SIMPLE_PHONE_IN_USE_INDICATOR

Published:2009/7/14 3:32:00 Author:May | From:SeekIC

The LM741 op amp is used as a voltage comparator, comparing the telephone-line voltage to the battery voltage. The telephone line drops below 9 V when it is in use. That drop turns the op amp on, which lights the LED. Resistor R1 prevents the circuit from loading the telephone line excessively. In my location, the in-use circuit drops the line voltage approximately 5 V. That leaves quite a bit of re-serve because a telephone line operates at approximately 40 to 48 V. Resistor R2 limits the current to the LED. A 1000-Ω resistor will brighten the LED; however, the total circuit current draw will in-crease. The circuit (as shown) consumes 1.15 mA in standby and 3.80 mA when indicating that the line is in use.



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