PHONE_IN_USE_INDICATOR

Published:2009/7/10 23:08:00 Author:May | From:SeekIC

The circuit receives its power from a 15-V wall adapter (not shown). The circuit takes advantage of the fact that the phone line voltage drops from 48 to 10 V when an extension is taken off the hook. When the voltage on a line drops, the optoisolator/coupler is turned off so that the inputs to the line-1 hexinverters (U2 pins 1. 3. and 5) float high. The corresponding outputs (U2 pins 2. 4. and 6) go low and light the line-1 LEDs.



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