Failsafe electrical circuitry

Claim 2

An electrical control circuit comprising an input terminal; first amplifier means arranged to amplify signals received at said input terminal and to provide an output in accordance therewith; first inhibiting means arranged to inhibit operation of said first amplifier means; second amplifier means arranged to receive as an input said output of said first amplifier means and to provide an output in accordance therewith; second inhibiting means arranged to inhibit operation of said second amplifier means; first comparator means arranged to nullify said first and second inhibiting means in response to the value of said signals received at said input terminal lying between two predetermined limits; second comparator means arranged to receive said output of one of said first and second amplifier means and to be conductive in response to the value of said one amplifier means output lying between two predetermined limits; and third comparator means arranged to receive said output of the other of said first and second amplifier means and to be conductive in response to said other amplifier means output exceeding a predetermined value; said electrical control circuit deriving from said signals received at said input terminal a control output comprising signals derived from conduction of said second and third comparator means.

Claim 3

A control circuit according to claim 1, wherein said second comparator means is arranged to receive the output of said first amplifier means, and said third comparator means is arranged to receive the output of second amplifier means.

Claim 4

A control circuit according to claim 1, wherein said first and second inhibiting means each comprises a depletion-mode field-effect transistor.

Claim 5

A circuit according to claim 1, for use as a fail-safe control circuit for an electrical system, comprising a light-emitting diode arranged to be energized on supply of electrical power to said electrical system, and a phototransistor associated with said diode, said phototransistor being electrically connected in series between said first comparator means and said first and second inhibiting means and arranged to be conductive thereby to nullify said first and second inhibiting means only when said light-emitting diode is energized simultaneously with the value of said signals at said input terminal lying between two predetermined limits as determined by said first comparator means.

Claim 6

A control circuit according to claim 1, comprising flip-flop means arranged to be conductive in response to output signals received from both the second and third comparator means to provide, together with said output signals of said third comparator means, said control output of the circuit.

Claim 7

A control circuit according to claim 5, wherein said flip-flop means is a D-type flip-flop.

Claim 8

A circuit according to claim 6, for use as a fail-safe control circuit for an electrical system, comprising light-emitting diodes arranged to be energized in response to output signals of said flip-flop means and said third comparator means respectively, and respective phototransistors associated with said light-emitting diodes, whereby conduction by said phototransistors provides said control output enabling operation of said electrical system.

Claim 9

A circuit according to claim 7, comprising further transistors electrically connected across respective ones of said phototransistors, and means screening said further phototransistors from said light-emitting diodes, thereby to eliminate the effect of ambient light incident on said phototransistors.

Claim 10

A control circuit according to claim 7 wherein the amplifier means comprises two amplifiers in series with each other and the inhibiting means comprises two inhibiting arrangements associated with respective ones of the amplifiers.

Claim 11

A control circuit according to claim 9, wherein each inhibiting arrangement comprises a depletion mode field effect transistor.

Claim 12

A control circuit according to claim 9, wherein further comparator means is arranged to receive output signals from one of the amplifiers and to be conductive in response to the value of said output signals lying between two predetermined limits.

Claim 13

A control circuit according to claim 1, comprising flip-flop means arranged to be conductive in response to output signals recieved from both the further comparator means and said comparator.

Claim 14

A control circuit according to claim 12, wherein said flip-flop means is a D-type flip-flop.

Claim 15

A control circuit according to claim 11, wherein said further comparator means comprises two comparators connected in parallel with each other.

Claim 16

A control circuit according to claim 7, wherein the control output is in optical form.

Claim 17

A control circuit according to claim 15, wherein the optical output is derived from light-emitting diode means.

Claim 18

An electrical system comprising switch means for conducting electrical power thereto, first light-emitting diode means conductive on closure of said switch, two independent phototransistor switch means, said system being operable only on conduction by said three switch means; the system further comprising an input terminal for receiving signals for deriving optical outputs for supply to said phototransistor means, amplifier means for amplifying signals received at said input terminal, depletion mode field effect transistor means arranged to inhibit operation of the amplifier means, first comparator means for receiving said input signals and providing a nullifying signal for switching off said field-effect transistor means when the value of said input signals is between predetermined limits, further phototransistor means arranged to conduct said nullifying signal in response to light from said first light-emitting diode means, thereby to enable operation of said amplifier means, second comparator means for conducting a first output signal from said amplifier means when said first output signal lies between predetermined limits, third comparator means for conducting a second output signal from said amplifier means when said output signal exceeds a predetermined value, flip-flop means connected to receive said first and second output signals from the second and third comparator means respectively, and to provide an output signal in response thereto, second light-emitting diode means for deriving an optical output response to said second output signal of said third comparator means, third light-emitting diode means for deriving an optical output in response to said output signal of said flip-flop means, and wherein said two phototransistor switch means each comprises a pair of phototransistors, one phototransistor of one said pair being exposed to said second light-emitting diode means and the other phototransistor of said one pair being shielded from said second light-emitting diode means, one phototransistor of the other said pair being exposed to said third light-emitting diode means and the other phototransistor of said other pair being shielded from said third light-emitting diode means, each said other phototransistor of said pairs shutting off its respective said one phototransistor upon the receipt of ambient light thereby to prevent spurious operation of said phototransistor switch means.

