SPEAKE(a)R: Turn Speakers to Microphones for Fun and Profit [pdf] (2017)

What’s wild is that most things having to do with light, magnetism, and/or electricity are interchangeable and reversible. Put electricity through a wire and it’ll create a magnetic field, or wave a magnetic field near a wire and it’ll create electricity. That means that putting electricity into an LED creates light and a magnetic field, or putting light into the LED creates electricity and a magnetic field, or waving a magnetic field near it will create electricity in the wires and light from the LED. Granted for that last one you’ll need a spinning magnetar nearby, or just add some more wire to the LED and it becomes a kitchen counter experiment.

Same interchangeability with solar panels, transformers, thermoelectric devices, etc. The effect might be big or small, depending on the setup, but the physics is happening either way.

I’ve spent time lost in space thinking about how much stuff is really just a copper wire in various configurations.

Have a copper wire - it’s an antenna, magnet, inductor, fuse, thermometer, heater, and strain gauge.

Put another copper wire near it - it’s a capacitor.

Curl one more than the other - it’s a transformer.

Put iron on it - it’s a thermocouple.

Put electricity through it - it’s a peltier cooler.

Add salt water - it’s a battery.

Put electricity through it - the iron is now a permanent magnet.

Wave the permanent magnet near it - it’s a generator and a microphone.

Put electricity through it again - it’s a motor and a speaker.

Heat it up and it’ll make Cuprous Oxide - it’s a solar panel and a diode.

Put electricity into it - it’s an LED.

It was so popular in studios to use an old Yamaha ns10 speaker as a kick (bass) drum mic that now Yamaha started making and selling an “official” version.

I personally used a toy guitar amp for this purpose https://music.stoyanstefanov.com/2017/03/30/diy-sub-kick-mic...

Chips only have a certain number of pins. It probably works out better economically if those pins can be used for either input or output. Chip manufacturers can thus make one product that will fit the needs of more customers instead of (say) 9 different chip variants with 8 inputs and 0 outputs, 7 inputs and 1 output, 6 inputs and 2 outputs, etc.

It could also be useful to the end user. Motherboards have a limited number of ports since the connectors cost money and take up space on the back panel. One user might want a line input (for digitizing old cassettes, for example)[1] and another user might want an extra surround sound output (for 7.1 surround sound instead of just 5.1 surround). With retasking, the motherboard can support both these niche use cases with a single shared port.

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[1] You can't use a microphone input for this because (a) it's mono and (b) it's a different voltage level.

That's all true. As a kid I was watching with sister a children's show in TV. The actor on the screen asked a question followed by silence. My sister yelled the answer at the TV, then the actor said something like "you're right kid". I was flabbergasted.

Dora the Informer.