which is proportional to the square of the voltage).

Other driving schemes use diodes to isolate non-driven solenoids, which allows only the disks whose state needs changing to be flipped, which certainly uses less power, and may be more robust.

The flip-disc display was developed by Kenyon Taylor at Ferranti-Packard in the 1970s. The displays were initially very expensiveFact|date=November 2007, 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 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.Fact|date=November 2007

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