MCHARRY%BNR.CA@MITVMA.MIT.EDU (J.) (10/06/88)
Some years ago I saw a nifty way of putting a 'busy light'
on a phone. Normal phone systems have the tip wire grounded
and the ring hooked to -48v. (Some systems fail to provide a
ground reference.) The trick is that both -48 and ground are
applied through about 200 ohms. When a phone on the loop is
off hook, tip and ring are nearly shorted together, and appear
at about -25v to ground. An led hooked through a resistor to
the tip side will come on if anyone is off hook. Of course,
there is a catch or two: 1) You have to make sure the ground
wire is connected and not just tip and ring. You may have to
hook it up at the building entrance block. 2) This will
likely introduce a large ground loop and a lot of hum. I haven't
tried this, but old ringers are usually rigged with two windings
having a blocking cap between them. Going from the tip side
winding to ground might stifle the hum. Alternatively, a good
sized choke in series with the led ought to kill it also. The
resistor is 25k / (number of mils to light the led).
In addition to the usual disclaimers, I am not Drano: your
pipes are on their own.
John McHarry McHarry%bnr.ca.bitnet@cunyvm.cuny.edu