This design is based on the requirements for a 25 Note Velocity sensitive Fatar keyboard. The keyboard encoder is arranged as a matrix of 8x5 (40), but only 25 keys are wired, Pins MK0 and BR0 are not connected. However the software scans the 40 possible key input positions but the 15 unconnected key positions do not produce any MIDI output. The connections can be made via a 20 way (2x10) Micromatch ribbon cable, which can plug in directly to the corresponding Micromatch socket on the Fatar keyboard PCB.Operation:
The 25 note range is from C3 (MIDI Number 48) to C5 (MIDI Number 72). The MIDI Channel is preset to MIDI Channel 1. However other values for the MIDI Channel and the note range can be pre-programmed into the firmware. The encoded switches are wired via IN4148 diodes. There are 2 switches associated with each key. They are called the Break (BR) and Make (MK) switches. When a key is pressed the Break contacts close first and then the Make contacts close, as the key is further pressed. The difference in time between these two switch closures provides the time which is used to calculate the MIDI Note On velocity for each keypress. Also the MIDI Note Off velocity is calculated in a similar way when the key is released. This unit can be connected to new keyboards or it can be used with an older non-MIDI keyboard by using magnetic/reed switches, or Hall Effect devices, to isolate the old and new scanning circuits. This will allow the older keyboard synthesiser to continue to operate as normal while also providing a MIDI output.
This unit can work in standard MIDI mode and via the USB connection, at the same time. It defaults to standard MIDI Baud rate of 31250. By replacing the original firmware on Mega8u2/16U2, the Arduino will act as USB-MIDI device (Standard Midi Class), you do not need to install additional device drivers on Windows, MaxOSX, and Linux, as the firmware acts as a device of Standard Midi Class. It will automatically install on the system as an Audio USB Device.Power Supply:
External power can come either from an AC-to-DC adapter or battery. The adapter can be connected by plugging a 2.1mm center-positive plug into the board's power jack.
The board can operate on an external supply of 7 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.