Peter Kirn recently wrote a very kind post on Bluetooth and MIDI, and so I figure it’s time to report on my attempts to reduce the cost of the MIDI-Bluetooth setup that I’ve been experimenting with, even though the design will probably undergo another revision or two.
The original version used a BlueSMiRF module, which is great for prototyping because it’ll accept voltages between 3.3V and 6V (according to specs, anyway) so that you can plug it into any reasonable setup and it’ll work. What’s not so great is the price — basically, the BlueSMiRF consists of a $20 Bluetooth module and $2 worth of level-shifting circuitry, but SparkFun sells it for $64.95. Clearly, significant savings are possible if you’re willing to do your own voltage conversion.
So, I went ahead and ordered a KC-21 Bluetooth module for about $20, only to discover that it wouldn’t support the nonstandard MIDI baud rate. Fortunately, Chris at KC Wirefree was extremely helpful and prepared a new firmware version that can be configured for arbitrary baud rates (thanks, Chris!). Updating the firmware was a breeze, and adding components for shifting voltage levels was easy, too, using schematics found here. The resulting unit is pictured above. The KC-21 module is on top; the parts on the left operate at around 5V, and the voltage conversion to 3.3V happens on the right, with a couple of MOSFET transistors hidden underneath the circuit board.
The resulting gadget works, for a total of less than $25, and preliminary measurements of latency and jitter suggest that it performs about as well as the original version. I’ve since been told by a real electrical engineer that there’s a much simpler way to convert voltages for the purposes of MIDI; maybe we’ll come up with a simplified design at next week’s Handmade Music Open Lab.
I’ve been meaning to try the same setup with an RN-42 unit for comparison, but I have yet to find the time to do that (maybe at the lab next weekend…). In the meantime, I’ve finally managed to install the original interface in a proper project enclosure. It took many attempts, made a big mess, and required the replacement of a couple of parts that I ruined with runaway epoxy, but the reward is a vastly more generic-looking device. Yay!