Discrete ringing detector for use in key telephone systems

A key telephone line circuit adapted for connection between a branch telephone exchange and at least one key telephone subset including signaling means, said line circuit further connected to a source of first and second signals and including ring detection means operable in response to incoming signals from said branch exchange representing a call originating in said branch telephone exchange to connect said first signals from said signal source to said key telephone signaling means whereby said signaling means are operated in a first distinctive mode and operable in response to incoming signals from said branch exchange representing a call originating outside said branch telephone exchange to connect said second signals from said signal source to said signaling means, whereby said signaling means are operated in a second distinctive mode, said ring detection means comprising:

• switching means normally completing a circuit path for said first signals from said signal source to said signaling means;
• and timing means connected between said branch exchange and said switching means operated in response to incoming signals received from said branch telephone exchange in excess of a predetermined minimum duration to operate said switching means to break said signal source first signal circuit path to said signaling means and complete a circuit path for said signal source second signals to said signaling means.