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.