We wanted to find some way, some how to reverse wireless charging. There are energy waves surrounding us at all times, so how cool would it be to leverage that!? PowerGrab uses one form of energy available - vibrations - and converts it in to a DC power supply.
What it does
A piezoelectric transducer captures the vibrational energy from a Pebble watch. The Pebble and transducer are tuned so that their natural frequencies are as close as possible and a rectification circuit converts the output of the transducer in to usable DC power.
How I built it
We spent many careful hours characterizing the piezo and finding its natural frequency, and then doing the same for the Pebble. Next we attached the piezo to the Pebble and began tuning the Pebble's frequency to achieve resonance (a very tricky process). As we approached resonance, the piezo's output voltage increased, so we continued until we got to a usable amount of voltage output with the Pebble vibrating. Next up was the rectification circuit. We needed to convert the AC signal to a DC supply so we implemented a wheatstone bridge with four diodes for full-wave rectification. Next, we tested varying smoothing capacitor values and picked the best of what we had. Finally, we were able to power an LED! (We would have loved to boost our DC voltage up to 4 V and feed it back in to the Pebble, but alas, we were missing a few critical hardware components for that.)
Challenges I ran into
Frequency tuning is a difficult process with the right equipment and piezo's do not give out too much voltage without resonance. Therefore, we had to spend hours upon hours trying to bring down the frequency of the Pebble to get closer to the natural frequency of the piezo.
When we finally got that working, the struggle continued in to the rectification circuit: we lost any voltage that we thought we had (we were getting less than 0.2 VDC, yikes). We had to take a step back and debug the circuit step by step, try out different hardware components, re-tune the frequency, change components, re-tune, and on and on. We weren't able to get the measly 0.2 V up to a usable 1.85 V until late in to the night.
Accomplishments that I'm proud of
WE POWERED AN LED WITH A PIEZO! (and we're excited about it =) ) This project idea was ... a stretch, to say the least. We couldn't find any examples of this being done online and thought we might be crazy to be embarking on a project that might be truly impossible scientifically. BUT, we saw a small opportunity for crazy cool power harvesting and the math said it was possible (though highly implausible), so we dove in. It took days of frustration and seemingly thankless hardware tuning, but in the final hour... WE POWERED AN LED WITH A PIEZO!
What I learned
Tuning natural frequencies is quite challenging and piezos are incredibly sensitive. A lot of variables need to be in line perfectly for this to work. Also...you can indeed get free power from the environment if you have a lot of patience and hot glue.
What's next for PowerGrab
We need to get our hands on a charge pump and boost this sucker up to 4.0 V (the minimum for charging a Pebble battery), just to see once and for all if we can get that beautiful charging icon to light up. Then we need to manufacture this piezo setup in large quantities, place them around noisy cities and construction sites, and grab some power. =)