The Tool Desk
Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEFix the driver behind crashes, sound loss and screen glitchesFind Drivers →You can turn two corded telephones into a private, voice-only intercom with a telephone line cord, a 9V battery, and a series resistor. The circuit carries speech between the handsets; it does not ring the other phone or connect to an active public telephone line.
How the two-phone intercom works
Each telephone supplies a handset speaker, microphone, and internal circuitry. A separate 9V battery powers the simple voice circuit: the battery and resistor are wired in series with one conductor of the cord linking the phones. Jason Poel Smith’s Make: Projects instructions describe using the red or green conductor; the other conductors are not needed for the basic arrangement. See the original project instructions.
This is a private voice path, not a complete telephone system. It has no call signaling, so lifting one handset will not make the other phone ring. Keep the circuit separate from an active telephone service line.
Parts and tools
- Two corded telephones
- A telephone line cord
- One 9V battery
- One 9V battery connector
- One 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
The tutorial does not compare phone models or report performance tests for the three resistor values, so it does not establish that every telephone pair will behave identically. Its list and circuit description are available in Jason Poel Smith’s Make: article.
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Build the voice-only circuit
- Choose the phones and cord. Set aside two corded phones and a telephone line cord long enough to connect them. Do not attach this homemade circuit to an active public telephone line.
- Identify a conductor. Use the red or green conductor specified by the tutorial. With a four-conductor cord, leave the two extra conductors unused for the basic voice circuit.
- Wire the power path. Connect the 9V battery and resistor in series with the selected cord conductor, then connect the phones through the cord as shown in the original project instructions. The tutorial specifies a 300-ohm resistor, with 270 or 330 ohms as alternatives; it does not provide comparative test results for those values.
- Insulate and house the connections. Use heat-shrink tubing over exposed joints and place the circuit in a small project housing. Follow safe soldering and cutting practices while assembling it.
- Try the link. Connect one phone at each end and test speech between the handsets. This circuit provides voice communication only; it does not add a ring or other call alert.
What the basic circuit does—and does not—include
Voice communication
The two handsets are the endpoints, and the battery-powered circuit supplies the described voice path. The tutorial is a simple reuse project, not a model-by-model compatibility guide, so it makes no guarantee for every pair of phones.
No ringing or call signaling
The basic arrangement does not ring the second phone. Smith notes that the two spare conductors in a four-conductor cord could be used for an optional buzzer or LED indicator. That would be an add-on requiring its own wiring; it is not part of the voice-only circuit described above.
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Battery-powered setup
The specified build uses a 9V battery and battery connector. Smith mentions an AC adapter as a possibility for a more permanent system and points to a separate regulator project, but the instructions cited here do not establish an adapter voltage or a validated supply design for this circuit. Do not substitute an arbitrary adapter.
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