How to Use 2 Telephone HANDSETS as Intercom?

I understand how to use 2 telephones (USA) with 9v battery and
resistor and possibly .47mfd cap across resistor to make 2 station
intercom
BUT
How do you use two HANDSETS(USA modern not old carbon type) to make an
intercom? I cannot get into the handsets to see if earphone and the
mic are connected to anything such as a diode. I have a red & green
lead that I believe is earphone and a yellow (+) and black leads is
mic. I have tried experimently to connect them as if they were phones
and failed. I don't need much (gain/amplificarion if any) they will be
connected across 5 feet of wire.

PLEASE REPLY TO NEWSGROUP-MY SPAN FILTER IS VERY AGRESSIVE.

 

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? ;-)

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^^^^
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.

 

IF I do without the "sidetone"(mic voice reproduced in earphone of
same handset) what do I have to do AT A MINIMUM to make 2 handsets
into "intercom" ?

 

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