gbell@pnet12.cts.com (Greg Bell) (04/28/89)
This is in response to the person who was talking about lighting an LED from
the 50 VDC phone line voltage as an indicator for hook status:
That's the easy project! But, how 'bout making a circuit that will light an
LED when the line voltage DROPS (ie. the phone goes off hook). I tried it
once, and got it to work moderately well. I used a combination of voltage
dividers to keep a transistor off if the voltage is 50 or so. Worked alright
until the phone rang... the circuit answered the phone! Got a little
annoying.
I'm still amazed at how phones run entirely off the phone line power. They
have lit dials, and audio amplifiers... and I had trouble lighting an LED!
(trouble means the phone company equipment gets loaded down enough that it
shuts off your line for a while!).
Can anybody offer insite on the secrets of running audio amps and chips off
the meagre phone line power? How 'bout the ammount of current the phone line
can source?
Greg Bell_________________________________________________________
Hardware hacker |
Electronics hobbyist | UUCP: uunet!serene!pnet12!gbell
EE major at UC San Diego |larry@kitty.UUCP (Larry Lippman) (04/30/89)
In article <636@serene.UUCP>, gbell@pnet12.cts.com (Greg Bell) writes: > This is in response to the person who was talking about lighting an LED from > the 50 VDC phone line voltage as an indicator for hook status: > > That's the easy project! But, how 'bout making a circuit that will light an > LED when the line voltage DROPS (ie. the phone goes off hook). I tried it > once, and got it to work moderately well. I used a combination of voltage > dividers to keep a transistor off if the voltage is 50 or so. Worked alright > until the phone rang... the circuit answered the phone! Got a little > annoying. No kidding! :-) That's what I'm been trying to tell sci.electronics readers for quite some time. > I'm still amazed at how phones run entirely off the phone line power. They > have lit dials, and audio amplifiers... and I had trouble lighting an LED! > (trouble means the phone company equipment gets loaded down enough that it > shuts off your line for a while!). > > Can anybody offer insite on the secrets of running audio amps and chips off > the meagre phone line power? How 'bout the ammount of current the phone line > can source? Again, here are some assorted comments about telephone loop voltages and currents: 1. The on-hook (i.e., idle) voltage of the vast majority of central office telephone loops ranges between 48 to 52 volts, with 50 volts being a pretty typical value. However, this is NOT ALWAYS the case, since on-hook voltages of lines supplied from various subscriber line concentrators, subscriber line carrier apparatus and subscriber lines on long loops with loop extenders can vary from a low of 6 volts (that's right, 6 volts using Continental FDM AML subscriber line carrier) to a high of 96 volts (using Cook Electric or equivalent loop extenders). A few electronic PABX's may have 24 volts instead of 48. There are two morals to the above: (a) Only a 500-type telephone set with passive network will work on all telephone loops at all times; anything else will fail under some circumstances. (b) If in doubt, measure the on-hook voltage of the telephone loop. 2. In general, good telephone circuit design practice dictates that no less than 100,000 ohms should be bridged across a telephone line in an on-hook state. That means you can draw a maximum of 0.5 milliampere of current with _assurance_ of not causing trouble with your telephone and not causing telephone company automatic line insulation test (ALIT) apparatus to print false trouble reports. 0.5 mA is, by the way, sufficient current to keep a well-designed reperatory dialer memory or clock circuit alive. 3. In general, good telephone circuit design practice dictates that no current path to ground should ever exist between tip and/or ring of the telephone loop and ground. From a practical standpoint, a value of at least 100,000 ohms leakage resistance can be used. The only exception to the above is equipment for use on ground-start PBX loops and for test purposes. An additional exception to the above involves party line circuits, but most telephone company tariffs prohibit the use of ANY apparatus on a party line circuit that is not directly obtained from the operating telephone company. 4. From a practical standpoint, bridging less than 20,000 ohms across a telephone line (i.e., a current flow of more than 2.5 mA) in an on-hook state is an immediate invitation to trouble, especially with respect to false ring-tripping as described in the referenced article. 