Not built yet. This is my plan and parts list. When it's done I'll add what actually happened.
What to expect
There's no amplifier anywhere in this. Every bit of sound at the far end comes from the energy of someone's voice. That sets some hard limits.
- It's quiet. You hold the speaker to your ear in a quiet room or garden. It won't work across a noisy workshop or a busy road.
- Short to medium runs. Somewhere between 10 and 200 metres is the sensible range. Longer runs get quieter because the wire's own resistance eats into the signal.
- No ringer. Nothing can make a bell ring with no power. You agree a time, or a signal like a sharp tap or a loud whistle.
- The simple version is one-way at a time. With one speaker per end, you talk into it and then move it to your ear. The four-wire version below fixes that.
Why this way
A speaker is a coil on a paper cone, sitting in a magnet. Push current through the coil and the cone moves. Move the cone with your voice and the coil makes a current. Same part, both jobs.
A small, light cone moves easily with your voice, so it makes more signal for the same effort. Four of them cover the full two-way version.
Cheap, thin, easy to hide along a fence, and made for exactly this kind of low-voltage run. A 100 m drum is enough for a long two-wire line or a shorter four-wire one.
A box behind the speaker makes it easier to hold to your mouth or ear, protects the wiring, and stops the cable pulling on the speaker tags.
Push the lever, drop the wire in. You can unhook the handset and take it indoors without tools or unsoldering anything.
Two wires is simplest and fine for passing messages. Four wires gives each end a separate mouthpiece and earpiece, so you can talk and hear at the same time, like a real phone.
How it works
When you talk, sound waves push the cone back and forth. The coil glued to the cone moves in the magnet's field, and that makes a small alternating voltage, a copy of your voice. The wire carries it to the other speaker, where the current moves that coil and cone, and you hear the voice again. Much of the energy is lost along the way, which is why it's quiet.
The four-wire version uses two separate pairs. Each pair carries sound one way only: from one end's mouthpiece to the other end's earpiece. That's the same idea as a real telephone handset, with a part for your mouth and a part for your ear.
What to buy
This is a cheap build. Listings change, so check each one before you order.
These are affiliate links. If you buy through them I make a small commission, at no extra cost to you. It helps cover the cost of running this site. This is a hobby, not a shop.
The phones
The line
Small extras
You'll also need: a soldering iron, a drill, a hole saw or a round file, some strong glue, cable clips or ties for the run, and a scrap of thin cloth or mesh to protect each cone.
Build it
Six steps. Do the table test first: if it doesn't work there, it won't work at the end of the garden.
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Test on the table
Clip two speakers together with two crocodile leads. Put one in another room, or ask someone to hold it to their ear. Talk clearly into the other from a couple of centimetres away.
Why: this proves the speakers are good enough before you drill anything. It also shows you how close to talk and how quiet the result is, so the finished set won't surprise you.
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Cut the speaker hole
Measure your speaker. The 57 mm speaker is nearly as wide as the 60 mm box, so it won't drop inside. Instead, it sits on top of the lid, facing out, with its back pushed through a round hole. Cut that hole just big enough for the back of the speaker to pass through, so the flat front rim rests on the lid. The hole will be close to the full width of the lid, so go slowly: use a hole saw, or drill a ring of small holes and file it smooth.
Check the speaker's depth too. The magnet must fit inside the box with the lid on.
Why: mounting the speaker by its rim is how speakers are normally fitted to a panel. The rim spreads the load and the box behind it seals the back of the cone, which helps the sound go forward.
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Fit the terminal
Drill the hole or holes for the spring terminal in one end of the box, so the levers face outwards. Solder a short wire from each speaker tag to each terminal tag. If one speaker tag is marked + or red, use the same terminal for it on every box.
Why: getting the + the same way round on every box doesn't change the sound much, but it makes any fault much easier to trace later.
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Close it up
Put the lid on with the speaker's back and wires inside. Glue the speaker rim to the lid. Stretch a scrap of thin cloth or mesh over the cone and glue its edges down.
Why: the cone is paper and splits if you poke it. A thin cover keeps fingers, rain and insects off without blocking the sound.
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Run the wire
Lay the bell wire along fences, walls and hedges, clipped or tied every metre or so. Keep it off the ground where it crosses a path, and well away from mains cables. Bring it in through a window gap or an existing hole. Leave a little slack and a downward loop where it enters, so rain drips off rather than running inside.
For the four-wire version, run two lengths side by side and mark one of them with tape at each end, so you know which pair is which.
Why: running beside mains wiring can add hum. Wire lying across a path gets tripped over and pulled out. Marked pairs save a lot of confusion when you wire up the far end.
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Connect and test
Clip the line into the terminals at each end. For four wires, follow Fig 2: pair 1 joins the mouth box at end A to the ear box at end B; pair 2 joins the mouth box at end B to the ear box at end A. Mark each box MOUTH or EAR.
Why: on the four-wire set, if you wire a pair mouth-to-mouth, you'll each be talking to a speaker nobody is listening to. Labelling the boxes stops that.
Using it
- Talk close and clearly. Mouth two or three centimetres from the cone, normal-to-firm voice. Shouting tends to distort without helping much.
- Seal the ear. Press the box against your ear and cup your hand round it. Keeping outside noise out matters as much as the signal coming in.
- Calling. There's no ringer. Agree times, or a signal: a sharp tap on the box or a loud whistle into it comes through as a click or a squeal at the other end. Tap the box, not the cone.
- Simple version etiquette. Say "over" when you finish and move the speaker to your ear. It feels old-fashioned and it works.
- Looking after it. Bring the handsets in when it rains, or keep them in a bag. Check the line for chafing where it runs over edges a couple of times a year.
If it's not working
| What you see or hear | Likely cause | Fix |
|---|---|---|
| Nothing at all | Broken wire or a loose terminal | Clip the two handsets together with short croc leads. If that works, the fault is in the line. |
| Worked on the table, very faint on the line | Long run of thin wire | Shorten the run if you can, or use thicker wire. On long runs the wire's resistance starts to rival the speakers' 8 Ω. |
| Loud hum | Line running near mains cables | Move the wire away. Cross mains cables at right angles rather than running alongside them. |
| Crackles when the wire moves | Bad joint or damaged insulation | Resolder the tags and check where the wire rubs on edges. |
| Four-wire: one direction silent | Pairs swapped at one end | Check pair 1 runs mouth A to ear B, and pair 2 runs mouth B to ear A. |
| Buzzy, rattly voice | Torn cone or loose cover | Swap the speaker. Glue the cover edges down flat. |
Safety
- Trip and neck height. Keep the line out of paths, and never strung at head or neck height where someone could walk or cycle into it.
- Mains and phone lines. Never connect to mains wiring or to the household phone line, and don't attach the wire to power or telephone poles.
- Storms. A long outdoor wire can pick up a charge from nearby lightning. Unclip the handsets and leave the wire outside when a storm is close.
- Soldering. Use a stand for the iron and work somewhere ventilated.
Notes
Navies have used sound-powered telephones on ships for a long time, because they keep working when the ship loses electrical power. Their handsets use transducers designed for the job, which make much more signal from a voice than a cheap loudspeaker does. This build is the hobby version of the same idea, so expect it to be quieter.
If you want more range later, the usual trick is a small matching transformer at each end, so the line runs at a higher impedance and loses less in the wire. I've left it out to keep this build as simple as possible.
Related guides: Morse code without batteries, the crystal radio, and the foxhole radio.