Are Drum Beats Enough to Charge an iPhone?

Spend a little bit of time in one of those “stupid questions” subreddits, or use the Wayback Machine to browse Yahoo! Answers, and you’ll see someone ask something like: “why don’t we just use the energy from people walking to power things?” That’s actually not a stupid question at all and it is something Real Scientists™ and Also Real Engineers© have researched, falling under the general umbrella of “energy harvesting.” But can otherwise wasted energy be used for anything useful? Will decided to find out by trying to charge an iPhone using drum beats.

Energy harvesting (EH) refers to the collection of small amounts of wasted energy for useful work. That’s somewhat distinct from energy recovery or recapture techniques that are common in industry for making use of waste heat. When we talk about EH, we’re referring to the energy from stray radio waves, body heat, artificial light, and vibrations.

But there is very little energy there and even with perfect efficiency, a small solar panel would probably provide more. So, EH is usually only seriously considered for niche applications, like powering biomedical implants.

Even so, Will wanted to know if all of the energy from his drumming practice could do more than annoy the neighbors. Soundwaves are just vibrations, so he decided to collect them directly at the source: the drum head. Piezo elements exist for exactly the purpose of converting vibrations into electricity (normally for sensing) and were the obvious choice. After some initial experiments comparing a snare and tom, Will found that the tom produced more voltage.

But with a load (just a small resistor), Will discovered that the amount of power (voltage multiplied by current) produced by a single piezo was miniscule. That led him to an obvious upgrade: more piezos. He ended up with ten in total. He also glued large nuts to those, which added inertia to increase power output.

However, that still wasn’t enough for an iPhone to even register charging. To get to that point, Will used a rectifier to turn the piezo AC output into DC input for an array of capacitors. Those charge up until they have enough power, then release it all at once to the iPhone. A clever switch circuit puts the capacitors in series for easy charging, then in parallel for higher-output discharging. A custom PCB tidies up all of that circuitry.

With those enhancements, Will was able to get his iPhone to register charging. It only did so for a moment and probably only received a barely detectable amount of power, but it technically meets the criteria and so we’re calling it a success.