> Thinking about it, I don't know that I would use pressure sensors unless
> you wanted pressure measurements. I don't know if that would matter for
> pedals, as the pressure is going to be checked only once you have
> already pushed it down. Simple dial resistors on each pedal would tell
> you how far down they were pressed; optical encoders would work too
> (dead mice scroll wheels, if you want those cheap). For more accuracy
> than that, I would have to think about it more. Magnetic sensors might
> work, but you would have to calibrate the whole set.

I've made a few sets of pedals for car racing games (where I think the 
forces on the brake pedals are much higher than what would typically be used 
to activate an organ pedal).  The usual construction is to have the pedal 
rotating on a bearing with a big stiff spring giving the resistance force, 
then a simple rotary potentiometer (variable resistor) is wired up as a 
voltage divider (0V on one end, 5V on the other end) on the pedal axle with 
an ADC (as you say often part of the microcontroller itself) converting the 
centre tap into a digital signal that can be sent to the PC via USB.  This 
setup is accurate enough to give good control over controlling the brakes in 
a car racing game without any noise or spurious signals, so it should be 
fine for controlling an organ pipe :-)

It would be a nice little project, you can buy ready-made PCBs with USB 
interface and analog/digital interfaces if you don't feel like doing too 
much of the electronics and microcontroller software yourself.

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