jim@mmsac.UUCP (Jim Lips Earl) (06/26/90)
>What I *am* looking for is a simple circuit to let me know if any phone on >my home line is off hook, preferably by lighting an LED or any sort of >lamp. I haven't figured out how to do it, but I know it can be done. A friend of mine bought a desk phone that has just what you are asking for - a "in use" LED - that lights up when any phone in the house is picked up. And here is the best part: it works off of phone line power! No batteries, or AC adaptor. What I'd need to make such a thing work is a device that works "opposite" of a zener diode. That is, it only conducts when the voltage across it gets BELOW a certain level, rather than ABOVE a certain level, like a zener does. So when the phone voltage dropped below, say 10 volts or so, it would conduct, allowing the line voltage to flow through it and a LED+dropping resistor. The only reason I can think of that this wouldn't work, is that if it drew too much from the phone line, it would keep the line "off hook". If you ever figure it out, let me know! -- Jim "Lips" Earl UUCP: ucbvax!ucdavis!csusac!mmsac!jim KB6KCP INTERNET: mmsac!jim@csusac.csus.edu ======================================================================= The opinions stated herein are all mine.
dt@yenta.alb.nm.us (David B. Thomas) (06/26/90)
jim@mmsac.UUCP (Jim Lips Earl) writes: >What I'd need to make such a >thing work is a device that works "opposite" of a zener diode. That >is, it only conducts when the voltage across it gets BELOW a certain >level, rather than ABOVE a certain level, like a zener does. So when The secret is not voltage but current. An led is placed in the loop so that it turns on when loop current flows. When the phone is off hook, that subcircuit isn't connected, so the led is off. This works for one phone only, however. With voltage comparators, it is easy to make a circuit that simply monitors the voltage on the line, lighting and led if the voltags falls between, say, 5 volts and 20 volts (typical off-hook voltage is 9 volts). I have built and used one of these, and will happily send schematics. This method of course requires external power (I used one of those little wall power supplies), but what is neat about it is that you can use the two unused wires in the phone cable to carry the led signal, and just install an led and resistor at each telephone. Another neat phone circuit (VERY simple) uses an scr, a couple of resistors and a button. If you wish to switch phones, you hold in the button (gating the scr), then hang up. The scr maintains loop current. An led could be included in series to show that the line is being held. Then you can take your time, walk to any phone, and pick it up. The extra load on the line will lower the current through the scr to the point where it unlatches itself. No more extra trips to switch phones. You need one of these at every phone you are likely to answer, but not want to talk on. I'll send this circuit to anyone interested as well. David (yenta!dt@bbx.basis.com)
brian@ucsd.Edu (Brian Kantor) (06/27/90)
The simplest way to do it is to cut one wire of the phone line after it enters the house before it branches to any in-house extension, and insert a sensitive relay with a capacitor across the coil in series with the cut line. That way the current drawn by any phone being off-hook will close the relay, and you can use some of the spare inside wire conductors to run a lamp at each phone. That's pretty much the way old CO equipment did it. (You need the capacitor to prevent the impedance of the relay from attenuating voice levels.) You could add another relay and have a hold circuit too. If you don't want to do it that way, you can design a little widget running from a 9-volt battery and using a CMOS voltage comparator to sense the voltage across the phone line. That will typically be more than 30 volts when all phones are on-hook, and less than 20 when one or more are off-hook. It'll have to be a rather robust circuit, since ringing voltage is more than 100v if you're close to the CO. Probably about $10 worth of parts if you bought them at retail. - Brian
jones@optilink.UUCP (Marvin Jones) (06/28/90)
Perhaps one solution here would be to use a Low Voltage Detector, such as the
Seiko 8054 series or the Motorola MC34164. These are typically used to provide
an open drain pulldown at the output (for driving processor reset lines)
when the input voltage "browns out" below 4.7 volts or so. With a divider
network on the input, this could be adapted to 10 volts or whatever
threshold you desire. The open drain output could pull down an LED/limit
resistor to indicate "in-use". The system should reset itself automatically
when the voltage increases again. One would need to provide a zener clamp or
resistive current limiting to protect the device from up to 60 volts of open
circuit "on-hook" voltages.
Hmmm. Just might work.