5. In the off-hook state power to operate amplifiers and other circuit elements may be obtained from the voltage drop across a series resistor (i.e., in series with the talk circuit). From a practical standpoint, any such series resistance must be limited such that no less than 30 mA of loop current flows in an off-hook state. Taking an example value of a 250 ohm resistor on a 30 mA loop, we have 7.5 volts available with a DC power of 225 mW. 225 mW is actually a decent amount of energy to play with (in this day and age of CMOS circuits) - but you will ONLY have it available when the telephone is off-hook. If you attempt to obtain a larger voltage drop through a higher series resistor, such that the off-hook loop current drops below 30 mA you run the risk of: (a) having the central office apparatus fail to detect an off-hook condition; (b) being unable to trip ringing; (c) being unable to send rotary dial pulsing signals to the central office; and (d) having insufficent current available to operate telephone set DTMF dialing and talk circuits. 6. The "series resistor" described in (5) above may be effectively replaced with solid-state circuit elements, such as a constant current regulator set for a minimum of 30 mA. 7. Any series element for the purpose of obtaining DC power as described above should be bridged with a non-polarized capacitor to minimize the voice-frequency insertion loss of such element. It should be obvious from the above guidelines that any BRIDGED connection of an LED will draw excessive on-hook current, but that a series connection (i.e., "in use" indicator) is okay. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"
markz@ssc.UUCP (Mark Zenier) (05/01/89)
In article <636@serene.UUCP>, gbell@pnet12.cts.com (Greg Bell) writes: > > I'm still amazed at how phones run entirely off the phone line power. They > have lit dials, and audio amplifiers... and I had trouble lighting an LED! They don't , If you dig around in your installation, you'll probably find a plug in the wall transformer to run the incandescent dial lights. Mark Zenier uunet!nwnexus!pilchuck!ssc!markz markz@ssc.uucp uunet!amc! uw-beaver!tikal!
dr@skivs.UUCP (David Robins) (05/02/89)
In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes: > >I'm still amazed at how phones run entirely off the phone line power. They >have lit dials, and audio amplifiers... and I had trouble lighting an LED! >(trouble means the phone company equipment gets loaded down enough that it >shuts off your line for a while!). I don't know how it is now, but the old Princess and other Bell System phones had dials with incandescent bulbs, whcih were powered off AC by a small wall transformer. The power was carried throughout the house phone wiring on the yellow/black pair (red/green reserved for signal). However, my phone dialer IS powered by the phone line, but no light. -- David Robins, M.D. (ophthalmologist / electronics engineer) The Smith-Kettlewell Institute of Visual Science, *** net: uunet!skivs!dr 2232 Webster St, San Francisco CA 94115 *** 415/561-1705 (voice) The opinions expressed herein do not reflect the opinion of the Institute!
greg@bilbo (Greg Wageman) (05/02/89)
In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes: > >I'm still amazed at how phones run entirely off the phone line power. They >have lit dials, and audio amplifiers... and I had trouble lighting an LED! >(trouble means the phone company equipment gets loaded down enough that it >shuts off your line for a while!). An off-hook telephone is *required* to draw a minimum amount of current. I believe that it is permitted to draw about 20 milliamps from the line in an *off-hook* condition. The Central Office's switching equipment takes current draw above a certain limit as an indication of an off-hook condition. If you were trying to light that LED with the phone on-hook, you were actually taking the line off-hook. After 15 seconds or so, the dial tone generator would time out and you would have an open line. Please note that the (Western Electric) telephones (e. g. "Princess") with lighted dials are supplied 6 VAC by a small line transformer which is typically connected to the black and yellow "unused" wires in the phone jack. Multi-line phones are a whole 'nother ballgame, with a closet full of electronics and power supplies to support them. >Can anybody offer insite on the secrets of running audio amps and chips off >the meagre phone line power? How 'bout the ammount of current the phone line >can source? I've used line-powered modems. They use all CMOS parts to keep current consumption down, and as I said, since an off-hook device is required to draw a certain amount of current, why not use it to power the modem? Don't expect to be able to use phone line current and keep the circuit on-hook, however. Even if you could, you would have to cope with the possibility of an incoming ring signal, which is typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house current.) Longish .signature follows. Skip now, or don't complain. Greg Wageman DOMAIN: greg@sj.ate.slb.com Schlumberger Technologies UUCP: ...!uunet!sjsca4!greg 1601 Technology Drive BIX: gwage San Jose, CA 95110-1397 CIS: 74016,352 (408) 437-5198 GEnie: G.WAGEMAN ------------------ "Live Free; Die Anyway." ------------------ Opinions expressed herein are solely the responsibility of the author.
