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From: larry@kitty.UUCP
Newsgroups: mod.telecom
Subject: Re: (none)
Message-ID: <8511092024.AA29169@ucbvax.berkeley.edu>
Date: Sat, 9-Nov-85 15:24:35 EST
Article-I.D.: ucbvax.8511092024.AA29169
Posted: Sat Nov  9 15:24:35 1985
Date-Received: Mon, 11-Nov-85 06:03:20 EST
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> An active line dectector should not be to hard for someone to design.
> The phone line has a potential of about 5 volts when off the hook. Its
> on hook voltage is considerably higher.  All that need be designed is
> a simple circut that detects the low voltage.  When the device sees
> this lower voltage is draws a very small amount of current for the LED.
> I am unfortunatly unable to take it beyond this stage.  I would
> however be interested in the plans if someone can handle the next step.

	It would be far better to have a line status indicator which was
powered from a small alkaline or mercury battery, and which used the battery
power to drive the LED.  If you used a CMOS voltage detector IC, like the
Intersil P/N ICL7665, the quiescent battery current would be well under
5 microamperes.  This IC will drive an LED directly (with a suitable current
limiting resistor).  Only a handful of components are needed for the circuit,
but the circuit is too complex to reproduce over the Net.  If you get a copy
of the Intersil data sheet, the circuit should be self-explanatory (assuming
you have some basic electronic experience).  You should use a full-wave bridge
rectifier across the telephone line to make the circuit polarity-independent;
in addition use at least a 50 kilohm resistor in series with the telephone
line (i.e., AC) side of the bridge, and clamp the DC output side with a 4.7
volt zener diode to prevent ringing current damage.  You will have to then
interpose a voltage divider between this zener diode and the input circuit
to the ICL7665.
	In general, you should assume that a telephone line has a nominal
48 to 52 volt on-hook voltage, and that ringing is a nominal 105 V RMS 20 Hz
superimposed on 48 volt DC.  The nominal DC resistance of a 500-type telephone
set is 300 ohms, which means that if you are close to the central office, you
could see ALMOST 20 volts DC in an off-hook condition; do NOT assume a nominal
5 volt DC off-hook condition!  So, to be safe, you should set your on-hook/
off-hook threshhold voltage at 20 volts.
	If you are located some distance from the CO, your line could have
what are called loop extenders on it, which means you could see an on-hook
voltage of as high as 96 volts DC (72 is more common, though).  Also, if
your line is served through line concentrator equipment or through subscriber
line carrier, then anything is possible.  These latter two possibilities are
not very common, but they do exist, and you should be aware of the likelihood
in the event that some circuit you build really weirds out.
	In general, you should not draw more than 2 milliamperes of BRIDGING
current from a telephone line, nor place any BRIDGING resistance of LESS than
20,000 ohms.  If you fail to abide by this condition, you run the risk of
premature ring tripping, and having automatic line test apparatus in the CO
think you have cable trouble.  Ideally, you should never bridge LESS than
100 kilohms across the telephone line, which limits your bridging current to
0.5 milliampere.
	The above paragraph is why you should really use a low-power active
voltage sensing circuit such as the Intersil part.
	We won't talk about FCC Part 68 certification...

===  Larry Lippman @ Recognition Research Corp., Clarence, New York        ===
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