Film switching system for mutually isolated circuits

Claim 2

A switching system for simultaneously connecting selected respective impedances in a plurality of mutually isolated circuits, said switching system comprising a plurality of groups of film switches, each such film switch comprising a temperaturesensitive resistOr formed as a film of resistive material extending between two terminals and having a resistivity that changes by at least about two orders of magnitude when the temperature thereof goes through a transition temperature that is substantially above ambient temperature, and a heater element of electrically conductive material extending between two terminals and thermally coupled to said film of resistive material and electrically insulated therefrom, the temperature-sensitive resistors of each group of film switches being coupled to respective ones of said impedances for a respective isolated circuit, power supply means for supplying current through said terminals of said heater elements to heat the respective films of resistive material substantially above the transition temperature of said material, and selective means for selectively connecting a heater element of one of said groups of film switches in series with a heater element of another of said groups of film switches and to said power supply means thereby placing a respective selected impedance in each of said mutually isolated circuits.

Claim 3

In a multifrequency dial signal generator for a telephone set including a plurality of tone generators, each such tone generator including a plurality of frequency determining elements, a switching system comprising a plurality of groups of film switches, each such film switch comprising a temperature-sensitive resistor formed as a film of resistive material extending between two terminals and having a resistivity that changes by at least about two orders of magnitude when the temperature thereof goes through a transition temperature that is substantially above ambient temperature, and a heater element of electrically conductive material extending between two terminals and thermally coupled to said film of resistive material and electrically insulated therefrom, the temperature-sensitive resistors of each group of film switches being coupled to respective ones of said frequency determining elements of a respective tone generator and acting to place the frequency determining elements selectively in circuit to produce different predetermined frequencies in the respective tone generator, power supply means for supplying current through said terminals of said heater elements to heat the respective films of resistive material substantially above the transition temperature of said material, and selective means for selectively connecting a heater element of one of said groups of film switches in series with a heater element of another of said groups of film switches and to said power supply means whereby said respective tone generators operate at respective selected predetermined frequencies.

Claim 4

In a multifrequency dial signal generator for a telephone set including a plurality of tone generators, each such tone generator including a plurality of frequency determining elements, a switching system comprising a plurality of groups of film switches, each such film switch comprising a temperature-sensitive resistor formed as a film of resistive material extending between two terminals and having a resistivity that changes by at least about two orders of magnitude when the temperature thereof goes through a transition temperature that is substantially above ambient temperature, and a heater element of electrically conductive material extending between two terminals and thermally coupled to said film of resistive material and electrically insulated therefrom, the temperature-sensitive resistors of each group of film switches being coupled to respective ones of said frequency determining elements of a respective tone generator and acting to place the frequency determining elements selectively in circuit to produce different predetermined frequencies in the respective tone generator, power supply means for supplying current through said terminals of said heater elements to heat the respective films of resistive material substantially above the transition temperature of said material, and selective means for selectively connecting a heater element of one of said groups of film switches in series with a heater element of another of said groups of film switches and to said power supply means whereby said respective tone generators operate at respective selected predetermined frequencies.

Claim 5

The switching system of claim 2 wherein said selective means includes a plurality of single closure switches each operating when closed to connect a unique combination of heater elements in series whereby upon closure of each such single closure switch said tone generator operates at a unique combination of different frequencies.

Claim 6

The switching system of claim 3 including a plurality of push buttons each associated with a respective one of said single closure switches for actuation thereof.

Claim 7

The switching system of claim 2 including an output enabling gate for transmitting the outputs of said tone generators when enabled by an enabling signal, and an additional film switch like the other film switches with its heater element connected in series with the heater elements of one of said groups of film switches and with its temperature-sensitive resistor coupled to said enabling gate for producing an enabling signal therein upon closure of one of said single closure switches.

Claim 8

The switching system of claim 2 wherein the temperature-sensitive resistors of a group of film switches are coupled to respective taps on a tapped series of resistors comprising said frequency determining elements.