larry@uunet.uu.net> (08/24/90)
In article <11203@accuvax.nwu.edu> julian@bongo.uucp (Julian Macassey) writes: > > I was thinking of putting together a linesman's handset here, and I > > was wondering, is it really as easy as putting good-quality Alligator > If you want to build your own buttset, here is something I > have done. Get a Western Electric (AT&T) Trimline phone ... > Attach test leads to the Tip and Ring. ... > What you have now is a buttset that is touchtone only and has > no monitor function. To add the monitor function you need to add a 2 > uF (4 is better) 250V mylar cap. This cap is switchable in series with > Tip and Ring. When the cap is in series, you can listen but not speak. > A miniature toggle switch can fit in the lower part of the earpiece, > the cap is best made up of units distributed in nooks and crannies > round the set. A word of caution on creating a monitor function through the above technique... This will work, but it will create a monitor function that will bridge a rather low impedance across the telephone line under test. A very noticeable "click" will be heard if the telephone line is in use, in addition to a noticeable degree of attenuation. Furthermore, should a data call be in progress, you may cause interruption - unless there are error-correcting modems in use. "Real" buttsets are carefully designed to create as high an impedance as possible in the monitor mode - typically 6,000 ohms or better. The "talk-monitor" key switches more leads that just a capacitor in series with one side of the line. A transformer is switched in and out of the circuit, with the transformer providing coupling between the receiver element and the telephone line. A series capacitor is also used; however, in better quality buttsets this capacitor is typically 0.02 uF - which is a far cry from 2 uF. While a trimline telephone set makes a good poor man's buttset, there is almost no room to attempt any circuit modification to improve monitor mode sensitivity by increasing its impedance. Incidentally, here is an old trick to minimize click when connecting a buttset in monitor mode. Connect one lead first, then use your finger to bridge the gap between the other lead and its terminal; then make the connection. The presence of your finger will usually provide enough series resistance to charge the monitor circuit series capacitor, thereby minimizing the click. Larry Lippman @ Recognition Research Corp. "Have you hugged your cat today?" {boulder||decvax||rutgers||watmath}!acsu.buffalo.edu!kitty!larry VOICE: 716/688-1231 || FAX: 716/741-9635 {utzoo||uunet}!/ \aerion!larry