Path: utzoo!attcan!uunet!lll-winken!netsys!vector!nobody From: kitty!larry@cs.buffalo.edu Newsgroups: comp.dcom.telecom Subject: Submission for comp.dcom.telecom Message-ID: Date: 9 Oct 88 13:44:52 GMT Sender: chip@vector.UUCP Lines: 71 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 159, message 2 X-Submissions-To: telecom@bu-cs.bu.edu (TELECOM Digest Coordinator) X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) In article kaufman@polya.stanford.edu (Marc T. Kaufman) writes: > >I'm looking for a relay to place in series with a telephone which will > >open (or close) when the set is offhook. The closest I've come is with > >a 200 ohm 4.5 mA relay. It didn't affect the phone operation too much, > >but wouldn't quite pull in. If I pressed the armature in, it would > >latch. Close, but no cigar. > > You want to use a voltage comparator IC to measure the voltage between the > wires (through a full wave bridge rectifier, so you don't have to care which > way you hook it up). Open circuit (on-hook) voltage is about 48, off-hook > voltage is about 1-2 volts. I am not sure what "hold" voltage is, but it is > higher than 2 volts. Simpler, for a single phone, is to see if the phone has > a spare set of contacts on the hook switch (many do), and use them. A few words of advice on the design and construction of such a circuit: 1. In general, it is good practice to isolate any voltage comparator circuit by using a 100,000 ohm resistor in series with the tip lead and the ring lead (i.e., two resistors, 200 K effective impedance across telephone line). This means that the voltage comparator circuit needs to have a high input impedance to function with such a high input series resistance. This is really not a problem for a well-designed circuit. The input circuit to the comparator or op amp (i.e, following the two 100 K resistors) MUST be protected against voltage transients caused by ringing voltage, dialing pulses, etc. Such protection can readily be accomplished with a pair of back-to-back zener diodes. In addition, a parallel capacitor should also absorb some transients and prevent spurious signals. While the capacitor will result in a time constant when used in conjunction with the 100 K series resistors, this is desirable - a time constant between 300 and 600 ms should be fine since the output of the voltage comparator does NOT want to follow rotary dial pulsing in a status indicator application. 2. The use of the 100 K series resistors protects both your equipment and the telephone network. Under these circumstances, a short between the comparator or op amp input and Vcc or ground will cause neither harm nor appreciable degradation of voice signal. 3. The statement above that "off-hook voltage is about 1-2 volts" is not correct. Off-hook DC voltage typically ranges between 6 and 25 volts, depending upon the resistance of your CO loop and the internal resistance of your telephone set. From a realistic standpoint, a threshhold of 35 to 40 volts (some hysterisis is desirable to prevent transients from causing spurious signals) is reasonable for the design of a voltage comparator circuit. 4. A well-designed status indicator circuit is polarity insensitive. You can do this with a full-wave bridge rectifier prior to the voltage comparator, or with multiple comparators (may be more convenient when using quad comparators or op amps). 5. In general, DC holding resistors range between 200 and 400 ohms so that "hold" voltages approximate those of off-hook voltages. A realistic range is 5 to 25 volts. 6. Higher off-hook voltages will usually be present if you are using the above status circuit on a PABX extension. In general, -48 volt battery and ground is still fed through a total of 400 ohms (or electronically current-limited to an equivalent value), but there is very little loop resistance. So, a 500-type telephone set as a PABX extension will generally show an off-hook voltage between 20 and 25 volts. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {att|hplabs|mtune|utzoo|uunet}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"