Abstract: A circuit tests the telephone line for polarity, line voltage and open/short circuit. The polarity test is accomplished using opposite polarity LED elements which generate a visual indication of correct or erroneous polarity. The line voltage is tested by comparators which compare the line voltage to predetermined limits and generate a visual indication if the voltage is within or without the limits. The tip or ring ground short circuit is detected by monitoring the AC voltage while in the ON HOOK state. If a ground short is present on either tip or ring of the telephone line, the AC voltage becomes high due to power frequency coupling. A comparator detects this voltage and generates a visual indication if the voltage is greater than a predetermined value.

Abstract: A two line telephone available for the home or the small business employs five pushbuttons for line selection, conference, hold and intercom. The line circuits and the conferencing switches are opto-isolated so that a single transformer coupled power supply is used. The usual illumination is provided in all of the pushbuttons to indicate that their function is being implemented. The conference function is implemented using the transformer coupled SCR circuit of U.S. Pat. No. 3,470,867.