A piezoelectric sensor would be excellent, quite possibly. I'm sure I could find just the piezo part, free of the phidgets board. (Quick skim indicates that the Phidgets need to be run to a computer or processor. I'm looking for a "dumb circuit" implementation, or maybe just an IC or two.
I'll throw an idea out now:
With the resulting voltage (big question is how much), I could trigger a MOSFET and start a circuit running. What would be an approach for making the circuit keep itself running until turned off? ... i.e. once the MOSFET is first triggered, bypass the MOSFET completely until switched off. There's probably an IC that can do that, aye?
Mind's rolling here:
An IC with two inputs, one being connected to the battery, and the other connected to the MOSFET. IC turns on when both give voltage high, and only resets itself when the first voltage goes low?