How to Use 2 Telephone HANDSETS as Intercom?

^^^^ As far as I can tell, there are a lot of cell phones out there that don't provide this 'sidetone,' and people don't seem to get too confused using them.

---Joel

You need to amplify the signals from the mics and feed it to the earphones. But the sound from each mic needs to be fed to both earphones, because you need to hear yourself as well as the other person in your earphone. The sound which reaches your ear becomes very confusing if you do not hear your own voice as well as the other person.

You can use a transformer to mix all input signals and create the sum of all signals for all the outputs. Use separate windings for each input and each output. The mic signals need to be amplified before the mixing in the transformer, and maybe amplification for the output signals too.

I think there is an even simpler solution, mix all input (mic) signals using an op-amp, and connect all earphones to the output of the op-amp.

I understand that you are into tinkering with stuff, etc (after all, this entire newsgroup is about that;-)) , but a phone set actually has couple important features, such as echo and feedback cancellation as well as amplification, that aren't too easy to implement using just discrete electronics. On the other hand, a brand new analog phone can be had in Wall Mart for maybe $7 to $10, and I cannot think of eBay not being able to offer the two that you need for maybe $10 or less. Maybe you may want to skip this basic part and instead concentrate on the actual application you need the intercom for? Or am I violating tinkering spirit of a member of this group by suggesting leaving some things to Wall Mart? ;-)

Yes, I realize now that you are right about that.

That is the reason why inexperienced cell phone users often shout into the phone.

The old telephone system had this sidetone, for reasons which were judged to be important. You could hear your own voice, so you could adjust the volume of your voice through feedback.

Okay, it is up to the constructor to decide if he thinks the sidetone is important or not. I explained two ways to realize it, in any case, so he knows how to do it if he wants it.

If you use one op-amp you can have sidetone for no extra cost.

Mix the two mic signals on the input of the op-amp, send the output to the earphones.

Separating the signals so you get no sidetone will cost more components. You will need two amplifiers (op-amps) then.

The mics you have there are probably electret mics, which need a power supply to work, but you can hopefully fix that from the same battery which drives the opamp. Use a battery instead of a power supply, it will reduce noise and hum problems and make the circuit portable.

Look up a standard op-amp circuit called audio mixer.

So the minimum is one op-amp and a few resistors and capacitors, and a battery.

But a quad op-amp in one capsule is just as cheap as a single op-amp, so I would recommend to use one op-amp for each mic as mic amplifier, one op-amp as audio mixer, and the last op-amp as loudspeaker amp, in case you will need it in the future.

Look up electret mics power circuits, study the mics you have and find out how they need to be supplied with power.

Or describe the equipment you have in detail, and hope for somebody to give you a readymade circuit schematic. :-)

Don't you have any specifications on these headsets, what does it say, technical details like impedances, mic power supply, connectors details, etc..

THANKS TO ALL