Keypad caller for a telephone set

Claim 1

In a keypad caller for an automatic telephone set including a keypad with at least ten keys, each suitable for putting one column out of four in contact with one row out of four, the columns being supplied from telephone line voltage via a fullwave rectifier bridge and a voltage regulator, a ringing circuit of the set being upstream from said bridge and the microphone and receiver circuits being downstream therefrom, a two-frequency oscillator, said keypad including at one end of each of said rows a transistorized switch for earthing a corresponding resistance of said two-frequency oscillator which emits one of the four lower frequencies and including at the opposite end to the voltage regulator of each of said columns, a transistorized switch for connecting to the low impedance of said regulator a corresponding resistance of said two-frequency oscillator which further emits one of the four higher frequencies, the emission of the group of two frequencies, lower and higher, being made possible only by contact of said key, the improvement comprising: first and second comparators, and wherein the other ends of said rows are connected to the non-inverting inputs of said first and second comparators in parallel whose inverting inputs are connected to said voltage regulator and whose respective outputs constitute a positive and negative voltage supply source of said two frequency oscillator only when a single key is depressed, said first and second comparators both supplying a negative voltage which inhibits said two-frequency oscillator when no key is depressed and both supplying a positive voltage which inhibits the two-frequency oscillator when two keys are depressed simultaneously, and a third comparator whose negative or inverting input receives the output signal from said first comparator and whose positive or non-inverting input is connected to the common point of two resistances disposed between earth and said voltage regulator, so as to deliver an on/off control voltage depending on whether at least one key is depressed or released to a first transistorized switch disposed to conduct in a first direction between the microphone-receiver and one of the wires of the telephone line and wherein said first comparator delivers an on/off control voltage depending on whether at least one key is depressed or released to a second transistorized switch disposed in parallel with said first switch to conduct in the opposite direction to said first direction, thereby providing an analog inhibiting system for the two-frequency oscillator when two keys are depressed simultaneously.