I’ve finished the hardware part of my Teensy 2.0 based iPad MIDI interface project.
Getting it to fit in the smallest possible Hammond 1551 box was a challenge. I rebuilt the circuit no less than three times before getting it to fit in the the 1551H enclosure. The 8-pin opto-isolator used on the proto was replaced with a 6-pin Sharp PC900 to save space.
Update 1: Supply current is about 35mA. Not sure if this can be reduced below the 20mA limit.
Update 2: I’ve managed to get the operating current down to about 22mA without changing hardware. A simple code change – to take advantage of the idle mode when waiting for events – saved a few milliamps. About 10mA savings comes from setting the clock to 8MHz instead of the default 16MHz.
According to the Teensy documentation, another 7mA of savings is possible using a 3.3V linear regulator IC that can be installed on the back side of the board. That should drop the operating current well below the 20mA limit.
See below for more pics and schematic.
The circuit is simple: A Sharp PC900 (PDF datasheet) provides the input-side electrical isolation required by the MIDI spec. Its output connects to the Teensy UART RX input. The MIDI out is driven directly by the Teensy UART TX output pin.
The circular DIN jack is wired as a combination MIDI-OUT and MIDI-IN, but should be considered a MIDI-OUT when using it with gear that is not wired for bidirectional MIDI. Pins 4 and 5 are MIDI-OUT, and pins 1 and 3 are MIDI-IN. This is the same arrangement as used on the MFC-101 MIDI footcontroller’s MIDI-OUT jack, and should be compatible with the Axe-FX and Axe-FX II.
I actually used a 7-pin DIN jack, and removed the contacts for pins 6 and 7. This lets me use the same 7-pin MIDI cables that I use with my MFC-101.
Pins 1 and 3 of the DIN jack are soldered to the board with short pieces of resistor lead. This is the mechanical weak point of the design. Flying leads between the DIN jack and board (e.g. the green, yellow, and purple wires) would be better.