Be careful with circuits that are detecting on/off hook by measuring loop
voltage.  FCC 68.312 (below and at http://hallikainen.com/FccRules)
requires the DC resistance of terminal equipment to be 5 meg between tip
and ring for all DC voltages up to 100 volts.

Harold


Sec. 68.312  On-hook impedance limitations.

    (a) General. The limitations in this section that involve 2-wire
network ports apply to tip and ring of the public switched network. For
4-wire network ports (tip, ring, tip 1, and ring 1) with loop-start or
ground-start signaling, the limitations apply when tip and ring
conductors are connected together and treated as one of the conductors
of a tip and ring pair and the tip 1 and ring 1 conductors are connected
together and treated as the other conductor of a tip and ring pair.
    (b) Limitations on individual equipment intended for operation on
loop-start telephone facilities, including PSDS Type II in the analog
mode:
    (1) Registered terminal equipment and registered protective
circuitry shall conform to the following limitations, for each Ringing
Type which is listed as part of its Ringer Equivalence:
    (i) The dc resistance between tip and ring conductors, and between
each of the tip and ring conductors and earth ground, shall be greater
than 5 megohms for all dc voltages up to and including 100 volts.
    (ii) The dc resistance between tip and ring conductors, and between
each of the tip and ring conductors and earth ground shall be greater
than 30 kilohms for all dc voltages between 100 and 200 volts.
    (iii) During the application of simulated ringing, as listed in
table I below, the total dc current, shall not exceed 3.0 milliamperes.

On Sat, 6 Feb 1999 19:15:53 EST "Karl A. Uscroft" <KUscroft@AOL.COM>
writes:
>Old circuit i've seen involves one rectifier (B250C1500) with the
>output
>across a 22k resistor and a high efficiency diode.  With a line
>voltage of
>~40V coming from the phone lines when not busy and the LED will light.
> The
>rectifier means that it doesn't matter which way round you have it,
>also
>polarity protection is of no consequence.  When in use the LED will
>dim.
>Using a zener as mentioned in another e-mail will stop it lighting
>when the
>phone is in use.  This could easily be altered for use with a op-to
>isolator
>for input into a PIC for timing.
>
>PS.. In most countries it is not allowed to connect anything other
>than
>approved equipment across the phone lines.  Hence talk to the phone
>company to
>see if you can get the Okay.  They check using line resistance or
>something
>like that.
>
>I hope this is of any use.
>
>Karl  U.
>

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