wheels@mks.UUCP (Gerry Wheeler) (05/02/89)
In article <636@serene.UUCP>, gbell@pnet12.cts.com (Greg Bell) writes:
" I'm still amazed at how phones run entirely off the phone line power.
" Can anybody offer insite on the secrets of running audio amps and chips off
" the meagre phone line power? How 'bout the ammount of current the phone line
" can source?
You'd probably get lots of info on stuff like that by posting to
comp.dcom.telecom if you can. They discuss all kinds of telephone things
there.
--
Gerry Wheeler Phone: (519)884-2251
Mortice Kern Systems Inc. UUCP: uunet!watmath!mks!wheels
35 King St. North BIX: join mks
Waterloo, Ontario N2J 2W9 CompuServe: 73260,1043zantow@tellab5.tellabs.CHI.IL.US (Al Zantow) (05/02/89)
In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes: > >This is in response to the person who was talking about lighting an LED from >the 50 VDC phone line voltage as an indicator for hook status: > > >I'm still amazed at how phones run entirely off the phone line power. They >have lit dials, and audio amplifiers... and I had trouble lighting an LED! >(trouble means the phone company equipment gets loaded down enough that it >shuts off your line for a while!). > >Can anybody offer insite on the secrets of running audio amps and chips off >the meagre phone line power? How 'bout the ammount of current the phone line >can source? > There are some problems with trying to use phone power to monitior the phone line. The phone company equipment "watches" the current in you phone line, and if it goes above a few mA's it assumes you are off hook and want dial tone. When you realy are off hook the current in the line is limited to about 50 mA's. This is partly for your protection so you don't cook if you get across the phone line, but mostly since bipolar transistors are current operated devices. A "regulated" current makes them happier than a regulated voltage source, which allowed the earlier touch tone phones to be more stable. The dial lights and such are probably powered by the yellow and black wires that have about 12 VAC on then from a transformer wired into your house (or aprtment) AC system. This is so the phone company does not need to provide all the power for the "goodies", just for the actual phone circuits. The red and green wires are the ones that have the connection to the phone company, and normaly are not dependent on comercial AC power to work. This is so that you can use the phone after power is knocked out to yell at the electric company for being such clods and letting the lights go out ;-) Not all houses have the "extra" AC power circuit wired up, you can always check this with a meter though. The power availabe from the seconday AC is abount a few hundred mA's, so you can power just about anything from it. "tell me, tell me about Subscriber Trunk Dialing !!!" from Time Bandits (the Devil was asking one of his minions for really EVIL technology like fast breeder reactors, comercial television, ect)
larry@kitty.UUCP (Larry Lippman) (05/03/89)
In article <920@snjsn1.SJ.ATE.SLB.COM>, greg@bilbo (Greg Wageman) writes: > >Can anybody offer insite on the secrets of running audio amps and chips off > >the meagre phone line power? How 'bout the ammount of current the phone line > >can source? > > I've used line-powered modems. They use all CMOS parts to keep > current consumption down, and as I said, since an off-hook device is > required to draw a certain amount of current, why not use it to power > the modem? Interestingly enough, the Western Electric 113-type data set in its "plain vanilla" form was totally telephone-line powered. This is a 300 baud originate-only modem which was developed around 1970. It used NO IC's whatsoever, and employed discrete germanium and silicon transistors. I always thought it was a pretty clever design, but also sort of silly to go to all that trouble for a box twice the size of a 500-type telephone set that was ALWAYS used where AC line power was available. Musta been someone's pet project at Bell Labs... :-) > Even if you could, you would have to > cope with the possibility of an incoming ring signal, which is > typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house > current.) Sorry, but you canNOT ring a phone with "house current". Virtually all conventional telephone ringers are frequency selective, and operate only between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older party line systems (like the "harmonic", "decimonic" and "synchronic") were _very_ freqeuncy selective, with a "bandpass" of 5 Hz or less. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"
