Abstract: This invention relates to an electronic ringing circuit for telephone systems. The inventive circuit provides a dual tone output with the tones changing at a predetermined rate. The circuit consists of a rectifying circuit connected across the telephone lines which receive the normal 90VAC/20 Hz telephone ringing signals. The rectified signal is then connected to one input terminal of a voltage comparator, the other terminal of which is connected to a reference voltage source. When a ringing signal is present, the rectified signal will be greater than the voltage of the reference source so that the voltage comparator will provide an output. This output is fed to the control terminals of electronically controlled switches, and when the switches are closed, the oscillators will be grounded through frequency determining capacitors associated with the respective oscillators.

Abstract: The invention relates to an electronic circuit for a speakerphone which, when in the balanced condition, will eliminate acoustic feedback as between the microphone and the speaker of the speakerphone. The arrangement consists of a bridge having four legs. Input from telephone lines are fed into the bridge between two adjacent points (i.e. across one leg) of the bridge. The microphone is connected between one of the input points and a point opposite to the one input point. The legs on either side of the opposite point of the microphone connection contain equal impedances, and the fourth leg, between the input leg and one of the equal impedance legs, contains a variable impedance. To balance the bridge, the variable impedance is adjusted such that the microphone signal, appearing in opposite sides of the bridge, will be canceled out and thereby prevented from exiting on the speaker circuits. However, because of the imbalance relative thereto, the microphone signal will be passed up the telephone lines.