http://arduino.cc/en/Reference/AnalogReference

Couldn't you also use this?

Isn't the rasp-pi actually cheaper than a high-end (genuine) Arduino board? If cost is a huge factor here I doubt changing over to the pi would make an enormous difference. And they're tiny As long as you can learn how to do what you want on that platform, you can't lose.

You just know... the day after you throw it away, someone will write a decent library for exactly that part.

Well, for my project idea (which may or may not ever get built!!) I was thinking of two different approaches to accomplish the same thing - do an analog read on 8 pads and act accordingly.

As my project plan was for a modular system similar to the old Simmons kits, my first plan was for a master trigger module, and its job would be managing the input pads and MIDI/trigger signals to other stuff. This is how I imagined such a module MIGHT work (it also might very well NOT work, as I have had no time to build, code and test it):

Going by the highest midi timing resolution feasible on whatever micro I ended up using, first perform a sequential analog read on all inputs to determine which pads were hit in that "time slot" and how hard. Let's say for argument's sake that the 8 inputs could be polled sequentially every millisecond or so, and the 8 values obtained put in an array, which could be passed to another function responsible for generating the MIDI data.

My idea to test this if I ever got around to it would be to write a Processing app consisting simply of 8 coloured boxes which could change from green to red when a pad is triggered and display the velocity (0-127) in the box, and hope that if it LOOKED like it was responding correctly and quick enough, it would sound "in time".

I suppose with all the libraries available for midi/serial stuff, it wouldn't be too hard to take the pad data and assign it to MIDI notes... but then, this is all just pure speculation and untested by me.

I have attached a schematic from a project called "EDrum" which demonstrates how to hook up the pads to your micro's analog inputs. Perhaps you'll find it useful!

I know that feel Work is running me ragged at the moment so while I have just about enough time to read about stuff, I rarely get a decent amount of time to break out the breadboards. I've had a sampler/drum machine project "on the cards" for about 6 months now and have done precisely dick, as far as building something goes!! I have more project ideas than viable lifespan left, I think 

You could take a look at the Rocket 8 synth's filters and see where you get to copying those...

http://hackmeopen.com/rockit-build-info/rockit-schematic/

When I *finally* get around to buying a scope and doing some analog stuff I was considering getting some SSM2044 VCF chips to play with and see what happens. Probably not the best IC choice in the world as far as long-term availability goes, but the circuits look simple enough and one or two won't break the bank

I'd just do it in software by sending the appropriate commands to the 7219. Why add extra components and complexity if you don't have to?

Do you want to actually synthesize the drum sounds, or just make a trigger pad to midi converter? One's relatively hard, the other's supposed to be quite easy.

Looks like a P-Channel MOSFET. I'll bet there's a cheap "standard" one people tend to use for this sort of thing, but I don't have a part number off the top of my head to give you!

EDIT: You could try a parametric search on the component supplier's website of your choice, to find which ones are suited to your requirements

Wouldn't the bog-standard BC547 be ok for this? The max. collector current is 100mA and the maximum voltages are more than high enough.

Majenko, I whole-heartedly agree with your advice on printing multiple copies... You ALWAYS offer a sacrifice to the fuck-up fairies

There's this...

https://www.sparkfun.com/products/10743

Not quite the same form factor and it has pain-in-the-arse connectors (although they do sell leads to fit them), but maybe that'd do the job?

I just checked on a 555 calculator, and according to that it's 66.6% duty cycle and 21.8KHz. Oh well, it was fun to play with anyway... with a few tweaks I might be able to make something useful out of it. I quite fancy playing with an IV-18 (or similar) myself, they're cool looking tubes!

I tried the 555-based step up circuit last night, just to see how I got on with it using the parts I had on-hand... it worked surprisingly well! For the 555 stage I used:  

R1 & R2 = 100ohms
C = 0.22uF

And for everything else:
Diode = UF4007
MOSFET = 2SK2718 (pulled from an old dot matrix printer)
L = 470uH

Setting my bench supply to 7.2V I was getting well over 700V on the output with a current draw of around 100mA, and it charged a small bank of photoflash caps very quickly Think I might build a permanent version, it's got to be useful for something!

The 74HC595 is the standard choice for that sort of application.