larry@kitty.UUCP (Larry Lippman) (05/03/89)
In article <1325@tellab5.tellabs.CHI.IL.US>, zantow@tellab5.tellabs.CHI.IL.US (Al Zantow) writes: > There are some problems with trying to use phone power to monitor > the phone line. The phone company equipment "watches" the current in you > phone line, and if it goes above a few mA's it assumes you are off hook and > want dial tone. The minimum off-hook threshhold current ranges between 10 and 20 milliamperes, depending upon the type of central office apparatus. > When you realy are off hook the current in the line is limited > to about 50 mA's. I don't know about AT&T #5ESS, Northern Telcom DMS-series or other current production electronic central office apparatus, but older ESS and electromechanical central office apparatus had no active current limiting whatsoever. Current limiting was achieved by a combination of a 200 ohm resistance on the ring lead to central office battery, and a 200 ohm resistance on the tip lead to central office ground. Assuming a 50 volt office battery, the maximum short circuit current in the central office at the distributing frame is 125 mA. Loop resistance of outside cable plant further reduced this 125 mA to lower levels; however, if you were close to the CO, you could easily get 100 mA of short-circuit current across the telephone loop. > This is partly for your protection so you don't cook > if you get across the phone line, but mostly since bipolar transistors are > current operated devices. A "regulated" current makes them happier than a > regulated voltage source, which allowed the earlier touch tone phones to be > more stable. This is simply not true; the central office apparatus regulated absolutely nothing. Earlier touch-tone telephone sets used a _single_ germanium transistor to generate the DTMF tones. This tone generator was an _extremely_ clever design which would oscillate under virtually ANY condition (provided the tip/ring polarity was correct; later telephone sets were equipped with bridge rectifiers called "polarity guards" by WECO to solve this problem). > The dial lights and such are probably powered by the yellow and black > wires that have about 12 VAC on then from a transformer wired into your > house (or aprtment) AC system. The voltage is actually 6 to 8 volts AC for WECO sets using a 2012A transformer. Interestingly enough, in the early 1970's GTE/Automatic Electric manufactured a "Trimline" lookalike telephone called "Styaline" which had a dial illuminated by an electroluminescent (EL) panel. The EL panel was powered by a "plug" adapter which plugged into the AC power line and contained two 1 megohm resistors in series with each leg. NO TRANSFORMER - just two series resistors. Hard to believe, but true. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"
jgk@cbnewsc.ATT.COM (joseph.g.klinger) (05/03/89)
Newsgroups: sci.electronics Subject: Re: Using "Free" telephone power Summary: Expires: References: <636@serene.UUCP> <920@snjsn1.SJ.ATE.SLB.COM> Sender: Reply-To: jgk@cbnewsc.ATT.COM (joseph.g.klinger) Followup-To: Distribution: Organization: AT&T Bell Laboratories Keywords: In article <920@snjsn1.SJ.ATE.SLB.COM> greg@sj.ate.slb.com (Greg Wageman) writes: >In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes: >> >>I'm still amazed at how phones run entirely off the phone line power. They >>have lit dials, and audio amplifiers... and I had trouble lighting an LED! >>(trouble means the phone company equipment gets loaded down enough that it >>shuts off your line for a while!). > >Please note that the (Western Electric) telephones (e. g. "Princess") >with lighted dials are supplied 6 VAC by a small line transformer >which is typically connected to the black and yellow "unused" wires in >the phone jack. I have an AT&T 1300 feature telephone which only has connections to tip and ring (I took it apart), no transformer, no yellow/black pair. It also has two high intensity (and obviously high efficiency) LEDs which light the translucent pushbuttons when the phone is off hook, drawing current from the telco line. It's a current design, 1986. Joe Klinger att!iexist!jgk
