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In this project, I show you how you can make a simple intercom system from a pair of old corded telephones. The phones already have all the circuitry that is needed for processing a voice call.  All you need to add is a 9 volt battery and a resistor. This is an easy electronics project for beginners and it is great to do with children. Try it out and have fun.

Here’s the full build on MAKE: Projects and on Instructables.

See the full series here.

Jason Poel Smith

Jason Poel Smith

My name is Jason Poel Smith. I have an undergraduate degree in Engineering that is 50% Mechanical Engineering and 50% Electrical Engineering. I have worked in a variety of industries from hydraulic aerial lifts to aircraft tooling. I currently spend most of my time chasing around my new baby. In my spare time I make the how-to series “DIY Hacks and How Tos.”

  • Jonathan Williams

    Any way to make it ring?

    • Jeremy Akers

      When a phone line is not in use there should be about 50-60 volts DC on the line. To make it ring, you apply around 90 volts of around 20 Hz AC signal.

      When you pick up the phone to answer a call, the voltage drops to 9-12 volts modulated to carry the voice signal. That’s why this project only needs a 9 volt battery because it doesn’t need to make the phone ring, it only re-creates the actual call portion of the phone signal which only needs 9 volts.

      • M. T. Gonzalez

        So if I use 90 volts I`ll make the other phone ring when I pick up the phone?

        • Jeremy Akers

          No, it’s not quite that simple. 90 volts applied to an off-hook handset would probably damage your phone unless there was a resistor in place to drop the voltage, but that would drop it to the phone on the other end and prevent it from ringing.

          Also, you need around 9 volts of DC voltage to transmit audio signal, but the ring signal is around 90 volts of AC 20HZ (Alternating Current). Alternating Current can’t transmit audio so once they pick up for have to switch back to the 9-12 volts DC.

          So you’d need to connect two power sources, one for 90 volts AC to make it ring, one for 9-12 volts DC for when they pick up. Then wire up a push button for the 90 VAC and when you push the button the other phone will ring, and then when they pick up you talk.

          • M. T. Gonzalez

            Thank you very much for the great answer!!!