Help with Piezo Circuit

[EricSchuman]

Hi,
I'm helping my son build his 8th grade science project, which is an array of Piezo disks for collecting energy from sound, which will be used to light LEDs.   The plan is to wire ~40 piezo disks together (parallel and serial) to increase the voltage/amperage.  The leads would be connected to a small LED flashlight (3v DC.)  We tested some of the disks and they were putting out ~100-400 mV AC.  The array of disks will be attached to the bottom of his snare drum.

Questions:
1. Polarity.  Because the polarity of the piezos may not be in sync, will the voltage of some disks cancel out the voltage from others?  How do we get around that problem?  Will a rectifier circuit solve the cancellation problem, and if so, does each piezo needs it own rectifier, or can we feed all of them through one circuit?

2.  Construction.  Are there ready-made components that we could use, rather than building these circuits from scratch?

3.  Design.  How can we add a capacitor so that the light has a steady beam, and not flicker?  How do we maximize the power output?

4.  Cool Factor: Rather than lighting LEDs, can we construct/buy a board that will allow a cell phone to be charged?

We (I) have reached the limit of our electronics knowledge, and internet searches haven't yielded any comparable projects that we could copy.  Any help would be immensely appreciated. 

Eric and Tristan Schuman

POST:  I've added a rough sketch of our proposed circuit.  I don't know what diodes or capacitor to use.

[mariush]

There are energy harvesting chips to which you can connect a piezo and the chip will output a stable voltage. For example see linear's page: http://parametric.linear.com/Energy_Harvesting

Something that should work for you would be this: http://www.linear.com/product/LTC3588-1  See details and datasheet.

They're expensive though, maybe 5$ a piece, so forget about 40 piezos with such chip. 

You could connect a supercapacitor at the output so that the energy gets "cached" into the supercapacitor. Then you could use a boost dc-dc converter to boost the voltage to whatever voltage is needed.. 3v for a led or 5v for a charger.
Boost regulators like LT1307 will do about 3-5v output with as low as 1v which you can obtain with piezos in a short amount of time... how fast i guess it depends on the power of the piezo.. 100-400mv ac doesn't tell me anything. And you can also configure such dc-dc converter in sepic mode, so you can get 1-8v dc in, whatever output voltage you want, with good efficiency.
How much a led lasts depends on how much current you set the led to... as for charging a phone, yeah, you could configure it to output 5v but there's a huge amount of energy required to charge a phone. it may take minutes to recharge a phone for a few seconds.

ps. The Signal Path blog has a very good video in which some of those energy harvesting chips are reviewed and the theory of operation is explained, it's worth your time : http://www.youtube.com/watch?v=lrELRVFEx_s