Two-Point Intercom Circuit: How to Build One

Updated 5 November 2026 · Standard formula, shown below

A simple two-point intercom lets people at separate locations exchange voice over a wired connection. This guide builds a low-voltage, half-duplex system: one person speaks at a time using a push-to-talk switch, and the other listens. You will need basic soldering skills and two matching audio modules; keep the circuit battery-powered and do not connect it to household mains wiring.

What You’ll Need

Two identical low-voltage audio amplifier modules, each rated for a 9 V supply and able to drive an 8-ohm speaker. Choose modules with a microphone-level input, or use a separate microphone preamplifier at each end.
Two 8-ohm speakers, preferably 0.5–1 W or higher.
Two electret microphone capsules, if the amplifier modules do not include microphone inputs, plus suitable preamplifier circuits.
Two momentary, normally open push-to-talk (PTT) buttons.
Two 9 V battery holders and fresh batteries, or a safe, regulated low-voltage DC supply compatible with both modules.
Two enclosures, insulated hookup wire, a two-conductor cable between stations, soldering tools, and a multimeter.
Optional: 100 nF ceramic bypass capacitors and 100–470 µF electrolytic capacitors, if the module instructions recommend local supply decoupling.

Use modules designed for the same supply voltage, and follow their pin labels and datasheets. The circuit below uses one amplifier and speaker at each station. A PTT switch selects whether that station’s speaker is connected to its amplifier output (listen) or its microphone signal is sent to the other station (talk). This arrangement avoids trying to use one amplifier output as an input.

Step-by-Step

Plan the stations and cable. Label the two ends Station A and Station B. Use a two-conductor cable between them: one conductor carries A’s microphone signal to B, and the other carries B’s microphone signal to A. Each station’s amplifier, speaker, microphone, battery, and PTT switch stay at that station. For long runs, use shielded cable and connect the shield to circuit ground at one end only; the two signal conductors still carry the two directions.
Check the audio modules. Identify each module’s positive supply, ground, audio input, and speaker output. Confirm that its input can accept the microphone preamplifier’s output and that it supports the chosen battery voltage. Many small amplifier boards have bridged (BTL) speaker outputs: neither speaker terminal is ground. Do not join either speaker terminal to ground or to the cable.
Build the microphone circuits. Connect each electret capsule to its matching preamplifier exactly as its circuit specifies; electret microphones are polarity-sensitive. Feed the preamplifier output to the PTT switch’s talk contact. If your module has a proper microphone input and bias supply, use the specified wiring instead. Do not connect a bare microphone directly to a speaker output.
Wire the PTT changeover at Station A. Use a double-pole, double-throw (DPDT), momentary switch, or an equivalent switch arrangement. In the released position (listen), connect A’s amplifier input to the incoming microphone signal from B, and connect A’s speaker to A’s amplifier output. In the pressed position (talk), disconnect the speaker from the output and connect A’s preamplifier output to the A-to-B cable conductor. Keep the amplifier input from receiving the local microphone signal while talking. Check the switch’s terminal diagram; pin layouts differ.
Repeat at Station B, reversing the cable direction. In B’s listen position, connect B’s amplifier input to A’s microphone signal. In talk, send B’s preamplifier output on the B-to-A conductor and disconnect B’s speaker. The cable’s two signal conductors must remain separate. Connect the stations’ signal grounds together over a third conductor if the cable has one; otherwise use a cable and grounding arrangement specified by the module maker. Never rely on a shield as the only safety-earth connection.
Connect power and decoupling. Connect each module to its own correctly polarized battery or regulated supply. Add bypass capacitors close to the module supply pins if recommended: a 100 nF ceramic in parallel with a suitable bulk electrolytic capacitor is a common approach. Observe electrolytic polarity. Keep audio wiring away from switching power supplies and speaker wires.
Inspect before powering up. With power disconnected, check for solder bridges, loose strands, reversed electrolytic capacitors, and accidental connections between speaker outputs and ground. Use a multimeter’s continuity setting to verify that the PTT switch changes the connections as intended. Confirm that no speaker is connected during transmit and that the correct remote microphone reaches the listening amplifier.
Test at low volume. Set both amplifier gains low. Power both stations, leave both PTT buttons released, and have someone speak into A’s microphone while listening at B. Press A’s PTT to speak, then release it to listen. Repeat in the other direction. Raise gain gradually only until speech is clear, then secure the boards and wiring in insulated enclosures.

Common Mistakes

Using mismatched modules: Different supply voltages, input sensitivities, or output types can cause distortion or damage. Check the specifications before connecting them.
Grounding a bridged speaker output: BTL amplifiers drive both speaker terminals. Grounding either one can short the output stage.
Leaving the local speaker connected while talking: Sound from the speaker can feed back into the microphone, producing a howl. The PTT switch should disconnect it in talk mode.
Mixing up signal and speaker wires: Speaker outputs are amplified and must never be connected to the remote microphone input.
Ignoring microphone polarity or bias: A reversed electret capsule or missing bias resistor can result in silence or very weak audio. Follow the microphone circuit instructions.
Running unshielded signal wires beside power wiring: This often introduces hum, buzz, or radio-frequency interference.

Tips

Put the PTT button where it is easy to hold while speaking, and label each station’s talk control.
Use a twisted pair for the two audio signals where possible, and keep the run as short as practical. For longer distances, consider a balanced audio interface rather than extending an unbalanced microphone signal.
Mount each microphone away from its speaker and point them in different directions. Start with low gain to reduce feedback.
Use strain relief where the cable enters each enclosure. Insulate exposed solder joints and secure batteries so they cannot shift.
If you want hands-free, simultaneous conversation, this PTT design is not suitable. That requires a more complex full-duplex circuit with separate transmit and receive paths and echo/feedback control.

Troubleshooting

No sound either way: Check battery voltage and polarity, module ground connections, speaker wiring, and the PTT switch’s common and throw terminals. Verify that each microphone preamplifier has power.
Only one direction works: Trace the failed direction from microphone to preamplifier, through the cable conductor, across the receiver’s listen contact, and into its amplifier. Test continuity with power off.
Very quiet or distorted speech: Confirm that a microphone-level input is not being fed a speaker signal, that the preamplifier and amplifier gains are appropriate, and that the battery is not depleted. Lower gain if the sound clips.
Howling or squealing: Make sure the listener’s speaker is disconnected when that station’s PTT is pressed. Increase the microphone-to-speaker distance and reduce gain.
Hum or crackling: Check for loose grounds, poor solder joints, depleted batteries, and unshielded cable near power wiring. Connect a cable shield at one end only to avoid a ground loop.

Frequently Asked Questions

Can I build this with two speakers and no amplifier modules?
No. A speaker produces only a small signal when used as a microphone, and a microphone cannot drive a speaker directly. Each station needs suitable amplification.

Can I use ordinary telephone wire?
It can work for a short run if its conductors are kept separate and the modules tolerate the cable’s noise and capacitance. Shielded or twisted-pair cable is generally better for audio.

Can both people talk at once?
Not with this half-duplex PTT circuit. Only one station should transmit at a time; simultaneous speech needs a full-duplex design.

How far apart can the stations be?
There is no single reliable distance: cable length, wire type, interference, microphone level, and grounding all matter. Test the actual cable run before installing it permanently.

Can I power it from a wall adapter?
Yes, if it is an isolated, regulated low-voltage DC adapter that matches the module’s rating and current needs. Do not connect the circuit directly to mains, and unplug power before changing wiring.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.