Tariff data counter for communication systems - accepts pulses of set polarity and level for feeding to relay

摘要

The system removes the need for conventional split coils and bridge rectifiers where the call tariff is measured by counting a frequency superimposed on, but not interfering with, the speech signals. The toll circuit is from an RC network across the floating telephone lines and consists of a PNP and an NPN transistor in cascade, the emitter of the latter being held by a zener diode at a fixed level about earth. The input transistor only accepts signals of a given polarity and of a specified level, feeding the second transistor controlling a standard toll recording relay.