Phone-line-voltage monitor meets FCC specs

Posted on Jun 29, 2012

The circuit in Figure 1 shows a method of meeting this challenge with 100% margin. In other words, it presents approximately 10-M leakage resistance across the line. The basic relaxation oscillator has the LED of an optoisolator in the discharge path of the main capacitor. It delivers a pulse train, the frequency of which varies with the voltage on the phone line. By measuring the period between pulses, the equipment`s microcontroller can easily determine the approximate line voltage. C1 is the timing capacitor. It should be a film-type capacitor rather than ceramic for good results. It slowly charges through R1, a 10-M resistor. When the voltage across C1 reaches approximately 3V, the remainder of the circuit turns on, causing C1 to rapidly discharge through R4 and the optoisolator`s LED.

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