jack@csccat.UUCP (05/02/87)
I am looking for some ideas or info on where I can find a curcuit that will tell me if a telephone line is in use. I need such a curcuit for use with a modem that will tie into an outgong 800 number but this number is also used for voice. As out lines are on a first come first serve bases I would like to look at the highest line on our rotary and tell if it is in use and if not seize siad line and use it for data communication and prevent any voice communications on that line until the conversation is complete. I other words I want to steal a 800 line while nobodys looking :-). Thank in advance. Jack Hudler -- See above (214)661-8960
coltoff@burdvax.UUCP (05/11/87)
With all the other phobe questions I was going to post this today anyway. This is from the TI LInear and Interface Circuits Applications book, 1985, pgae 7-23. The circuit is as follows. Use a TLC271 Op amp (available at Radio Shack). There is a 5.1M resistor from each phone wire to pins 2 and 3 of the op amp. Across the phone lines is a 4.3M resistor and a 0.01uf cap. Pin 3 of the 271 is connected to ground through a 2M resistor. Pin 4 goes right to ground. Pins 7 and 8 are tied together and then to Vcc. (In this case it is 9v). Between pins 2 and 6 there is a 2M resistor and a 0.001uf Cap in parallel. Pin 6, the output, goes through a 1.5M resistor. HOW DOES IT WORK DR. SCIENCE? Well Rodney, your ignorance of telephony appauls me. When the phone is off the hook a 1K load is placed between tip and ring. This removes the differential input to the op amp and causes its output to go low. When the phone is hung up the impedance is removed and the differential voltage across the line causes the op amp output to go high. The TI example uses a TCM1520 as a ring dectector. This chip is designed for just that purpose and needs 2 resistors and 2 caps to interface to the phone line. Radio Shack sells a phone ringer, the TCM1512, I believe which can also be used as a ring detector.
coltoff@burdvax.PRC.Unisys.COM (Joel Coltoff) (05/12/87)
I realized last night that I left one detail out of the phone is use circuit. Not that it really matters but tip goes to pin 2 of the TLC271 and ring goes to pin 3. - Joel