Phone blocker

[Mark Musone]

Yea, that phone light is all over the place and their cheap too, I'll
look around today and let you know where I find them..

(all it is is a resistor and an LED, it essentially works on the
principal that when the phone is on-hook (hung up) the voltage across
the phone lines is 48V DC. The off-hook voltage is 6-8v. 


Below is an excerpt I just found on building a circuit for it..i'm still
looking for a commercial product..

Support for 5 of these units per phone line:

This requirement puts a limit on both the off-hook and on-hook
loading for the circuit.  Off-hook detection usually occurs at a
threshhold of about 15mA, with 23mA being the required low-end
supply for proper phone service to the off-hook phone.  Back to
on-hook detection is often around 10mA.  So with 5 of these, I
need to keep the LED current below 2mA -- I've chosen 1.5mA as
my limit here, so that the off-hook draw of each of these
circuits is 1.5mA max.  Also, just to be safe, I want the
on-hook draw to be negligible -- I've chosen 1/5th of the
on-hook current draw, or 300uA as my on-hook maximum with an
on-hook voltage of -52V.

What I came up with is this:

  TIP >---x------x-------------x--------x--------,
          |      |             |        |        |
         gnd     \ R4          |        V D2     |
                 / 330k        |        -        |
                 \             |        |        \ R1
                 |           |/e Q2     v D3     / 470
                 x--/\/\/----|   PNP    -        \
                 |  4.7M     |\c        |        |
                 |  R2         |        v D4     |
                 |             |        -        |
                 |             |        |      |/e Q1
                 |             x--------x------|   PNP
                 |             |               |\c
                 |           |/c Q3              |
                 x-----------|   NPN             |
                 |           |\e                 |
                ___/ D5        |                --- ~
               // \  15V       \ R3             \ / ~
                ---  zener     / 220k           ---
                 |             \                 | D1
                 |             |                 |
 RING >----------x-------------x-----------------'

My thoughts were:

D1 is a 2mA type LED, operated at about 1.5mA or so.  R1 sets
this current level, with a kind of flimsy current source built
out of Q1, where it's base voltage is limited by the D2..D4
diode chain so that R1's voltage is relatively constant.

Q3 could just be removed and R3 directly connected to Q2's
collector.  But then the current gets kind of high through it
when the phones are on-hook, as the voltage gets between 48 and
52 volts.  So Q3 lets enough current through, up until some
limit is hit at around 60uA or so.  Beyond that, I don't need
any more sink capability so using Q3 this way helps limit the
current loading for on-hook situations.

D5 is a zener, maybe built stacking two transistors' BE
junctions, or so -- or maybe using something like the BZX84C15L.
Anyway, it does double duty, acting as a stable voltage
reference for Q3's base (a current sink) as well as a mechanism,
coupled through Q2's base, of detecting the off-hook/on-hook
situation.

Off-hook detection occurs here when the phone line voltage goes
below about 15.5V, or so, in magnitude.  And it can supply
current to the LED down to about 1.5V, at which point it runs
out of headroom and can't do much.  But the phone line voltage
is supposed to remain over 3V, when in normal off-hook
operation, so I think this is okay.

Jon
-----Original Message-----
From: owner-nflug at nflug.org [mailto:owner-nflug at nflug.org] On Behalf Of
Robert Meyer
Sent: Friday, August 15, 2003 7:28 AM
To: nflug at nflug.org
Subject: Re: Phone blocker

Well, if you really wanted to mess with the phones...

You could have the incoming line from the house connect only to the
modem in
the computer and then have the rest of the house tied in on the 'phone'
connection on the modem.  When the modem is on, the rest of the phones
would be
cut out.  I've never heard of the devices that you mention but I'm sure
they
are not terribly difficult to design.

Cheers!

Bob
--- Asheville Joe <josephj at main.nc.us> wrote:
> Hi.  This is a little off topic, but I hope someone knows about it.
> 
> I share a dial up line for Internet with a couple of voice users who 
> occasionally pick up the phone and knock me offline.  In the past, I
saw 
> a device that you could plug into any phone jack in the house that
could 
> tell if the phone was in use elsewhere in the house and block access
to 
> the line.  Does anyone know what it's called or where to get it?
> 
> Thanks.
> 
> Joe
> 


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