How to wire telephone as an intercom circuit

[psychshop]

I need to link an observation room (one way glass ) to the other side via a direct wired phone. This will not connect to any external phone system. Just two regular phones, a power supply, and whatever else is needed to generate the ring tone. I thought I had seen a diagram to do this years ago I wondered if someone could steer me to the answer.

[myp71]

Try this one then if thats what you are looking for then let me know how it goes because I was going to build it.<p>
http://electronic-circuits-diagrams.com ... ckt2.shtml

[Chris Smith]

Tie the two phones together on the same two wires, and apply around 3 to 6 volts. The ringer can be any AC circiuit up to around 100volts with very low amperage in the Ma range. 20 hz is the standard, 30 and 60 hz work as well. Straight injection into the two wires via a cap on each leg.

[Dimbulb]

A better approach migt be to purchase an intercom or modify an intercom to be a telephone. It is difficult to modify the newer telephone to be an intercom. With the price of intercoms what they are the notion of using older telephone sets has not been a favorite method.<p>There is little advantage to try to work around the chipset trying to add features to a modern telephone set as if the telephone exchange and all of its equipment were not involved or did'nt matter. In computer terms the server side intensive network functioning from a dumb terminal would be a waste of time.<p>At a beginners level the kitsRus has a workable solution in a kit that addresses some of the basics of telephone technology which could be enhanced theoretically to add more complexity such as a link to an observation cell.

[Externet]

Hello.
The simplest way is to install a 9V battery in SERIES with ONE of the two wires between the telephones, and run buzzers with pushbuttons on a separate circuit, using the SECOND pair of a telephone 4 conductor cord... or bang on the glass to call the other side...
Miguel

[psychshop]

For my purposes Chris Smiths suggestion seems to be closest. I have a ready supply of old phones that can't be used on the university PBX. I should have mentioned that the observer needs to speak privately to the interviewer which is why a standard intercom is not an option.

[Chris Smith]

Its very easy to do, just connect both phones on the same two wires and a small battery or filtered power supply across the lines and they start working right away crystal clear. Voltage should be able to be adjustable or preset between 3 volts or higher if your lines are very long, say over 300 feet or so. A DC blocked injection of a AC voltage [50 to 100 volts] or signal through two small ceramic caps will inject a ring signal, and polarity on old phones mostly isn’t an issue. Minimum parts needed. Power draw from the phones is in the very low MA draw, so if you chose to use batteries, they last a long time between charges.