Read the clue description first.
The bones keyboard is an electromechanical device that uses two AVR re-burnable microcontrollers to read the inputs from the near 30 white keys and then light up 27 LEDs that correspond to letters in a correct pattern. One microcontroller was on the back of the light board, and controlled the output of power to the LEDs. The other was contained in the piano box and it monitored the inputs from the keys and sent pulses to the first microcontroller through a detachable set of wires. A 9 volt regulated to 5 V powered each microcontroller and the piano itself was powered by 6 AA batteries.
Using the fact that piano keys are already spring loaded, the box that housed the piano contained a large flat piece of metal that stretched across the tops of the white keys and was attached to ground. Each key had a stripped wire contact attached to it which was connected to one of the inputs of our piano microcontroller. This means that while no keys are played, all the inputs read 0 volts. When a key was pressed, its contact would be removed from the metal strip and the connection would be broken. The input would no longer read 0 volts for that key and the chip would know which key was pressed.
This piano microcontroller was programmed to listen for a certain sequence of keys and for each correctly pressed key, it sent a pulse to the light microcontroller. If a wrong key was pressed, it sent a pulse to the reset input of the light controller, starting it program over. At the end of the sequence, or when a wrong key was pressed, the piano microcontroller would then begin listening for the first correct note again.
The light controller just waited for pulses, and lit up the lights in sequence as it received them, until it reached the end of the sequence or was reset.