Two-tone generator using switching transistors in LC resonant circuits controlled by push-button

Claim 1

A tone generator, for producing a multifrequency signal in the form of two selectable tone signals, comprising: a two-tone oscillator having two LC resonance circuits each comprised of a coil and a capacitor; a plurality of switching transistors having the collector-emitter paths thereof coupled to corresponding ones of said two resonance circuits to switch the values of the inductances of the coils in response to the turning ON of the collector-emitter paths, said switching transistors comprising a first group of NPN transistors having their respective collector-emitter paths each connected in common between the capacitor and respective taps progressively along the coil comprising a first of said resonance circuits, and a second group of PNP transistors having their respective collector-emitter paths each connected in common between the capacitor and respective taps progressively along the coil comprising the second of said resonance circuits; a plurality of switches individually controllable and each having a contact connected in series between the bases of a respective pair of transistors one of which is respectively included in the first group and one in the second group; and current means electrically coupled to said switching transistors for flowing, in the base-emitter paths of respective selected ones of said switching transistors of the two LC resonance circuits, base currents greater than the peak values of respective AC currents developed in the two LC resonance circuits to turn-ON said selected ones of said switching transistors and switch the inductances of said coils thereby to select the tones developed by said oscillator, said selected switching transistors being selected by closure of the contact of a selected one of the switches to electrically connect their bases and complete a flow path for the current supplied by said current means.