Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!seismo!rutgers!ames!ucbcad!ucbvax!kitty.UUCP!larry From: larry@kitty.UUCP Newsgroups: comp.dcom.telecom Subject: Submission for mod.telecom (Ringing Detection Circuits) Message-ID: <8704261613.AA00055@seismo.CSS.GOV> Date: Sun, 26-Apr-87 12:13:26 EDT Article-I.D.: seismo.8704261613.AA00055 Posted: Sun Apr 26 12:13:26 1987 Date-Received: Sat, 2-May-87 04:47:28 EDT Sender: daemon@ucbvax.BERKELEY.EDU Distribution: world Organization: The ARPA Internet Lines: 97 Approved: telecom@xx.lcs.mit.edu While the Texas Instruments TCM1520A is a nice IC, it is possible to build simple and reliable ringing detection circuits by other means. Here are some suggestions which may be helpful in the design of circuits which detect ringing: 1. When a telephone goes on-hook and off-hook during hookswitch (i.e., line switch) operation, a voltage transient is generated whose voltage is the same order of magnitude as a ringing signal. When a rotary dial is used, each dial pulse is a momentary line open which also generates these voltage transients. A poorly designed ringing detector circuit will falsely detect the above voltage transients as ringing signals. To avoid this problem, ALL reliable ringing detector circuits require a time constant. NO ringing detector circuit (unless it is has frequency discrimination - which is extremely rare) can tell the difference between on-hook/off-hook transients and the ringing signal itself based upon a voltage threshhold ALONE. Such a time constant can be established by three means: (1) integrating the rectified voltage from the telephone line with a resistor-capacitor before it drives an LED or relay; (2) using a thermistor in series with the LED or relay (a traditional design approach, but the "right" thermistor is difficult to obtain); (3) providing a specific timing circuit which looks at the output of the optoisolator or relay, and requires that a signal be present for a minimum period of time before asserting an output logic line. A reasonable integration time constant is between 200 and 600 milliseconds; i.e., the ringing signal must be present for this time period before a detection logic line is asserted. 2. All ringing detector circuits should have their telephone line connection electrically isolated from ground, and should be coupled to the telephone line using a series capacitor. In general, the value of this series capacitor should not exceed 0.68 uF, and such a capacitor should be rated at 200 WVDC. Excessive capacitance will cause voice-frequency attenuation on the telephone circuit, and may also result in premature "ring tripping" and dial-pulse distortion. In general, the effective DC resistance of a ringing detector circuit - EXCLUDING the capacitor - should be a minimum of 1,000 ohms. Following the above capacitance and resistance constraints should result in a ringing detector circuit which has a REN of less than 1.0 on the "B" scale, and consequently should not interfere with proper operation of the telephone line. 3. Optoisolators are nice for ringing circuit detection, but proper and reliable ringing detector circuits can be made with relays. Use a sensitive "plate" relay of 2,500 to 10,000 ohms resistance. Connect a full-wave bridge rectifier to the telephone line using a series capacitor; connect the DC output to the relay in series with a resistor, and place a capacitor across the relay winding to provide an integration time constant (be sure to have this capacitor rated at at least 100 WVDC!). If your application is a ringing "extension" circuit - like to drive an AC line horn, bell or light - you may find a plate relay with a contact current rating sufficient for the job. This makes for a pretty simple circuit. Plate relays with the required resistance and sensitivity are often available surplus for a couple of dollars. Do NOT use an AC relay rated for 120 VAC; AC relays of this type generally do not have enough sensitivity and a high enough resistance for telephone applications. Also, note that some plate relays (like certain Sigma models) have their body as the common contact - so these relays MUST be properly insulated from the case and outside world. The use of a relay to directly detect ringing and control an AC power line circuit is a well-established design technique; however, use extreme CAUTION when wiring such a circuit so that faulty construction does not permit accidental connection between the telephone line and AC power line! 4. If you are serious about designing telephone circuits, take the time to study the operation of a telephone line using a storage scope with differential inputs (i.e., one input for TIP, one input for RING - NEVER ground either TIP or RING). You will notice that -48 volts DC is ALWAYS present on the telephone line, even during the actual ringing. The 20 Hz ringing voltage is actually superimposed across the -48 volts DC; this is referred to as "superimposed ringing". Superimposed ringing is done to assure rapid operation of the "ring trip" relay in the central office trunk circuit. Generally, telephone ringing is 1 second on, and 3 seconds off (i.e., the "silent interval"). If you are using PBX extensions to "play with" for telephone circuit design, beware that their behavior may NOT be the same as central office telephone lines. For example, some PBX's use 30 Hz rather than 20 Hz; and some PBX's do not superimpose the ringing signal on -48 volts DC in the same fashion as a central office. Also, PBX's generally provide a "hotter" ringing signal than a central office because your loop resistance to the PBX is generally << 100 ohms. 5. None of what I have said applies to party lines. You should never attempt to design telephone circuits for connection to party lines. Not only might you be detecting ringing for other parties, but improper design or connection might also result in YOU getting billed for THEIR telephone calls! <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"