Hardware Reference
In-Depth Information
Figure 6-18.
The Alcyone
In the Lowrey design, the pedals convert the circuit to a CV/Gate signal, which was a
standard before MIDI was created. However, CV/Gate was never formally specified,
so different manufacturers use different implementations. CV/Gate normally con-
verts to a voltage: the first note might be +1 V, the second note +2 V, and so forth.
However, this is a convention and not a standard, and the Raspberry Pi has neither
analog inputs nor the capability to handle +13 V.
Thus it was simpler to use as little of the existing mechanism as possible, which meant
relying solely on the rocker switch and wiring custom digital I/O. As the Pi has only a
limited number of GPIO pins, the circuits for the rocker switches are wired to a set of
I2C chips. These use a serial protocol to map GPIO pins. The MCP23008 offers eight
individually addressible GPIO pins, and the MCP23017 offers 16.
The I2C chips can be wired such that seven of them are on a single serial connection.
Thus, if you use seven MCP23008 chips in a series, you have 56 uniquely addressable
pins. With the MCP23017, you could have 112. There are likely to be other I2C chips
available, and you don't have to have a homogenous chain.
The physical electronic circuit is thus simple: three MCP23008s in series with two of
them dedicated to pedal input and one driving three LEDs for runtime status.
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