Abstract: An off-hook/on-hook status check circuit is adapted to be connected in series with either the tip (2) or the ring lead (3) of a telephone subscriber loop (1) at a location remote or apart from the telephone station set (13). The circuit comprises an opto-emitter subcircuit (7) in series with a capacitor (4). These are connected in parallel with a pulsing contact (K.sub.1) which alternately permits the charging of the capacitor by the telephone central office voltage or its discharge through the opto-emitter subcircuit. As the capacitor may only be charged when the telephone station set is off-hook drawing loop current, the opto-emitter subcircuit provides a predetermined output when the capacitor discharges, identifying the off-hook condition of the telephone station set. No output from the opto-emitter subcircuit during the capacitor discharge interval identifies the on-hook condition of the set.

Abstract: The circuit includes a signal input circuit having a light emitting diode that is optically coupled to a phototransistor in a signal output circuit comprising a noise suppression circuit and a Schmitt trigger circuit. The noise suppression circuit comprises a switch portion including a transistor switch and a timing portion including a capacitor. The transistor switch is connected in parallel with the capacitor, and in the quiescent condition of the line isolation circuit, the transistor switch is turned on so as to maintain the capacitor in a discharged condition. When the light emitting diode is illuminated by an input signal exceeding a particular threshold, the phototransistor is turned on, causing the transistor switch to turn off. The charging of the capacitor thereupon commences.

Abstract: A telephone with electronic telephone alerting circuitry which includes a telephone ringing detector, a sound synthesizer activated by the ringing detector and an amplifier-speaker arrangement responsive to the synthesizer. Whenever ringing is detected, the synthesizer is activated and caused to generate a specific signal sequence programmed into the synthesizer. The synthesized signal is amplified and applied to a transducer, such as a speaker, which produces the alerting sound. The synthesizer comprises a synthesis controlling processor and a memory for storing the parameters of the programmed signal sequence. The memory may be alterable or replaceable.