DATAMATH  CALCULATOR  MUSEUM

This invention relates to hand-held calculators, and in particular it is concerned with a calculator having transparent background for a liquid crystal display and preferably also for number and function input keys, enabling the calculator to be used with an overhead projector to demonstrate operations of the calculator, along with calculated results, on a screen to a classroom or group.

U.S. Pat. No. 4,154,007 disclosed a hand-held calculator having a transparent window within which a liquid crystal display. (LCD) is positioned. This enabled light to be projected through the LCD window and onto a screen or wall to reveal the numbers or symbols as dark elements on the screen with the background left white. The calculator disclosed in the patent was particularly useful with an overhead projector in a classroom, whereby numerical results obtained using the calculator resting on the light platform of the projector could be projected for all in the classroom to view.

Since the above patent was issued, there has been a development by Epson of a pressure-sensitive conductor arrangement useful to produce a clear touch key panel, wherein the pressure of a finger on a key member printed on the back of a transparent membrane will effect sufficient change in conductivity in etched channels of a transparent plate below to act as a pressure switch. The slight change in conductivity is sensed by a circuit which effects a desired result. This technology was dedicated to the public in 1985.

This transparent-key technology has been applied to calculators of the type disclosed in the above referenced patent, with number keys and function are transparent except for the display of a symbol of the number or function. Thus, most of the calculator was transparent and could be projected onto a screen or wall with an overhead projector. In this way, not only numerical results but also the entry of numbers and the pressing of function or operation keys could be demonstrated on the screen using the overhead projector. One such calculator has been marketed by the present applicant, under the trademark "The Educator". A model of that calculator has employed a heavy, opaque grid of lines outlining and delimiting the number and function keys.

Such transparent calculators had certain problems. The transparent key technology required that a glass laminate be used, making the calculator quite fragile if dropped. The calculator could be inadvertently pushed off the glass surface of an overhead projector, for example.

Another problem with these calculators has been heat buildup from the lamp in the overhead projector. Liquid crystal displays are quite susceptible to temperature change, and an increase in temperature will often cause changes in the display and misreadings in the displayed numbers of an LCD calculator of the type described.

A further problem has been in relation to the accuracy of the keys to the applied finger pressure. Due to the nature of the circuitry for implementing these transparent keys, when keys are made smaller and closer together there tends to be some crossover of the effect of pressure on a key, particularly when the pressure is not applied precisely centrally. The result is inaccuracy in the entry of the data or functions to be calculated.

SUMMARY OF THE INVENTION

In accordance with the present invention, a substantially transparent calculator for overhead projection includes new features which overcome the problems described above.

As in previous calculators described above, the hand-held calculator of this invention has a transparent window carrying a liquid crystal display, and also preferably transparent number and function key locations such that when placed on an overhead projector, the calculator can be used to demonstrate to a classroom all operations performed in a sequence of mathematical calculations.

In a preferred embodiment of the present invention, the plastic and glass casing of the calculator has one or more small holes passing through the casing at corners, so that a string or cord (such as a fine nylon fishing line) can be used to tether the calculator to the overhead projector, preventing damage to the calculator from a fall to the floor.

Another feature of the calculator is an improved keyboard of relatively small domain size for each key, with a spacer grid on the back surface of the keyboard overlay element or membrane panel. This spacer grid is effective to provide pressure isolation of an active key from adjacent keys, thereby to prevent mistaken entries. it also prevents extraneous pressures on a preferred black divider grid from being read as key entries by contact with circuit trace paths on the rigid plate below.

A further feature of the calculator is a protrusion or foot at each corner of the back of the calculator casing, for holding the back surface of the calculator slightly above the light platform of the overhead projector. It has been found that this small spacing will enable the liquid crystal display to be cooled by air passing between the light table and calculator, thus avoiding overheating of the LCD display and consequent malfunctioning. In a preferred embodiment the protrusions or feet are rubbery for high-friction engagement with the surface of the projector light table.

Finally, the calculator in one preferred embodiment has a plurality of different transparent background colors for different groups of the transparent function keys, so that in following the operations of the calculator on the screen, the observer can more quickly identify a function by the color group in which the instructor's finger appears in carrying out the calculation. The instructor similarly is aided by the color grouping.

It is therefore among the objects of the invention to overcome problems of previous transparent calculators with features which add to the accuracy and reliability of the calculator, avoid damage to the calculator and provide a more colorful display of the calculator on a screen so as to assist visually a class of observers in following the demonstrated operation and functions carried out. These and other objects, advantages and features of the invention will be apparent from the following description of preferred embodiments considered along with the accompanying drawings.