Connect two corded telephones with a telephone line cord and a separate 9V battery circuit to make a basic voice-only intercom. The circuit described by Jason Poel Smith uses a resistor in series with the battery; it does not ring the other phone or provide call signaling.
What this intercom does—and does not do
The two corded phones act as the endpoints: each already has a speaker, microphone, and processing circuitry. A battery-powered circuit lets them carry voice over a connecting telephone line cord. This is a private setup, not a connection to active public telephone service.
The basic arrangement does not make either phone ring. A buzzer or LED indicator is described as a possible extension using the two spare conductors in a four-conductor cord, but it is not part of the voice circuit.
Parts and tools
- Two corded telephones
- Telephone line cord
- 9V battery and 9V battery connector
- 300-ohm resistor; the tutorial also names 270-ohm and 330-ohm resistors as alternatives
- Heat-shrink tubing and a small project housing
- Wire strippers, a sharp knife, soldering iron, and screwdriver
These are the parts listed in Jason Poel Smith’s Make: Projects instructions. The tutorial does not compare phone models or establish that every pair will work identically. For the original project instructions, see Simple Intercom From a Pair of Old Corded Phones.
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How the circuit is arranged
Put the 9V battery and resistor in series with one conductor of the telephone cord, then connect both telephones through that cord. The tutorial identifies the red or green conductor for the circuit. If the cord has four conductors, the other two are unused in the basic voice-only arrangement.
The resistor value is a component choice, not a performance ranking: the tutorial lists 300 ohms and offers 270 or 330 ohms as alternatives, but supplies no comparative test results for those values.
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- Basic Operation: Pick up handset to answer incoming calls and return to base when conversation is complete
Build and check the setup
- Identify the cord conductors. Use the red or green conductor identified by the tutorial for the circuit. Leave the extra conductors in a four-conductor cord unused unless adding a separate indicator extension.
- Make the series battery connection. Connect the 9V battery connector, 300-ohm resistor, and selected cord conductor in series, following the tutorial’s circuit layout. The named alternatives are 270 ohms or 330 ohms.
- Connect the phones. Attach both corded telephones through the telephone line cord so they share the described circuit.
- Insulate and enclose the wiring. The project materials include heat-shrink tubing and a small housing; use them to protect the assembled connections.
- Test voice communication. Speak into one handset and listen at the other. Do not expect a ring or standard call signaling from this circuit.
Optional signaling and longer-term power
Smith notes that the spare pair in a four-conductor cord could support an optional buzzer or LED indicator. That requires an added signaling circuit; the basic voice arrangement alone does not provide one.
The project also mentions replacing the battery with an AC adapter for a more permanent installation, but points to a separate regulator project. Its instructions do not establish a suitable arbitrary adapter or a validated adapter design for this intercom, so do not substitute an unspecified power supply.
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Make: published a companion overview of the project on February 4, 2014: DIY Hacks & How To’s: Make a Simple Intercom From a Pair of Corded Phones.
Quick Recap
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