--
=== Marvin Jones === Optilink Corp. === Petaluma, CA === 707.795.9444 X206 ===
=== UUCP: {uunet, tekbspa, pyramid, pixar}!optilink!jones ====================
=== CIS: 71320,3637 or 71320.3637@compuserve.com === AOL: mjones =============ee5391aa@hydra.unm.edu (Duke McMullan n5gax) (06/28/90)
This message is empty.
wiml@milton.u.washington.edu (William Lewis) (07/02/90)
In article <904@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes: >In article <1558@yenta.alb.nm.us>, dt@yenta.alb.nm.us (David B. Thomas) writes: >> Another neat phone circuit (VERY simple) uses an scr, a couple of resistors >> and a button. If you wish to switch phones, you hold in the button (gating >> the scr), then hang up. The scr maintains loop current. An led could be > >Ummm, can you post this one? Enough of us do occasional phone hacking that >I'd like one as well. I second that. It would be extremely useful around here ... Anyway, re: tying phone lines together to make a DIY conference calling set-up: So far, I haven't tried it with three lines (it would require stringing more wire, rather than just hooking up the unused pair in the 4-conductor cable already in place), but it works great with two lines, well enough that someone on the other end of one line can touch-tone dial the other line after I throw the switch. As it is, I'm just shorting the lines together (being careful about polarity =8) ). The off-hook voltage goes up a little when both lines are going into one phone (from 5+ volts to 7 or 8), but it seems to have no adverse effects upon the system. I do feel a little uncomfortable just shorting the lines, so as soon as I get a pair of capacitors of a reasonable value I'm going to put them in. I tried it first with a pair of capacitors I use for power-supply bypassing on digital circuits (0.1 uF? the numbers on the side are 104) and the signal was pretty faint but understandable. (I didn't have any more reasonable values lying around =8) ) I'll keep the net informed if the circuit does anything interesting. -- wiml@blake.acs.washington.edu Seattle, Washington | No sig under (William Lewis) | 47 41' 15" N 122 42' 58" W |||||||| construction
barber@beowulf.ucsd.edu (Terri Barber) (07/03/90)
In article <4633@milton.u.washington.edu> wiml@milton.u.washington.edu (William Lewis) writes: >In article <904@eplunix.UUCP> raoul@eplunix.UUCP (Nico Garcia) writes: >>In article <1558@yenta.alb.nm.us>, dt@yenta.alb.nm.us (David B. Thomas) writes: >>> Another neat phone circuit (VERY simple) uses an scr, a couple of resistors >>> and a button. If you wish to switch phones, you hold in the button (gating >>> the scr), then hang up. The scr maintains loop current. An led could be >> >>Ummm, can you post this one? Enough of us do occasional phone hacking that >>I'd like one as well. > > > I second that. It would be extremely useful around here ... Okay, let's see if I can remember how to do it (I didn't post the original, I just want to stick my $.02 in.... Please excuse the ascii graphics.... -------------------+------+---- | | | / | / momentary switch | | | / | \ 1200 Ohm | / -----____| / \ /___\ SCR | / \ 600 Ohm / | ^^ ----- // / \ // /___\ LED | | -------------------+---------
raoul@eplunix.UUCP (Nico Garcia) (07/09/90)
In article <4546@milton.u.washington.edu>, wiml@milton.u.washington.edu (William Lewis) writes: > When the voltage gets high enough (ie, off-hook condition), > the Zener starts to conduct, dropping the voltage oacross the > LED to less than 1.6 or howevermany volts. Maybe another resistor > directly in series with the LED would help... > This seems simple enough, or am I missing something? You sure are: the phone company supplies ~48 Volts at their end of the phone wires to an on-hook/OPEN-CIRCUIT line. This is what you will measure on your phone lines. When you are off-hook, current flows (which used to latch the relays in older phone systems) and the DC voltage on your phone drops to about 10 Volts due to line losses. With your circuit, the LED or the Zener will *always* conduct current, and the phone company will never hang up your phone. So remember: high voltage = open circuit = ON-HOOK. Otherwise the phone company would have to supply current to all the hung up phones in the world, a wasteful technique. -- Nico Garcia Designs by Geniuses for use by Idiots eplunix!cirl!raoul@eddie.mit.edu