jgo@mcgp1.UUCP (John Opalko, N7KBT) (05/04/89)
In article <2829@skivs.UUCP>, dr@skivs.UUCP (David Robins) writes: > > I don't know how it is now, but the old Princess and other Bell System > phones had dials with incandescent bulbs, whcih were powered off AC by > a small wall transformer. The newer phones use an electroluminescent panel that is, indeed, powered by the CO battery. What I'm trying to figure out is how you folks who hang voltage dividers and LEDs across your phone pairs keep from frying things when the ~100 VAC ringing current arrives. I imagine you can get a big, honking capacitor that has a low reactance at 20 Hz and shunt it around the LED, but... :-) John Opalko uunet!nwnexus!thebes!mcgp1!jgo PS: How much do 8,000 mfd non-polarized caps cost, anyway? :-)
greg@bilbo (Greg Wageman) (05/04/89)
In article <3139@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: > >> Even if you could, you would have to >> cope with the possibility of an incoming ring signal, which is >> typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house >> current.) > > Sorry, but you canNOT ring a phone with "house current". Virtually >all conventional telephone ringers are frequency selective, and operate only >between 16 and 32 Hz; 60 Hz won't cut it. Some ringers used on older >party line systems (like the "harmonic", "decimonic" and "synchronic") were >_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less. You are correct that the ring signal's frequency is nominally 20 Hz, but you can ring a telephone with house current. I have done it with several different (Bell) telephones of varying vintage. It isn't very efficient, since the coils are tuned for 20 Hz, but it works. (Of course the set must be on-hook for this to work.) Longish .signature follows. Skip now, or don't complain. Greg Wageman DOMAIN: greg@sj.ate.slb.com Schlumberger Technologies UUCP: ...!uunet!sjsca4!greg 1601 Technology Drive BIX: gwage San Jose, CA 95110-1397 CIS: 74016,352 (408) 437-5198 GEnie: G.WAGEMAN ------------------ "Live Free; Die Anyway." ------------------ Opinions expressed herein are solely the responsibility of the author.
ISW@cup.portal.com (Isaac S Wingfield) (05/06/89)
I'll bet that if you used a lead-acid gel cell or some Ni-Cads in your telephone, you could charge them all the time the 'phone was on hook, and have plenty of "free energy" during off-hook use. Isaac isw@cup.portal.com
larry@kitty.UUCP (Larry Lippman) (05/07/89)
In article <17996@cup.portal.com>, ISW@cup.portal.com (Isaac S Wingfield) writes: > I'll bet that if you used a lead-acid gel cell or some > Ni-Cads in your telephone, you could charge them all the > time the 'phone was on hook, and have plenty of "free energy" > during off-hook use. The maximum amount of current which one can draw from a telephone line while on-hook without running the risk of telephone line trouble is about 1 milliampere (this actually exceeds usual loop leakage specifications by a factor of two, but I'll be generous). (.001 ampere) X (50 volts) X (8760 hours) = 438 watt-hours per year At typical electric utility rates of 7 cents/KWH, this power is worth about 3 cents. Assuming that you can save 3 cents per year, let's do the ROI calculations for a 4 ampere-hour 48-volt battery string; this will give you the capability of saving up all your energy for a 2-hour consumption spree once per year. Using 4 Globe JC1240-1 12-volt 4-AH gel cells @ $ 34.00 will cost you $ 136.00. Neglecting the cost of money, changes in future energy costs and useful life of the batteries, the batteries should pay for themselves by the year 6522. :-) <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"