SPGDCM%cmsa.Berkeley.EDU@UCBVAX.BERKELEY.EDU.UUCP (12/05/86)
Years ago I saw (I think in an electronics hobbyist magazine) a very simple circuit which one could add to any phone to gain an illuminated "hold" function. It worked on regular twisted-pair phones; it worked only on the phone you wired this way. As best I remember it, it was something like a zener diode, a resistor, a temporary-connect push-button, and an led; maybe a neon light instead but I think not. The concept was that once your button made the connection, the zener diode effectively shorted the line enough to hold onto the circuit (one would need to draw the voltage down to say 10v). Then if any other extension went off-hook, it would draw the line down to 5v. which somehow dropped the zener out and it let go, as a good hold should. The LED got activated properly somehow so that it glowed on the held instrument until an intercept or that instrument went off-hook and dropped hold. Now I am really sad I lost the diagram, because I want to add hold to all my phones. Costs too much to upgrade them all when I could add this instead for say $2-4 each. Anybody know the circuit? Especially useful would be the exact component values, preferably within the set widely available such as Radio Shack, or if necessary with a source of supply if the values are rare. P.S.: unfortunately many commonly-sold phones still don't offer hold, but even worse, many offer what I would like to see called "false-hold": you can flip a dumb toggle or slide switch on the phone to "hold" the line, but must shut that particular switch off to unhold; really a mute toggle, not a hold. Some store's ads (such as San Francisco Macy's) call this hold, and you waste a visit to the store. Some mail-order houses have had to resort to calling the correct feature "true-hold". Hmmm... as if you had to see words like "real food" or "true-movies" in ads or else get cheated... Thanks, Doug
tj@utcs.toronto.edu.UUCP (12/16/86)
here is a guess... Looking at the components you suggest I could make one that should work. put a resistor, and LED, a Zener, and an SCR in series. The Gate of the scr goes through a resistor and a pushbutton to + phone line. Lets draw a schematic online... (Hope this works) -----------phone line + -------+-------------+ | | R | | | \___PB_______SCR | \_|_ Z \ +------| \_|_ R Z \ | | LED | | --------------- phone line - ---------+------+ The SCR is off until pushbutton PB is pressed making contact thus turning it on. Upper Zener is some voltage higher than the off hook voltage of the line. (say 10 volts) Until the line goes higher than 10 volts it is off, even though the SCR is on. Hold the push button as you hang up. This turns on the SCR and as the line voltage rises the zener conducts and the current through the SCR will hold it on (get a low hold current scr, say 3 ma) The resistor in series with the push button supplies the small turn on current. 5 k should do. The lower zener limits the voltage across the LED. Make say 5 volts. This would put the r in series with the LEd at about 150 ohms. Like no guarantees...
king@KESTREL.ARPA.UUCP (12/22/86)
From: tj <tj@utcs.toronto.edu> put a resistor, and LED, a Zener, and an SCR in series. The Gate of the scr goes through a resistor and a pushbutton to + phone line. Lets draw a schematic online... (Hope this works) -----------phone line + -------+-------------+ | | R | | | \___PB_______SCR | \_|_ Z \ +------| \_|_ R Z \ | | LED | | --------------- phone line - ---------+------+ The SCR is off until pushbutton PB is pressed making contact thus turning it on. Upper Zener is some voltage higher than the off hook voltage of the line. (say 10 volts) Until the line goes higher than 10 volts it is off, even though the SCR is on. Hold the push button as you hang up. This turns on the SCR and as the line voltage rises the zener conducts and the current through the SCR will hold it on (get a low hold current scr, say 3 ma) The resistor in series with the push button supplies the small turn on current. 5 k should do. The lower zener limits the voltage across the LED. Make say 5 volts. This would put the r in series with the LEd at about 150 ohms. Like no guarantees... I doubt this circuit would work, and it could be dangerous. There is no impedience between a 28 volt source and ground; that voltage will be pulled down to 15 volts. What would limit the current through the SCR and two zener diodes? -dick
larry@seismo.CSS.GOV@kitty.UUCP (12/26/86)
>> From: tj <tj@utcs.toronto.edu> >> >> put a resistor, and LED, a Zener, and an SCR in series. The Gate of the scr >> goes through a resistor and a pushbutton to + phone line. Lets draw a >> schematic online... [schematic delected for brevity] >> The SCR is off until pushbutton PB is pressed making contact thus >> turning it on. Upper Zener is some voltage higher than the off hook >> voltage of the line. (say 10 volts) Until the line goes higher than >> 10 volts it is off, even though the SCR is on. Hold the push button >> as you hang up. This turns on the SCR and as the line voltage rises >> the zener conducts and the current through the SCR will hold it on >> (get a low hold current scr, say 3 ma) The resistor in series with >> the push button supplies the small turn on current. 5 k should do. >> The lower zener limits the voltage across the LED. Make say 5 >> volts. This would put the r in series with the LEd at about 150 >> ohms. > I doubt this circuit would work, and it could be dangerous. There is > no impedience between a 28 volt source and ground; that voltage will > be pulled down to 15 volts. What would limit the current through the > SCR and two zener diodes? The circuit should work under most circumstances, and is by no means "dangerous". The current on a telephone subscriber loop is limited by the central office apparatus. In most central offices there is effectively 200 ohms between the tip of the line and ground, and 200 ohms between the ring of the line and -48 volt office battery. This results in a total of 400 ohms resistance so that from Ohm's law a short circuit across the loop sitting right at the CO would draw 0.120 ampere. There are two discreet resistances as described above so that the subscriber line will be longitudinally balanced with respect to ground. Since it is necessary for the CO apparatus to detect current flow in the subcriber loop to indicate an off-hook condition, the above resistances may be incorporated as two windings in a "line relay", ferrod sensor, repeating coil, etc. While a line relay and ferrod sensor sense current flow directly, the voltage drop across a resistor or repeating coil is used for current detection in other types of CO's. So the point is: 0.120 ampere _worst_ case isn't going to hurt anything or destroy a properly selected SCR (I would use 200 V @ 1 ampere to be conservative and handle surge voltages). A more typical short circuit current value for a subscriber loop (since most subscribers don't live next door to the CO) is .030 to .060 ampere. One minor improvement which could be made to the circuit is to make it polarity-independent and protect against possible false ring-tripping by placing a full-wave bridge rectifier (use 200 PIV @ 1 ampere to be conservative) between the circuit and the telephone line (the AC leads go to the telephone line). Also, a small resistance may be required in series with the SCR cathode to make certain that the circuit releases properly under some subscriber loop conditions. You could add say, a 500 ohm pot and know that you'll be in range. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|bbncca|decvax|nike|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?"
clark@beaver.cs.washington.edu@ssc-vax.UUCP (01/15/87)
I was thinking some more on the topic of hold circiuts and remembered that there was a article in a Popular Electronics a few years back that looked like a very good setup. It was an active type design, powered off the 120V line, which listened on the phone line for the '#' key. When this key was detected, a relay was activated, switching an audio transformer/resistor network across the phone line. This caused an off hook condition that held the line. The reason for the transformer was to provide the option of music on hold, by connecting a tape player or what ever. The controller in this design monitored the phone line, sensing when any phone was taken off hook, deactivating the hold relay. In addition, the controller had a time out that forced the hold function to switch off after about five minutes so that the line would not be tied up all day..... I was going the build the thing, but never got to it.... Any comments for someone that did build and test it ???? Roger Swann uucp: uw-beaver!ssc-vax!clark