atcomsystems.ca/forum Forums TDM Business Telephone Systems Panasonic Telephones & Systems tying TAW848 into a building intercom...

I've got a door card for the 848 which is working fine to ring my phones and let me know when someone is at my front door (I just have it attached to a spst doorbell switch and haven't actually wired the doorphone or door opener part orf things, and I don't intend to since I live in a doorman building and that's not necessary). Now it's time for something more ambitious.

As I mentioned, I live in a doorman apartment building and the doorman is able to call up to us to announce visitors with a building-wide intercom system. The handset that sits in my apartment is a Comelit Okay 2422 phone. Trouble is that it's not standard. Here are some differences from regular phones:

- the handset has two leads (not so unusual) but in order to call down to the doorman I need to push a button (which basically shorts the leads as far as I can tell).

I gather the ring voltage is 12 or 24VAC rather than 88VAC.

In addition, I measured resistance accross the phone when it is disconnected from the wires leading downstairs. I see that when the phone is off hook there is a resistance of about 27,500 ohms rather than something that looks close to a clean short to an ohm meter. However, as I said before when I push the button to ring the doorman downstairs that shorts the line.

What's the best approach to this? I was thinking I could either:

- connect the line into one of my CO ports so the 848 would treat it as another incoming line or

- connect it to my doorphone card in a way I haven't yet thought through.

If I connect it into one of my CO ports I imagine I will need a transformer to step up the ring voltage to something closer to 88VAC, correct? Or is there a way to have the 848 recoginize a ring that is only 12 or 24 VAC? Or do I need to do something more complex? I would also perhaps need to throw in a 27,500 ohm resistor in series with the phone so I don't mistreat the building system, correct? And finally I would need to set up a relay and assign a button on my phones to close that relay if I want to be able to call down to the doorman (but I never use that feature anyway so that's not so important).

If I try to connect it to my doorphone I haven't even begun to think about how I would go about it.

What I'm really after is advice on which approach I should seriously pursue (and of course any tips you might have beyond that would be greatly appreciated too!).


Thanks!

I did some more measurements and find that:

- the building intercom's line has a constant 9V DC running on it (I haven't measured the ringing a/c yet but from looking up stats about this product on the web I think the ringing voltage is either 12VAC or 24VAC as I mentioned earlier)

- a regular analog phone (POTS) appears to present about 1 million ohms when off the hook (this surprises me, since I would have thought the resistance would be reasonably close to zero), as compared to the resistance across the leads on the intercom when off the hook of about 27,500 ohms (or zero when shorted to call the doorman).

[edit: I just checked that via google and see that an offhook POTS should be more like 200-300 ohms, so I must be measuring this wrong (impedence problems?).]

This is harder than I thought (may be impractical). I just talked to a comelit distributor who tells me that these handsets are not remotely like a POTS. He says the ringing voltage is -13VDC (yes, that's DC) and talking is modulated over a positive DC voltage via an amplifier. So, if I hook up a POTS to the line nothing at all will work (the POTS phone won't ring when the doorman rings up and if I pick up and talk the doorman can't hear me).

I imagine there is some clever circuitry that would bridge a POTS with this, but that's over my head unless there are boxes out there (or circuit diagrams) that I could use.

Anyone know of any possible solutions for this or should I just give up?

you have some cahones, my friend! steer clear of your super. i hope you get it working. can u imagine the disaster if you blew out the building intercom!

Yup, I'd be out a lot of dough

But as I've thought about this more I think I can probably do this. I am buying a couple of spare handsets (they're only $36 apiece) that I can use as guinea pigs.

I realized I can easily get my phones to ring when the handset rings by sticking a relay in parallel to the handset with a diode in there to block positive voltage and allow negative voltage through. When the -13V ring voltage comes through ringing the handset the diode would allow the voltage through and the relay would close (which would be sensed by my doorphone card as someone pressing the doorbell).

And I can also use the doorphone card to short the two wires when someone presses a button on my 7736 in order to simulate pressing a button on the handset to call down to the doorman.

The last piece of the puzzle is how to get a regular phone to talk on the line and be heard by the doorman. I can the line into an incoming CO on my 848 but I think I'll need to mess around with some circuitry to make that work. I may just give up and do the ring part of things (which really shouldn't be hard or risky). I'll report as I proceed in case anyone else is interested.

checkout mikesandman.com
look at main page for
Telephone Accessories---Wizards Toolbox
lots of neat stuff.

listen to brokeda-- he fixed a chronic problem with my tva50/taw848 setup which had panasonic stumped for months!

Thanks for your input, brokeda.

I took a look at the relays on the site you mentioned, brokeda, and I'm trying to make sure they will work for this application but I'm not sure about it. The uses shown are all in series with the line (measuirng things like when the phone goes off hook) but that won't distinguish between positive 9VDC and negative 13 VDC (I don't think the relays reverse on negative voltage to give me a way to close relay leads only when the voltage is negative, do they??).

One thing I thought of is this: I could put the high sensitivity relay, a diode and a pretty large value resistor in series together and throw that contraption across the line (in parallel to the handset). That way, when the voltage is postive the diode would block things and the relay/diode/resistor would be invisible to the doorman system. And when the voltage turns negative, the diode would let current pass, the resistor would be of high enough value so that the relay/diode/resistor wouldn't load down the system too much and the relay (because it is so sensitive) would have enough current to click on.

Is that what you had in mind? If so, any feel for what kind of resistor I should be using? Perhaps I should be using a potentiometer and start with a very high value and then start cranking down till I find a place where it all works...then I could measure the potentiometer and get a resistor to match the resistance.

Or am I way off base?

Sounds like between Sandman and Radio Shack you can do it. Just be careful. Measure twice and cut once!

if i pulled this in my old building, Vinnie the Super would have me fitted for a pair of concrete shoes!