Magnet Powered Telephone




About: I'm a figment of my own imagination. ---- To Win the war on Terra Means to END the WORLD. How about a nice game of Chess? ---- I'm chilling, but forgive me for sometimes believing I invented everything.

This instructable will progressively show how to make at least one kind of telephone that sounds good enough to be fun and useful, without requiring batteries, AC utility company power, or utility telephone services.

I suppose it may be most appreciated by smart young children with "forts".

Step 1: Things You Will Need

Since I have done this before I know that the following supplies will be needed:

Two SMALL transistor radio speakers with BIG MAGNETS. Usually they are 8 ohms,
... higher resistance would be better, they could be (rare) 16, 32, 45, 100, or 600 ohms
as long as they are both the same ohms resistance.
The speaker size is almost perfect if both of them can hide under a CD. Bigger is not better.
Wire... Telephone wire or computer network wire or extension cord wire WITHOUT PLUGS.

Optional but very effective... some things like horns or cones.
I suppose you could use cans instead of horns.
But I'll try to show how to make a horn like those ancient wind up groovy-disc music boxes have.

Optional for long distance... high impedance matching transformer such as used in PA systems,
or which are designed for tube or transistor radios to match 8 ohm speakers to 1,000 or 10,000 ohm
amplifiers. Such transformers are also used in some speakers in systems for playing
background music and paging in supermarkets.
Distance is limited by how much wire you have and how far you can run it without annoying anyone,
especially the police.

You don't need all this stuff. Just one long pair of wire and two speakers will be enough.

Step 2: Connect 2 Speakers to the Ends of the Wire

Well you might want to run the wire first at least into the next room and find someone to talk to.
If you have no friends, turn on the video and the news, they usually talk a lot.

Then put the speaker wires on your "telephone wires" like in the picture
and talk and listen to your friend. If it's the video, be sure your "telephone" speaker is near
the video speaker and not a picture tube.

(If you magnetize a picture tube the colors get messed up and that's not good.)

Well this in the picture is the functional part.
When I add more steps they will be improvements mostly to make it pretty.
Like putting it in a box, adding horns, stuff like that.

WOW. Even I'm impressed how loud and clear this is already.
Maybe you'll have to holler on a really long wire but you won't be misunderstood!
I'm thinking that when I put horns on it I'll be able to eavesdrop on the other room.

BUT, If you try to talk to yourself you won't hear yourself.
Definitely get a friend and hook them up in a room that you usually can't hear them in.
And be very careful with the wires not to yank them off unless you can solder and collect speakers like me.

OH, I forgot to mention in case you didn't notice, you talk and listen to the same speaker.
It's used just like a cans-and-string phone, but there are magnets in it too.

Step 3: Make It Look Nice, and Louder Too.

Still working on horn plans. Gramophone type horns are magic the way they can make
really loud sound without any electricity.

The speakers will work better when mounted. There is even a noticeable difference in
loudness if you hold the speaker between your thumb and finger. And if you duct tape
it to the inside of a cardboard box at least. Really beautiful boxes cost more than
the rest of the project so I'd recommend making the box out of wood in antique-radio style
if you really want to use this project a lot and your mate can't stand ugly stuff.

Hot glue around the speaker (definitely avoid gluing the paper speaker cone! ) seems convenient
for mounting a speaker inside a box. I've made corrugated-box speakers before and it should be
obvious that a round hole slightly smaller than the speaker should first be cut in the box,
and optionally a mesh (screen) could protect the speaker from accidentally poking holes in it.

Besides the sort of junk pictured below that you could use as a case, well, someone just made
an instructable about making a decorative speaker out of a tennis ball. Check that out.

In the picture are just a few odd things that could, did, or do have small speakers in them
and could be used in this project.

Step 4: Miscellaneous Ideas About Using Horns, Etc.

At this point in time my plans to build a gramophone horn (for an unrelated project)
looks like it may not be so soon. Essentially I will try to make a cone or flower-shaped one by
cutting a large semicircle and/or long triangle out of copper flashing, bending it into
a cone or creasing it to make an octagonal pyramid shape open at both ends.
And soldering the seam together and perhaps to some copper pipe that goes to
-in this case it would be- the speaker(s). If I do that then I will put a picture of it here.

