I have, in storage, a commercially produced telephone accessory [cost about $50] to enable a single line telephone to operate two telco lines. Although the device could sit in view showing which line Ring indicators, 'line in use' indicators, AND HOLD indicators multi color LEDs; it could be placed out of sight because it had no buttons. You operated transitions by using ON HOOK/OFF HOOK, or 'flash' button on the keyboard. Very handy to place a present conversation on HOLD in order to answer the other line's incoming call. Plus, if the other person terminated the call while on HOLD; the unit sensed the hang up and released the line.
Something to watch for: Whether that one-line phone is the old type with p ower supplied through the phone line, or the new type with power supplied b y a nearby AC to DC converter. A solution for one type is unlikely to work for the other type, unless it has a switch to tell it which type you have.
All phones draw current from the line when off hook and respond to large AC by ringing. any that dont won't work correctly on ordinary telephone circuits.
Phones with individual powersupplies typically need them to power features that must run while on-hook (when the phne line can provide no significant power) or that require more power than the phone line can provide.
eg: the cordless phone base station must run the radio receiver to listen for the handset while on hook so the user can place a call. (and it probably powers most features of the phone)
The simplest solution is something based around ring detectors and a latching relay (as the device can be line powered), several people have proposed this, (I deleted my contribution before it was sent)
Such a device would probably behave badly when a second call arrives on the other line, unless significant complexity is added...
;-) Anyone can conjecture. Last year I had to assist in filing an action against a patent that was basically a single block diagram saying "this block is an RFID tag, and has these specs", thus is an invention... took more than a year but the patent court finally whacked them up aside the head ;-) ...Jim Thompson
| James E.Thompson | mens |
| Analog Innovations | et |
The ring signal is a strong (I think the specification is 96V p-p) 20Hz AC signal on top of the 48V DC to the phone. It's got enough oomph behind it to ring real, physical bells in several phones at once (or in my case, one honkin' big bell that can be heard all over my 5 acres when the shop door is open).
When you pick up the phone, about 48V/1200ohms flows through the phone, providing power to the phone and a signal to the central office that the phone is picked up.
However, for those interested. You can do an awful lot using the power from the telco, and not require a wall wart. You can take tiny bit of power when ON HOOK, assume around 10Meg to 20Meg is best [from memory the spec leakage requires more than 5MEG] you can use a couple of 2N3055 NPN's [MPS3055 now?] to do all kinds of things, like detect if another phone has acquired the line. ring detection is usually a separate bridge recifier circuit to give a sequence of appropriate pulses. Everything uses a bridge, but is not really necessary, except sometimes the telephone company will actually reverse the polarity of your call IN MID CALL!!! so best to always use a bridge. With today's new equipment they may not be doing that anymore.
A lot more power is available when OFF HOOK. To put the line on HOLD, requires something a bit husky like 1/2 watt R's and 2N2022 metal can to take the heat.
Circa 80's, I wrote an article that appeared in a Gernsbach Publication, the people who published Popular Electronics. Gernsbach was trying to launch a new publication called "Hands On Electronics". They wanted their new publication to contain DIY Electronic Projects, and 'how to' article's. The article was entitled "Hands-free Telephone Headset" [I think] which described a project for using parts around your lab and a bit of a tutorial on telco lines. For those of you who appreciate my writing 'ability' as demonstrated here, the Editor did NOT change a single word in the manuscript, nor edit any of the drawings and images. That's me in the photo using the hands free telephone on the first page. It all started when I went to buy a hands free telephone headset and the Plantronics unit was priced at a whopping $179 !! so I built my own with parts from the lab and $5 worth of connectors and a box.
And if you are 13 years old and holding the wires whilst hooking up a surreptitious phone when someone calls your house the 96V shock will surprise the crap (almost) out of you and you can't tell anybody about it because you were sneaking around installing the phone. Eric