atn@cory.berkeley.edu (Alan Nishioka) (02/06/91)
I've been trying to build a box to make key phone's lights flash and to implement a hold button. I am using a 1M/5M voltage divider across the line and some comparators to detect voltage levels and thus ring and off-hook. This isn't the best method, but I want it to coexist with other phones on the line in parallel and to be adjustable (ie. sans zener diodes) for now. (I forget what you call these phones but they have fifty wire cables, five line buttons and a hold button, ex-standard AT&T office phones) The problem enters when I add a 200ohm resistor and a 2n2222 transistor across the line to implement the hold button. (Common emitter, with a 47K base resistor to a CMOS latch output) When I call the line, a phone wired in parallel chirps shortly (a Ferrari phone, if that matters :-) and the ringing voltage ceases. The ringback is still heard on the calling line and the called line is still on hook and can be answered, but there is no ringing voltage. When I wire the base of the transistor low (off), the problem goes away. Is this some sort of protection that is tripping because there is a wierd impedance across the line? What could be causing it? My CO is a 1AESS, I think. It has been a long time since they installed it and offered tours. You hear a clunk when call waiting beeps. Alan Nishioka KC6KHV atn@cory.berkeley.edu ...!ucbvax!cory!atn 974 Tulare Avenue, Albany CA 94707-2540 37'52N/122'15W +1 415 526 1818
jon_sree@world.std.com (Jon Sreekanth) (02/08/91)
In article <16771@accuvax.nwu.edu> atn@cory.berkeley.edu (Alan Nishioka) writes: > The problem enters when I add a 200ohm resistor and a 2n2222 > transistor across the line to implement the hold button. (Common > emitter, with a 47K base resistor to a CMOS latch output) I'm not familiar with Key Switch Units, but assuming yours just simply distributes the incoming telco line, the specs on the DC battery and AC ring voltage are 56.5 V DC, 40 to 150 V RMS at 15.3 to 68 Hz (so called FCC B ring). 2N2222 has a Vceo of 40V, so it's almost certainly breaking down, and shorting ring. In a similar application, in one of my designs, I've used an MPSU10, which is a 300V rated NPN transistor. It does not have too high an hfe, so to drive it from an HC, I use an MPSA42 (also 300V) in a Darlington arrangement, so MPSU10 is guaranteed to saturate. Also, it's probably safe to preface this with a bridge rectifier (I use 1N4004's). This way, it can hold the line offhook, even if polarity is reversed. I've found several homes with tip and ring wired with incorrect polarity. Don't know about KSU's. > When I wire the base of the transistor low (off), the problem goes > away. This puzzles me, but try the high voltage transistor, anyway. Regards, Jon Sreekanth Assabet Valley Microsystems Fax and PC products 346 Lincoln St #722, Marlboro, MA 01752 508-562-0722 jon_sree@world.std.com
john@zygot.ati.com (John Higdon) (02/09/91)
Alan Nishioka <atn@cory.berkeley.edu> writes: > I've been trying to build a box to make key phone's lights flash and > to implement a hold button. Not to throw cold water, but I can think of two reasons for not bothering with this project. The first is that whatever you come up with SHOULD be certified to comply with the various applicable parts of FCC rules, since it will have to connect with the network directly. The other is that genuine 1A2 KSUs can be obtained for next to nothing (or nothing at all if you happen to be in the right place at the right time) and even if you save money you will be wasting your time. > The problem enters when I add a 200ohm resistor and a 2n2222 > transistor across the line to implement the hold button. (Common > emitter, with a 47K base resistor to a CMOS latch output) The transistor is conducting momentarily in the presence of the high ringing voltage. It just takes one spike to stop the ringing and then you have a "trouble" condition. If you do have a 1AESS this would be expected. This particular switch will protect itself from bogus ringing loads by killing the ring current if it exceeds a certain value. A transistor breaking down and putting a 200 ohm load across the line would create such current. Pac*Bell is running slimy ads for it $ENTREX service which shows old phone systems being thrown into this big junk pile. Just find this pile and pull out a KSU :-) Seriously, "fat wire" keys systems are so out of favor now that you should be able to obtain a KSU for nothing. Unfortunately, I have given my last one away. John Higdon | P. O. Box 7648 | +1 408 723 1395 john@zygot.ati.com | San Jose, CA 95150 | M o o !
jeffj@uunet.uu.net (Jeff Jonas) (02/15/91)
From TELECOM Digest: Volume 11, Issue 100, Message 6 of 8 >I've been trying to build a box to make key phone's lights flash and >to implement a hold button. The July, 1990 edition of {Modern Electronics} magazine features the schematic of a microprocessor based "phone mate" that provides hold, recall, timer and catched digits as they're dialed. The schematics should be helpful if you don't build the entire thing. Jeffrey Jonas jeffj@synsys.uucp synsys!jeffj@uunet.uu.net