I am trying to electronically simulate an "off hook" condition.

Long story (which I won't bore you with since its not really relevant) but I want to automatically answer a ringing phone, then automatically disconnect.

I'm good with the circuitry to detect a ringing condition, no problem there.

But once that "triggers" I need to know what to place between tip/ring to "answer" or simulate an "off hook" condition. I'm guessing this would be a resistor of some value, but am not sure. I'd like to do this in a TelCo friendly manner (as opposed to just connecting tip/ring together!!).

Thanks!

P.S. If anyone knows of a pre-existing off-the-shelf "answer and hang-up" unit, that would be great. From scratch, I can build this for about $40 in parts so I'm looking for a pretty cheap solution.

just a plain old short will do it

Is that TelCo friendly? I recall doing stuff like that a million years ago when I was a kid which "dropped a card" at the CO, which then prompted a visit to my house by Ma Bell, not pleasing my parents.

I was kinda looking for some resistance value (if that is what is needed) which would act like a real phone.

With phone unpowered, ohm meter between tip and ring with a set off hook, that will give you the value you are looking for.

Thanks for the reply, but unfortunately, I don't own an ohm meter.........

Most phone company POTS test are done with a 600 ohm termination, which is what the old phones put on the line. Todays phones put about a 1200 ohm temination on the line some of the cheap ones are even higher in resistance. You should be fine with 1200 ohms.

Was gonna say the same thing. 600 to 1.2K will work just fine. I believe the old 400E 1A2 cards use 600 ohms for the hold bridge.

I want to automatically answer a ringing phone, then automatically disconnect.

Read some of the threads here by people who wanted to know why they have a ring-trip situation and you will find that it's because they used a protector with a low clamping voltage (lower than the 90V ring). A 75V protector on the line will do what you want all by itself. Ring/click/disconnect.

-Hal

Captain - go back to your old TAPs and look up the "Answeroo" - it does exactly what you are describing. I'll look it up this evening, and PM you.

TTT -- Hey, anyone who remembers TAP is AOK with me! Mine were all lost several moves ago.

The problem with the Answeroo is that the phone doesn't ring at all. I need it to actually ring once for this application.

hbiss -- that's exactly what I am trying to do. Maybe I should have named the thread: "ring/click/disconnect!"

I did a search as you suggested and found some theads discussing clamping and line protection, but no real source for a 75V protector. Can you be a bit more specific because I think you have hit the nail on the head. Thanks.

What is TAP?

TAPS and TOPS were Bell System practices, one for installation and one for trouble shooting.

Jeff - sent you a PM regarding TAP

Captain - even a ring trip caused by a faulty or incorrect protector will squelch the ring cycle. You can simulate this with a 56V/5W Zener diode placed across the line. One direction will short the line totally, the other will do nothing until the ring comes in. 90V will overcome the 56V threshold and it will conduct to stop the ring in its tracks.

The elements in solid state protectors are basically two zeners back-to-back so as to give non-polarized (AC) clamping. So a single zener is going to do the same but will be polarity sensitive as you point out.

In many cases the caller will hear maybe a half ring although you may not hear anything.

If you wanted to allow a full ring (or more) then pick up the line you would have to get a little more complex. A delayed relay powered by the ring voltage that would put a short on the line comes to mind.

-Hal

I'm using an ELK-930 to detect ring, which pulls to ground when the ringing voltage is approx 50 volts. It then triggers a relay (ELK-960) which performs the task that I need (shorts two wires for one full second).

This works great with one exception.

The problem is that the ELK-930 follows the ring pattern. I need one-shot and done (at least for a few minutes) for my task.

Yes, I can hang-up after one ring -- that's what I do now and it works just fine. The ELK-930 triggers immediately, so there is a lot of time to hang-up, but I'd really like a more elegant solution.

I could put second relay in the middle with a delay which would ignore further input from the ring detector for a few minutes, but it sounds like the zener diode might avoid that complexity.

I think I understand what you are saying about how a zener diode works: A zener diode placed between the ring/tip just sits there and does nothing until it gets ringing voltage, higher then its zener rating. It then connects the ring/tip, causing a short which "answers" the phone. With the phone answered, the ringing voltage stops, so the zener stops shorting, thus the phone is "hung-up." Do I have that about right?

So that brings up several questions:

Will the zener diode prevent the ELK-930 from detecting ring?

Is it sound practice to just short the lines, rather than simulate a phone (like with a 1200 ohm resistor which was suggested)?

Any other reason this wouldn't work for my application?

Finally, is there an economical source for a 56V/5W zener diodes? I checked Radio Shack, but nothing close to those specs. Found a 56V/5W online for cheap, but the minimum order and shipping/handling was outrageous.

Really appreciate all the help -- I don't know that much about electronics, just enough to be dangerous and have some fun!

Try DIGI-KEY for the Zener Diode. Trouble is, again, you won't hear much, if any, of the ring.
What will work, and no I don't have a schematic, would be a Full wave bridge, rated at 1 amp and 400 volts or more, with the AC leads across T and R, with a, .47 ufd 200v or more rating, capacitor in series with either T or R. Use the dc output of the bridge to charge a capacitor. Use the charged cap to operate a low current, low voltage relay, with a resistor in series with the relay coil to slow the capacitor discharge, to apply a 600 to 1.2K ohm resistance across T and R. I'm guessing you'd spend about 10-15 bucks max on the parts. Trouble is, without specs for the capacitor, resistor' and relay, you'll spend 3 times that much buying assorted parts to try and get the line short timing where you want it. John C.

Thanks for all the help. I really appreciate it. I believe in closing threads (for future searchers) so here's the outcome.

Never did find a cheap source for a Zener Diode. Individual diodes were dirt cheap, but most places had minimum charges, minimum quantities, etc. I would have liked to try that configuration as it seems the most elegant solution.

Instead, I put a second relay in the circuit so now there are two relays which trigger simultaneously from the ring detection circuit. One does the work I need. The second momentarily shorts across tip/ring with a 1200 ohm resistor, 5 watts, thereby causing the answer/hang up condition I was looking for.

Thanks again for the advice on the 1.2K ohm resistor. Not sure whether the 5 watts was overkill or underkill, but it was what I was able to get through serendipity.