How to Use 2 Telephone HANDSETS as Intercom?

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? ;-)

^^^^ 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

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.

Joel posted:

But the sound from each mic needs to be fed to both earphones,

^^^^ 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. >>

--

The only time sidetone causes confusion is when there is echo. The combination causes one to stutter.

The advantage of sidetone is that it helps you regulate the loudness of your speech. Without sidetone you will talk louder, just as a hard of hearing person does.

By the way, *all* cellphone connections include digital echo cancelation. there is so much transmission delay in cellular switching and transmission, the echo would make it almost unusable otherwise.

Don

THANKS TO ALL