Hi,
Edrums eh? I've hoping to build some soon, haven't played drums in years...
Anyway, I can see two relatively simple ways to do this:
1) connect the MIDI OUT form your drums to a circuit, which reads the messages and makes various outputs high, namely LED's. This would need a microcontroller to read the midi, some MOSFET's to control each 'group' of LEDs, and is probably overkill.
2) For each drum sensor (your piezo element) connect its output to an opamp voltage follower, to give it a massive input impedence (so it doesn't draw the signal away from the circuit that reads the sensors and sends out the midi messages) and a comparator. The output of the comparator should have a capacitor to ground, say, 1uf? so that the breif 'spike' from the sensor lights the LED's for a bit longer, otherwise they would flash very quickly. Again, for 8 LED's, you'd probably need a MOSFET driver, and depending on the LED colour, and power supply available, try to run LED's of the same type in series.
That sounds complicated, but it really isn't, its just you'll need a circuit for each 'drum'. Each 'drum' would require 2 comparators (or 2 opamps, configured as comparators) both are available very cheap, in dual packages.
If the 'Edrum' is completely custom built, with a microcontroller, and you've got spare I/O pins, you could modify the code/circuit to output a pulse every time a particular drum is hit.
Theres some idea's, I will try to be of more help, given more information, wicked idea btw....especially if your drum heads are clear....they'll light up every time you hit one
Blueteeth.