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---------
LIGHT BOX
---------

By: The Night Owl AE

------------ 
Introduction 
------------

This file describes the operation and construction  of  a  device
that  will  signal  whether  or  not an extension of a particular
phone line is off-hook. It does NOT indicate  whether  or  not  a
phone  is  being tapped, and will light whenever any extension is
picked up.

---------
Materials
---------

        1 box.  Make it a nice one, preferably one with feet.
        1 phone cord with a modular plug at  one  end  and  spade
          lugs at the other.
        1 phone jack or Y adapter (optional).
        1 power cord or old extension cord.
        1 low current coil relay, double throw.
        1 5K fixed resistor.
        1 10K variable resistor.
        1 neon bulb with dropping resistor.
        1 panel lamp assembly.  Some come with bulbs.

------------
Construction
------------

The principle under which this device operates is that there is a
voltage  across  the  red  and green wires of the phone line (the
other wires are rarely used) that drops significantly when an ex-
tension  is  lifted.   Though this voltage can be used to power a
light on its own, the light would have to be of a very low  power
consumption in order not to keep the phone off-hook, and it would
only light when the phone is on-hook.

A simple way to get around the above problem  is  to  use  a  low
current  relay.   Radio  Shack  sells some nice 12VDC SPDT relays
that work well for this purpose.  Don't buy the red relays or the
subminis;  though  you want a low current coil, you also want the
contacts to be able to handle a  decent  current.   DPDT  (double
pole,  double throw) will also work, you just won't use the other
contacts.

For a 12 volt coil, the voltage will have to be dropped  so  that
the  relay  will  function  properly.   The  proper resistance is
around 10K ohms, but it is best to use a 5K  ohm  fixed  resistor
and a 10K ohm variable (standard values, any combination that can
cover the 10K range will work fine).  Adjust it to  approximately
10K, or halfway for the combination above.  Connect the resistors
in series with the coil, and we'll adjust it later.

Due to the nature of this device (or rather, its  simplicity)  it
will  operate  without having to be plugged into a phone, or vice
versa.  In other words, you can plug it directly into a jack any-
where,  and  it'll  work.  You'll want to get a modular plug that
terminates in spade lugs for this  purpose.   Connect  the  green
wire  to  one  side of the coil, and the red to the resistors (so
that it's all in series).  If you don't have an extra phone jack,
then you will want to a) buy a Y connector so that two phones can
be plugged into the same jack, or b) get another phone  jack  and
install it in the same box with the in-use light, so that you can
plug a phone into it.  Just  connect  the  spade  lugs  to  their
respective colors.

For the light itself, you can use almost anything.  Even  a  tiny
light  bulb and a battery will work fine, but after about 5 hours
of phone use, you'll have to replace the battery.  The best thing
to  use  is  a neon bulb powered by the 110V power available most
places.  Buy ones with dropping resistors, or use a 220K resistor
for  dropping.  You should also get a panel lamp assembly to keep
it steady and make it look pretty.  Since the  current  is  rela-
tively  low,  you  can use almost any power cord, or sacrifice an
old extension cord.  Connect it in series with the  NC  (normally
closed) contacts of the relay (in series with the dropping resis-
tor, of course).

            Your circuits should now look like this:

               /
---RED----!---- 5k --- 10k -----O    O-------------------!---110V
              /          |     |
                    coil    NC /
                   |     
--GREEN---!---------------------O    O-- bulb --- 220k --!---110V

Make sure that the phone circuit  (left)  and  the  lamp  circuit
(right)  are totally 100% separate, and in no danger of touching.
Use wire nuts for all connections in 110V that are not to a  ter-
minal  post,  and tape them.  For all wires that are exposed, use
electrical tape or shrink-wrap tubing to insulate them.  Glue the
relay to the box or mount it on a stable PC board.

If all looks well, hook it up.  It should work  immediately,  but
there are three things that can go wrong:

  1) The relay keeps the phone off  the  hook.  To  remedy  this,
     lower the  resistance  until it doesn't.  If you can't lower 
     the resistance enough, then you are using a resistor  with a 
     high  current  coil  and you'll have to get one with a lower 
     rating.

  2) The relay won't close.  The solution for this is the same as
     number 1.

  3) The relay stays closed.  Raise the resistance with the phone
     on hook until the light goes off.

When all is working, the lamp should be on when one or  more  ex-
tensions  are  off-hook,  and will be off when all extensions are
on-hook.  It will blink when the phone rings.

+++EOF