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Is there a device that can use the energy from a vibrating string (i.e. guitar string, etc.) to produce enough electricity to power a small LED?

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  • $\begingroup$ guitar pickups would be in the microamp ballpark IIUC, which is rather low. But you can couple the vibration to something more efficient through the bridge (think of the tremolo/vibrato bar mechanism) and perhaps extract more power from there(?) $\endgroup$
    – Pete W
    Commented May 9 at 21:51
  • $\begingroup$ Sure. Your vibration must be forced by something powerful enough to power the LED and any losses in between. For example, attach a strong spring to a string to return the spring to its original position. Attach a energy harvesting device (generator or even magnet and coil) to the string, spring combo. Your only remaining issue is sufficiently energizing the string repeatedly. Your average, highly entropic stuff will not likely suffice! $\endgroup$
    – Abel
    Commented May 9 at 22:29
  • $\begingroup$ You may also be able to "cheat" the stated requirement, by saving up a microamp scale output of the coil into a capacitor, then discharging it in a short burst with enough intensity to see easily $\endgroup$
    – Pete W
    Commented May 10 at 0:23

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The short answer, "No." It is easy with an amplifier but it is not going to happen without some source of external power.

In the meantime, if all you want is to light an LED, the electronics are not hard. Making an amplifier that sounds like anything requires some knowledge of for-real type electronics but to light an LED, all you need is a transistor and some resistors, the type of thing a bad amateur could put together in a few minutes.

How I arrived at my "NO" answer:

There are two ways of approaching this.

  1. How much energy is in the string? If you gather ALL of it, will there be enough to power an LED?
  2. How would you gather the energy? Could you gather enough to power an LED?

For point, #1, How loud is a string when plucked? Is it as loud as, say a bunch of people talking loudly? I don't think so but if it were, it would be about 80 decibels, dB. Audio dB are a logarithmic measure of sound intensity. A level of 80 dB would correspond go about 0.00001 watts per square meter--unless you gather a lot of sound from over a huge area, you are not going to have enough power to do anything with and a string, even many strings, won't produce any 80 dB at any distance.

For #2, I can think of half a dozen goofy ways of gathering energy from a moving string but the two simple ones that are in common use are, by a magnetic pick up from a steel string and by sound energy from any type of string.

Magnetic pickups, such as those used with an electric guitar, have a small permanent magnet and a coil. When the steel string moves near the magnet, it induces a small (as @PeteW observed, on the order of microamps) current in the coil. This is enough to pick up with an amplifier but not enough to see with an LED.

Some microphones work similarly to the magnetic pickup except measuring the movement of a diaphragm rather than a string. Again, not enough energy will be gathered to light an LED.

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