Subject: From: Date:

Re: [ECS] Telephone Deadman switch LEC1964.aol.com Thu, 27 Aug 1998 17:41:08 EDT

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Dan,

     This might be of some help for detecting phone service interruption.

     Using a multimeter measure your DC "idle" current. This can be done by
disconnecting either of the two conductors (usually a red and green pair) and
placing the meter leads in series. This must be done with all phones "on hook"
and as close to the phone service attachment as possible. Do not measure at a
point in between phone extensions. Record this value. Go "off hook" with one
extension. Measure and record this value.
     What I am thinking of doing is to add a resistor in series (in place of
the multimeter) whose value will be selected so that the voltage drop across
it will be around 5 volts with your existing idle current. Resistance =
voltage divided by current. Calculate it's value and and use a 1/4 watt carbon
resistor. In parallel with the resistor attach a 5 volt zener which is reverse
biased. This will clamp the DC voltage developed across the resistor to a
maximum of 5 volts. Be aware that the value of the resistor cannot be so large
that the phone company central office will not "see" your first extension
going "off hook". If this happens to be the case, another method of detection
or a modification will be needed.
     Now comes the difficult part. There is a possibilty the voltage developed
across the resistor/zener combination could be applied directly to a digital
input point. This is dependent on two things: 1. The input impedance of the
digital point. 2. How "stiff" the negative pulses of the ringing signal is.
You do not wan't to lose an input point. Also any voltage spikes on the phone
pairs from whatever source can damage your circuitry.
     The best way to protect your ECS hardware is to use a buffer/current
driver, possibly with optical isolation, driving a isolation relay whose
contacts will provide a potential to your digital input point. This circuitry
would monitor the 5 volts across the resistor. This would require some
assembly and an additional DC power source.
     A couple of comments. I don't believe any of the additional circuitry
will change the reactive components of your phone line. You might for example,
add a sensitve relay where its coil is placed in series (in place of the
resistor) and a zener is shunted around its coil to clamp the current. The
additional inductive reactance might severely limit data transmissions. I do
have another way to address the problem of phone service interruptions due to
a local cable cut that I am planning on using. It's a little "strange" but
requires nothing to be added to my phone line. If your interested I'll send
information on that idea.

Larry C.

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