A piano keyboard is set up as usual with one hammer per key and (to keep things simple) one string per hammer. All the strings are the same length and gauge.
The strings are attached to a resonating chamber / soundboard at the near end. The far end of each string is attached to a pulley with a constant-torque
system (the simplest being a weight dangling from the end of the string).
The line of pulleys is attached to sliders driven by a crankshaft, each pulley having its crank offset from the next by 10 degrees or so. The pulleys advance as close as 3 inches to the hammers and retract as far as 3 yards away as the crankshaft rotates.
The crankshaft is turned by bicycle pedals mounted under the keyboard.