Friday, April 06, 2007

Simplest Intercom

By leorick  |  4/06/2007 08:55:00 PM 11 comments

Using old phones

Using your old phones, you could create personal "micro" telephone exchange as simple as above circuit. I manage to solder the test circuit less than a minute. Than connect the phone and test it. It works. So why 39 ohm? From Epanorama Dot Net, the correct value of the resistor should be a resistor that can limit the circuit current around 30mA total loop. The method is by replacing the resistor in the circuit with a variable resistor, plug in the phones, apply power and tune the current with the variable resistor so the current flow is 30mA. After that take out the variable resistor and measure the resistance/ohm across it. Find the fixed resistor with the nearest ohm reading to it and replace it to the circuit.

Simplest Circuit

Testing In Progress


Davesp said...

not sure but I think some phone guy told me the british Isles phones have about 40v powering them. Not sure this would make a difference.

all the best from Ireland Dave

Anonymous said...

Nice - only problem is that there's no ringer so you still need to shout at the person at the other end to pick up the phone.

STEPHEN said...

could this be used to make a helmet to helmet intercom for motorcycles?

Cara said...

You have a lot of interesting stuff on your site. This one is my favorite though.

ignacio said...

I did it and it worked great! However, I had to keep the current around 15mA because one of the phones ' earpiece started getting hot :o

bobo said...

cool, i have four, old phone. i think i wanna play with my cousin. how the circuit to connect them? make circular ring serial ?. about ring call 40Volt, just add small alarm circuit each phone :-)

Great! Is it possible to realize this not with phones but whith headsets(3,5mm)?
Thanks for your answer!

leorick said...

I don't think we can do the same thing here with 2 headsets (3.5mm). Some reading [ here ] can help to understand why only land-line telephone can do this.

Anonymous said...

Do I do this the same with cordless phones


I see some schematics and stripboard designs on your blog.
What software do you use to made them?

Thank you very much,

leorick said...

Hi sebtux74, autocad :-)

Have you tried veecad? Better than autocad

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