rpluth@pnet01.CTS.COM (Ron Pluth) (07/25/87)
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 If you're interested in telephone electronics and circuits, as well as a somewhat thorough (but probably outdated) discussion of telephone function integrated circuits, pick up "Understanding Telephone Electronics" at Radio Shack.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 UUCP: {akgua, hplabs!hp-sdd, sdcsvax, nosc} | "We are all in the gutter, !crash!pnet01!rpluth | but some of us are looking ARPA: crash!pnet01!rpluth@nosc.mil | at the stars." INET: rpluth@pnet01.CTS.COM | -Oscar Wilde USPS: 13704 Celestial Rd. Poway, CA 92064TPBL