🚀 go-pugleaf

jjr@ut-ngp.UUCP (Jeff Rodriguez) writes:

>Where can I find answers to such questions as
>     1.  What voltage levels are used in phone lines?
>     2.  What is the signal that you get when you receive a call?
>     3.  What signal do you send back to signal off-hook?
>     4.  What signal do you send back to signal hang-up?
>
>P.S.  Any other ideas for a simple, fun, and useful toy that I can build?

Standard voltage levels for phone lines vary widely, depending upon where you
live, what kind of phone system is present, and how far you are from your
switch.  The normal telephone standard incorporates a voltage level of 24-48
VDC between tip and ring (the two conductors which transmit speech), tip more
positive than ring.  For the second question, the ring signal is a relatively
high *AC* voltage across tip and ring, normally from 90-150 VAC.  Obviously,
this is an AC voltage so the telephone circuit can distinguish it from the
normal talk voltage.  The off-hook "signal" is just a connection (I think it's
an impedance of about 600 ohms - the impedance of the telephone) between tip
and ring, causing current to flow between the two.  The on-hook signal,
conversely, is the breaking of that circuit, so that no current flows, except
for ring current.  

If you're interested in telephone electronics and circuits, as well as a
somewhat thorough (but probably outdated) discussion of telephone function
integrated circuits, pick up "Understanding Telephone Electronics" at Radio
Shack.  I'll see what I can do about picking up a schematic for a speaker
phone at work (I do telephones).  Also, if anyone is interested, drop me a
line about a *VERY* Mickey-Moused circuit for putting people on hold with
music from any phone.

		                    Ron Pluth

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