# HOW TO – Make the simplest intercom

LeorickSimon writes in –

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.

My Electronics Hobby: Simplest Intercom – Link.

Related:

• HOW TO – Use old telephones as an intercom – Link.
• Build your own intercom or walkie talkie out of two … – Link.

## 12 thoughts on “HOW TO – Make the simplest intercom”

1. kryten007 says:

You can use this principle to build an acoustic coupler for the modem-bearing device that you all undoubtedly carry. Get online with a payphone? Oh yeah.

2. Daenris says:

I’ve seen something like this before and was interested. I’m now wondering what would be the easiest way to set something up so that the other phone rings when you pick up the first one.

3. mc@uga.edu says:

All you need, to make 2 phones talk to each other, is a 9-volt battery. (Wire all 3 items in series: battery, phone, phone.) They limit the current themselves.

Ringing is a lot harder because it requires 90 volts AC.

1. t ball says:

Go to a site called gizmoplans.com—–they have plans for a phone intercom with a call buzzer that’s really simple.

4. Daenris says:

I suppose you could wire up your own ringing circuit. Rig it up to the hang-up button so that when you pick up the phone it triggers a circuit that then triggers a lower voltage buzzer on the other end.

5. Renee says:

do you all think you can repost this stuff so the under-dummies can understand it and be able to make a home intercom system?

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