A Tin Can Phone, But With Magnets

The tin can phone is a staple of longitudinal wave demonstrations wherein a human voice vibrates the bottom of a soup can, and compression waves travel along a string to reproduce the speaker in another can at the other end. All the parts in this electrical demonstration are different, but the concept is the same.

Speakers are sound transducers that turn electrical impulses into air vibrations, but they generate electricity when their coil vibrates. Copper wires carry those impulses from one cup to another. We haven’t heard of anyone making a tin can phone amplifier, but the strictly passive route wasn’t working, so an op-amp does some messy boosting. The link and video demonstrate the parts and purposes inside these sound transducers in an approachable way. Each component is constructed in sequence so you can understand what is happening and make sense of the results.

Can someone make a tin can amplifier transformer? We’d like to see that. In another twist of dual-purpose electronics, did you know that LEDs can sense light?

17 thoughts on “A Tin Can Phone, But With Magnets

  1. You don’t need a transformer; you just need a carbon microphone. That’s exactly how it was done 100+years ago in the first telephones.

    A diaphragm vibrates a “resistor” made with two metal plates filled with carbon granules. Its resistance varies with the sound waveform.

    This carbon microphone, an electromagnetic earphone, and a battery are hooked in series. Talk into the microphone, and the sound comes out the earphone at the other end of the wires. :-)

    1. It’s interesting enough to note that the carbon microphone derives from the earlier “coherer” which was a vial of metal filings to detect radio waves. There was speculation about why it works, and it finally turned out that the relatively high voltage coming off of the tuned antenna circuit would actually cause micro-welding in the filings, an effect which was amplified by the DC current that was connected to the tube. In other words, the tiny flecks of metal were oxidized on the surface, blocking the DC from flowing – but passing through the HF current from the antenna through the nanoscopic capacitors between the grains. This would cause enough localized reaction between the grains that the DC would start flowing and weld the grains together. This would then ring the receiver’s bell and cause it to tap on the vial to break the connection and re-set it. When they tried to do this with carbon dust instead of metal filings, it didn’t work, and nobody understood why – but they discovered the carbon microphone instead.

        1. The coherer or the effect behind it was discovered in the 1835, and found its use in radio around 1890. The carbon microphone was invented by at least three different people around 1878 while everyone was trying to make radio work.

          1. It’s worth adding that radio existed as a laboratory curiosity, hence the early date. It took some time to show it as practical. I guess that’s why some people think Marconi “invented radio”. He showed it was practiical for distance, and was interested in making money from it. So there may have been a time when Marconi was synonymous with radio. Early ship use, and that was about the first practical use, used Marconi supplied radio and operator, in effect a service rented by the shipowner.

    2. We still had carbon capsule transmitters (“microphones”) well into the 70s in our burgh. Granted, we still had party lines too, so we were a bit of a backwater.

      We got used to the other party saying we were getting faint; a sharp rap of the handset on the countertop or doorframe loosened up the carbon and restored the transmitted audio. A bit of a hack, so to speak.

      1. Carbon mics work well with party lines where the line voltage may get weak – they have no minimum threshold of operation so you just go fainter, whereas more modern handsets drop out.

  2. A better version of the pickup wraps the coil around a magnet – a weaker ferrite magnet will do better here – and then you glue a little iron button on the bottom of the cup. This reduces the moving mass and you get higher frequencies out of it.

    When set up that way, it works like an electric guitar pickup. The piece of iron changes the amount of magnetic flux going through the coils by reducing or increasing the reluctance of the magnetic path, which serves the same point as moving the coil or the magnet to induce a current. If you build a soft iron cage around the pickup, you can improve the effect considerably by concentrating the magnetic flux on that small air gap where the microphone diaphragm is.

  3. This works perfectly well if speakers are used as the transducers either end. Ideally a big light weight mid-range unit. Obviously there is a relatively inefficient power conversion step at each end (each about 1.5% efficient, thus a whopping overall 0.0225% ish overall or -36.5 dB). Which is roughly the difference between talking and whispering.

    Walkie talkies weren’t very good when I was a kid and big efficient speakers were plentiful. All it needed was 50 m of bell wire.

    1. Would it become louder if you pass a small DC current through the speakers? In an ideal case it would not, but in practical speakers when you bias the system like that, you can pull the voice coil deeper into the magnet where it should be more responsive to the signal, as opposed to pushing it outwards.

      1. No. Voice coils are symmetric an poke out of the magnetic gap equally (certainly the ones I speak of do). An asymmetry would not help and your method would simply leave more ineffective coils lurking about inside the motor than poking out the front.

        Covering the drivers with glitter would be equally effective, with the positives of not requiring power and being more sparkly. And, if a loud noise is made near one (popping a balloon right next to it perhaps), then whomever is at the other end will be bathed in a cloud of glitter. A plan with no flaws!

        (This may comment may have been influenced by the morning antics of a 6 year old girl who now needs to tidy up)

      2. A way to potentially enhance the effectiveness would be to increase the field through the voice coil. Plonking a backing magnet on the rear of the driver (such that it initially resists being pushed towards it), can enhance the BL product, which is directly proportional to the efficiency of the speaker. May gain a dB or so either end.

  4. Wow! You discovered sound powered phones. Sadly this has been around for a long long time. The military even used them. A dynamic microphone is essentially the same as a speaker. You can take 2 small speakers and connect them in parallel over 100′ of wire, these days you could be slick and put in an RJ45 female and use parallel runs of the conductors in a regular Ethernet cable, to hook the two ends together. You say something in one end, that vibrates the speaker that is acting like a microphone, the current passed down the wire, and in turn causes the speaker at the remote end to vibrate. You can say over at the end of each transmission to turn the device over so the other person can speak into the speaker while the other person listens or you can run 4 wires for truly two way communications. This reminds me of being 6 years old again…

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