Claim 19

An electrical system comprising switch means for conducting electrical power thereto, first light-emitting diode means conductive on closure of said switch, two independent phototransistor switch means, said system being operable only on conduction by said three switch means; the system further comprising an input terminal for receiving signals for deriving optical outputs for supply to said phototransistor means, amplifier means for amplifying signals received at said input terminal, depletion mode field effect transistor means arranged to inhibit operation of the amplifier means, first comparator means for receiving said input signals and providing a nullifying signal for switching off said field-effect transistor means when the value of said input signals is between predetermined limits, further phototransistor means arranged to conduct said nullifying signal in response to light from said first light-emitting diode means, thereby to enable operation of said amplifier means, second comparator means for conducting a first output signal from said amplifier means when said first output signal lies between predetermined limits, third comparator means for conducting a second output signal from said amplifier means when said output signal exceeds a predetermined value, flip-flop means connected to receive said first and second output signals from the second and third comparator means respectively, and to provide an output signal in response thereto, second light-emitting diode means for deriving an optical output response to said second output signal of said third comparator means, third light-emitting diode means for deriving an optical output in response to said output signal of said flip-flop means, and wherein said two phototransistor switch means each comprises a pair of phototransistors, one phototransistor of one said pair being exposed to said second light-emitting diode means and the other phototransistor of said one pair being shielded from said second light-emitting diode means, one phototransistor of the other said pair being exposed to said third light-emitting diode means and the other phototransistor of said other pair being shielded from said third light-emitting diode means, each said other phototransistor of said pairs shutting off its respective said one phototransistor upon the receipt of ambient light thereby to prevent spurious operation of said phototransistor switch means.

Claim 20

A fail-safe control circuit for an electrical system, comprising an input terminal; two integrating amplifiers connected to one another in series and arranged to amplify signals received at said input terminal; a respective depletion-mode field-effect transistor arranged to inhibit operation of said amplifiers; first and second comparators arranged to provide output signals in response to the value of said input terminal signals being respectively above and below predetermined lower and upper values; a light-emitting diode arranged to be energized when electrical power is supplied to said electrical system; and a phototransistor, the phototransistor being connected so as to be electrically conductive when supplied with said output signals of both said comparators and light from said light-emitting diode, whereby said amplifiers integrate said input terminal signals and generate a control output enabling operation of said electrical system.

Claim 21

A fail-safe control circuit for an electrical system, comprising an input terminal; two integrating amplifiers connected to one another in series and arranged to amplify signals received at said input terminal; a respective depletion-mode field-effect transistor arranged to inhibit operation of said amplifiers; first and second comparators arranged to provide output signals in response to the value of said input terminal signals being respectively above and below predetermined lower and upper values; a light-emitting diode arranged to be energized when electrical power is supplied to said electrical system; and a phototransistor, the phototransistor being connected so as to be electrically conductive when supplied with said output signals of both said comparators and light from said light-emitting diode, whereby said amplifiers integrate said input terminal signals and generate a control output enabling operation of said electrical system.

Claim 22

A fail-safe control circuit for an electrical system comprising an input terminal; amplifier means arranged to amplify signals received at said input terminal; inhibit means arranged to inhibit operation of said amplifier means; two comparators arranged to provide output signals to nullify said inhibit means in response to the value of said input signals being respectively above and below predetermined lower and upper values; light-emitting diode means arranged to receive said amplified signals from said amplifier means and to derive an optical output therefrom; and phototransistor means arranged to receive said light output and to derive therefrom a control output for enabling operation of said electrical system.

Claim 23

A fail-safe control circuit for an electrical system comprising an input terminal; amplifier means arranged to amplify signals received at said input terminal; inhibit means arranged to inhibit operation of said amplifier means; two comparators arranged to provide output signals to nullify said inhibit means in response to the value of said input signals being respectively above and below predetermined lower and upper values; light-emitting diode means arranged to receive said amplified signals from said amplifier means and to derive an optical output therefrom; and phototransistor means arranged to receive said light output and to derive therefrom a control output for enabling operation of said electrical system.

Claim 24

An electrical system according to claim 19, wherein said phototransistor means has associated therewith further phototransistor means, and means shielding said further phototransistor means from said optical control output, so as to cancel the effect of any ambient light.

Claim 25

An electrical system according to claim 19, wherein said light-emitting diode means comprises two independent light-emitting diodes, and said phototransistor means comprises two phototransistors associated with respective ones of the light-emitting diodes.

Claim 26

An electrical system according to claim 19, wherein said inhibiting means comprises depletion mode field effect transistor means.