There are probably many other sources of complete horns, and corrugated paper cardboard
and duct tape is a "el-cheapo" material to make one. Here are some ideas for where to get
Bicycle horn... Large vehicle horn... Broken or toy musical instrument... Traffic cone...
Totally rotten gramophone, or antique telephone or radio...
Actual field telephones that work the same way that this project does...
Surplus american fire sirens that usually consist of a fan with one or more horns around it...
UHF waveguides...

Untried idea... since another instructable used earbuds as microphones, they may work well
with horns. Earbuds may have tiny rare earth magnets and high resistance but my guess is
that they can not handle very much sound.

Untried idea... pair of telephone handsets with only a transistor-radio-speaker at the ear-end
may look and work more phone-like.
The parts that are usually inside the handsets will not be useful for this magnet-powered project.

The use of the transformers mentioned in step 1 are for long-distance lines and I'd imagine them being used
between two large crop farms. What they do is take the low voltage from the speaker and step it up,
and also make the speaker have a much higher resistance than the wire. It's necessary because
if the long wire is more than 8 ohms then it will cause a big loss of volume between two 8 ohm
speakers. But if the speakers are making a higher voltage at a resistance of 1000 or more ohms,
then the wire resistance is of no effect.

WARNING: Actually using these phones over such "long distances" is likely to attract lightning
and accumulate dangerous voltages.



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    28 Discussions


    4 years ago

    When i was a kid, my friends had a truck with a camper on the back, and they used this method as an intercom. cool instructable!


    4 years ago

    to epadgett: try reversing the polarity of the wires on one of the speakers, and remember that the longer the wires, the more the resistance.


    11 years ago on Introduction

    does this actually work? have any of you people posting "cool" comments actually tried it? I'm just wondering, don't get me wrong, but i can't see how this would actually work without batteries, and magnets couldn't (i think) supply power just sitting there.

    3 replies

    The official term is "Sound Powered Phones". They are used on naval vessels for emergency communications, as no power is required.

    I built them as a kid, using both terephone "receivers" and transistor radio speakers.


    Reply 11 years ago on Introduction

    Yes... The magnets are not supplying power "just sitting there", that's the whole point. Instead, they supply power only when the coil attached to the diaphram of the speaker is moved by your voice, creating electrical current in tune with the amplitude and frequency of the sound waves. This is reproduced at the other end using the opposite process.


    8 years ago on Step 2

    I am using two normal 8 ohm speakers and it does not work, could it be something to do with the length of the wires between? The length of the wire is about 10 inches.
    Help would be appreciated


    8 years ago on Step 4

    I've been known to cut the tops off 1 & 2 liter soda bottles for horns, I mostly use them on microphones, but they work as speakers too. Dad's Root Beer has a nice brown color. See my instructable on a blue light blocker mask. These bottles can be used by themselves or used as a mold to make a paper machet horn. An earphone is the right size to drive a soda bottle horn.


    8 years ago on Introduction

    a little longer the wwii.. look up the ee-8 sound powered handset (called the double e eight) with only slash wire between gun placements we had "acceptable" com.. that was a basic speaker for a transmitter.. your voice moved the small cone, that moved a coil over a fixed magnet.. no batteries, .. way before chips or even transistors made it to the field, great weekend project.. that unit had a hand squeezed generator (90v, 20 mil) that moved a fixed mag on the far end showing white through a small window.. i want to say we did hear the 20 cycle ring through the rec on the far end..


    8 years ago on Introduction

    I used to do this as a kid with old phones. You can just get some old phones and just take out the earpieces. You won't use the microphone so you'll actually need four handsets to make two phones. These are already high impedance and won't need any transformers. Simply connect two earpieces together and when you talk into on it generates a voltage which you can hear in the other one. It will work both ways (Over!) but as you've already go a couple of handsets, you can make up a phone-phone handset by replacing the microphone with another earpiece, which you cross wire to the earpiece of the other handset. Old fashioned handsets where both the earpiece and mic are the same size would obviously be easier. They will work with a reasonable sound level.


    10 years ago on Introduction

    well it works but it is very hard to hear. i always used a pair of portable tape decks, the ones you could get for $1.99. i would hook up a mic to the play head and a speaker to the headphone jack and wire them together with 4 conductor phone/intercom wire. it worked well even with several hundred feet of wire between them.


    12 years ago

    Old school simple electronics. I learned this on some show from the 60's.. Forget what is was called. Had a British host.

    1 reply

    12 years ago

    Well I have to work up slowly to the PMM. They don't teach this stuff in new school.

    1 reply