The circuitry for the hall effect magnetic sensors was relatively simple. The anologue devices sensor was the only one I could find that was programmable to be omni-directional or bi-directional. An omni-directional hall effect sensor would provide the full 10-bit resolution for the set-up on el lechero, where the keys face the sensor from only one side. Additionally, one could design a circuit with this sensors to vary the offset and gain of the sensor's sensativity. A diagram is shown below. Info about the sensor can be found here. The rest of the circuitry is the basic CREATE USB interface which can be founda at http://www.create.ucsb.edu/~dano/CUI/.