reyy@vax5.cit.cornell.edu (05/08/91)
Greetings: I need some information. I want to build a device that responds to a telephone ring. Can anyone tell me, or point me to some good lit., as to which one of those wires in the telehpone cable corresponds to the ringer, and at what voltage does it appear? Anything I should know about when i tap into that line? (I'm quite an amateur at this stuff, so complete explanations are the best. Also, responses via Email will prpbably be best.) Thanks, David S. Rowell REYY@Vax5.cit.cornell.edu (internet) .sig available upon request
grayt@Software.Mitel.COM (Tom Gray) (05/09/91)
In article <1991May7.211057.4572@vax5.cit.cornell.edu> reyy@vax5.cit.cornell.edu writes: >Greetings: > >I need some information. I want to build a device >that responds to a telephone ring. Can anyone >tell me, or point me to some good lit., as to >which one of those wires in the telehpone cable >corresponds to the ringer, and at what voltage >does it appear? Anything I should know about >when i tap into that line? (I'm quite an >amateur at this stuff, so complete explanations >are the best. Also, responses via Email will >prpbably be best.) > There are two wires in the loop - tip and ring. The tip is the green wire and is grounded though a resistance at the CO. The ring is the red wire and is connected to the ringing current generator at the CO. There is a telephone maxim that RED IS RING IS RIGHT. The red lead is the ring which is punched down to the right of the green lead (tip). This is all you need to know to install a telephone switch. As you know, the telephone loop is balanced. You must ensure that your circuit does not imbalance the loop. The differential resistance of your device must exceed 10K and better yet 30K ohms or there will be significant noise due to longitudinal currents.
grayt@Software.Mitel.COM (Tom Gray) (05/09/91)
In article <1991May7.211057.4572@vax5.cit.cornell.edu> you write: >Greetings: > >I need some information. I want to build a device >that responds to a telephone ring. Can anyone >tell me, or point me to some good lit., as to >which one of those wires in the telehpone cable >corresponds to the ringer, and at what voltage >does it appear? Anything I should know about >when i tap into that line? (I'm quite an >amateur at this stuff, so complete explanations >are the best. Also, responses via Email will >prpbably be best.) > >Thanks, >David S. Rowell >REYY@Vax5.cit.cornell.edu (internet) >.sig available upon request I forgot to put this in the original reply. AT THE CO. The tip is grounded. The ring has a nominal -48V (-41 to -56) supply attached to it. During ringing, the ring voltage of -48V has an additional AC voltage of 86v RMS superimposed on it. However your telephone is on the end of a long cable. This cable may havei a resistance from about 200 to 2000ohms. The voltage received at the end of the loop will vary greatly. For this reason most telephony circuits are designed to detect not voltage but current. A standard ringing detector is a differential current detector connected to a RC network with a time constant longer than the 50ms of the 20Hz ringing current. What application is this for? Is it in line with a telephone as an indicator or is it part of a bigger circuit. If you wish to terminate a telephone loop on some device, you can buy fully functional loop start trunk circuits from many manufacturers. This will provide you with ringing detection and all audio functions for a very nominal price (~10 to 20 dollars in unit quantities).