deanr@sco.COM (Dean Reece) (08/18/90)
In article <58953@bbn.BBN.COM> grossman@bbn.com (Martin Grossman) writes: > > >I would like to setup a mini phone service at my home connecting just 2 or 3 >(maybe 4) phones. What do I need to buy (radio shack) to do this. > >-------- -------- >|phone |------------------------|phone | >| |------------------------| | >-------- -------- >|ringer| |ringer| >-------- -------- > >MISC INFO: > > 1) There will be no connections to any regular telco phone lines. > 2) The little box's above marked ringer should be small and right next > to the phone(s). They should just have 1 button each and ring the > other phone(s) when depressed. > 3) The box's marked phone should be any phone that can be hooked upto > the standard (US) phone system. > 4) I understand just a little about how phones work... > a) I think its 50-60VAC at 20-30HZ to ring a phone > b) phone looks like inf resistance/reactance when on hook > c) phone looks like approx 680 ohms when off hook > d) there's a small DC voltage when off hook > 5) The dial (or push buttons) won't be used (ie just the ringer box) > 6) This will be used instead of multiple walky-talkies. > >USE: > 1) One phone will be in the basment and the other 1 or two will be > upstairs. I want to be able to signal (via the ringer) and talk > without running upstairs or shouting. Besides...its a great project. > >QUESTIONS: > 1) how do I connect tip and ring from each phone Connect All Tips together and All rings together. > 2) what do I need between the phones to act as the phone co. Buid a 60 VDC supply (< 1 amp) in series with a resistor (about 10k, try 25k linear pot). Connect the supply (-) to green, Supply (+) to the resistor, and the other resisitor lead to red. This should allow talking on any 2 (or perhaps 3) phones. Too many phones on the line at once would make the voltage drop too low; however, you can adjust the pot for desired volume. You want the voltage to be about 60 volts with all phones on hook, and about 6 volts with 2 phones off hook. Experiment, different phones will produce different results (sound quality). I personally like the old Carbon mic units since they don't sound so windy. > 3) what do I use for the rigner box 2 options: Dumb & cheap, or smart & expensive: 1) Dumb & cheap: Build another supply, either 3,6, or 12 vdc, again < 1 amp. Hook the supply (-) to the green (this will be your PBX's ground) and hook the supply (+) to the yellow (this will be ringer supply). Put this supply in with the 60 volt supply and position it near the center of the wire run. Now, for each ringer box, you buy a 4 wire modular jack, a buzzer or bell to match the supply voltage you picked and consuming < 200 ma, a SPDT or SPST(NO) push button switch (see below) and a small case. If you got a SPST(NO) button, then all buzzers will go (including the local one). Hook the switch contacts between Yellow and Black. Hook buzzer (-) to green and (+) to black. If you got a SPDT button, hook the black wire to the COM (center) contact of the switch. Hook Yellow to the NO side of the switch, and connect the buzzer between the NC side (+) and green (-). This arangement cuts out the local buzzer, so only the station(s) you are calling get buzzed. 2) Smart and expensive: This counts on the Touch Tone(tm) keyboard. You press a particular digit to ring a station. Big advantage is that a seperate case isn't necessary if you get desk type phones and remove the bell. Start as above (6 to 12 volt supply in main station). For each ringer, you'll need a piezo type buzzer with CMOS drive (a 3 lead device the Shack sells: 2 for power, 1 for gate) You then build a small dual signal detector based on 2 567 chips. You make a high impedance tap between tip and ring, and clean up the signal with an LM324. You tune each station to hear a DTMF(tm) pair and activate its buzzer when the pair is present. Put the detector/buzzer assembly where the bell was, and voila, just press a number and ONLY THE SELECTED STATION rings!! If you want schematics, I can mail them to you, but they are a but much for my ASCII artistry. > 4) what do I need to add if I want to hookup a third phone or forth. Just parallel them all you like. If you go with the Dumb buzzers, then buzzer current may get to be a problem. The other problem you'll have is if too many phones are off hook at once, volume will drop too low. You could build some intellegence into the 60VDC supply so that some lower resistance would replace the 10K resistor if the voltage dropped below, say 4 volts. Or you could just not allow many stations in use at once. > PS in now way am I trying to compeat agianst MA-BELL. I'll call the anti-trust lawyers right away :-) Hope it helps, sounds like a fun project. Good luck -deanr@sco.com ______________________________________________________________________ | Dean Reece Member Technical Staff |"The flames are all long gone | | The Santa Cruz Operation 408/458-1422 | but the pain lingers on" | |___________deanr@sco.com_______________|___________________Pink_Floyd_|