Phonline Diverter (my first project)

[Jberryman]

Hi everyone. I've just begun to learn about electronics and wanted to try my hands at making a diverter (or rather a variation on a design that I found), and I could use some help. The purpose of this circuit is to connect two phone lines together so that when you call LINE 1 you get the dial tone of LINE 2 and can send DTMF and talk just as if you had dialed out of LINE 2 directly. I appologize in advance for my butt-ugly illustrations and general newbiness; the drawings are meant to just show the basic logic, as that's enough for me to worry about at the moment.

This is the first sketch I came up with:


To explain: In the position shown, both lines 1 and 2 are "on hook". I dial up the number for LINE 1, the CO sends AC (ringing) current to line 1 and the capacitor allows current to flow into the full wave bridge rectifier, sending DC through the relay.
This closes both circuits in LINES 1 and 2, LINE 1 stops "ringing" and allows DC to flow through LINE 1 and through LINE 2. Now it is LINE 2 that is keeping the relay open. Both lines are now "off hook".
The gold blotch represents the undefined connection between the two lines that is the purpose of the circuit. I have been trying to find information about DC coupling but it all seems very complicated. The design that I initially found used a 600ohm-600ohm transformer, but is there an easier way? what would happen if you connected them with a bit of wire?
Now when the call is completed and whoever LINE 2 calls hangs up, the CO will initiate a polarity reversal on LINE 2 that will cause the circuit to collapse and reset. There is a problem if LINE 2 dials something that never hangs up, however...

Which leads me to v0.2:

Now this may be crazy, I don't know. But in this design, when the relay is thrown, LINES 1 and 2 are connected in a figure eight or infinity symbol shape, with RING of LINE 1 flowing into TIP of LINE 2 and vice versa. This way, a polarity reversal on either end will reset the circuit and I don't have to learn about coupling.

Does either design make sense? If not why? Any advice you can give me, even to tell me what to go about reading into would be great.
Thanks

[MP]

without an external power supply it looks like something that is going to cause you problems with your phone company. You will need to completely isolate your circuitry from the phone line with isolation transformers. You mentioned them, but I do not see them in your drawing.
If your phone line draws too much current from the system and causes someone else's phone to not work properly, you might be contacted by your local phone company with a bill for services to find the problem for the other customer. This is the reason for isolation transformers. Do not tap into the phone line to power your circuit and do not connect switches directly to the lines.

MP

[Jberryman]

Thanks for the response, MP. The transformer that I referred to was for bridging the two lines. Why is it necessary to isolate this from the phone line? Sorry for my ignorance, but could you explain quickly what you mean by "draw too much current"?

As for affecting the service of other phone customers, there's no way that anything connected to a POTS line can cause problems at the CO that would affect other people's service. Except for the connection between LINES 1 and 2, I don't see how this circuit differs from a basic telephone.

Thanks for your help

[MP]

I did not really follow your drawing well, but here is a link that can explain more about the telephone lines, what voltages they draw, why you need isolation, etc. Perhaps it will help. I would personally never connect to the phone system without isolation.

http://www.epanorama.net/circuits/teleinterface.html

MP

[Jberryman]

Thanks a bunch for the link, I've been looking for something like this. I'm sorry, the drawings are a bit confusing. If it helps any, I should have stated that the green dot represents the tip and the red dot represents ring, the pair on the left being line 1 and the pair on the right being ring and tip for line two.
thanks again

[audioguru]

Hi Jberryman,
I agree with you. There is no way that your loading of your telephone lines can affect the service of others. The "isolation" rules are probably meant to prevent laymen from putting their own hazzardous voltages on the line.

A very old phone presents a low resistance to the line when off-hook so that it can trip the CO's off-hook detector when it is at a far distance (high line resistance) from the CO. When this same phone is used on a line that is very close to the CO, a very high DC current will flow, and the CO's equipment is designed to handle this high current.
A modern phone uses the change of DC current to vary its amplification, so that it attenuates when close to the CO and amplifies when far. Your simple bridging of your lines may cause unacceptable attenuation, especially when both calls are weak.

The CO may have an alarm to detect if you draw too much ringing current. The telco can use this alarm to prosecute subscribers with too many ringers on the line. Select the value of your capacitor with care.
Your relay's coil will probably be designed for the high rectified voltage, so may not present a low enough resistance to hold both phone lines off-hook. A bridging transformer will solve this problem.

Your unfiltered rectification of the very low frequency ringing signal may cause your relay to chatter. I don't know how to prevent this since my experience involves high impedance optically-isolated properly filtered detectors.

In my location, free long distance bridged calls are provided by beer and newspaper companies. You call them, hear a short commercial then dial your "long-distance" number. It sounds illegal to me, but they do it for free by using their own network.
Aren't you trying to make your own network?

[MP]

...and there is a reason why they have an alarm to detect if you are using too much ring current.
BTW, the isolation also protects the end user. The 48V DC voltage in telephone lines does not cause immediate danger to the user, but the AC ring signal (70-120V AC) can give quite a shock.

jberryman: glad that the link was helpful. Good luck with your project.

MP