Pushbutton telephone dial

Claim 1

A telephone station dial comprising: a multifrequency tone generator having plural input paths each for receiving an initiating signal to generate a tone, a plurality of detector circuits and an RF oscillator connected to each said detector circuit, a control circuit connected between said source and and each said detector circuit, each control circuit comprising a rigid fixed capacitor plate and a movable compliant capacitor plate, means including a pushbutton for bringing each said movable plate toward and, when unobstructed, into contact with, the corresponding fixed plate thereby to effect a sufficient impedance change in said control circuit to cause said threshold value to be exceeded, and each said detector circuit comprising first, second and third FETs each having gate, source and drain electrodes; said first FET gate connected to said oscillator output; a capacitor connected across said second FET to a bias voltage source; said first FET drain, said second FET source, said third FET gate and said capacitor being commonly connected; means serially connecting said third FET source and drain electrodes in a selected said tone generator path: means driving said oscillator output voltage peaks above the threshold voltage of said first FET when the associated said button is undepressed, and below said threshold when sufficiently depressed; and means including said capacitor responsive to the latter condition for driving said third FET gate above its threshold, thereby to apply said initiating signal to said path.