Detection of line-current flow and indicating the flow to an electrically remote point is required in line status monitoring at a variety of points in theJelephone system and auxiliary systems. The line should be minimally unbalanced or loaded by the monitor circuit, and relatively high levels of 60-Hz induced voltages must be ignored. The HllAAl allows line currents of either polarity to be sensed without discrimination and will ignore noise up to approximately 2.5 mA. In applications where greater noise immunity or polarity- sensitive line-current detection is required, the HllAlO threshold coupler can be used.
Line-current-detector - schematic

This phototransistor coupler is specified to provide a minimum 10% current transfer ratio at a defined input current, while leaking less than 50 p.A at half that input current over the full -55°C to + 100°C temperature range. The input current range, at which the coupler is on, is programmable by a single resistor from 5 to 10 mA.

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