Schaltungsanordnung zur polaritaetsrichtigen Zuschaltung einer Zusatzspannungsquelle zur Speisespannung bei Tastaturwahl

摘要

The switching circuit, for switching a dialling voltage source in a pushbutton dialling device to the telephone line, uses opto-electronic couplers to connect the extra voltage source to the wire with the appropriate polarity. The base of each coupler's phototransistor is controlled via an RC parallel circuit. The base of the transistor whose collector-emitter path lies in the series with the extra voltage source is controlled from a variable resistor. The advantage lies in polarity selection being independent of the voltage source itself and of short voltage pulses on the line.