DIY Kit - Adding MIDI extension
MIDI is a good thing to have
MIDI is a standard, wide spread interface for connecting various music gear like keyboards, synthesizers, drum machines and other. It carries information about notes - their pitch and velocity, plus some effects, timings etc. (as opposed to a complete sound carried by other interfaces e.g. common analog or digital lines). It works pretty much like a digital representation of music score.
Since the MCU used in Gecho can drive both MIDI IN and OUT pretty well, you will be able (in certain channels) control Gecho with a MIDI keyboard, or - the other way around - control devices that have MIDI input. The USB-MIDI keyboard that connects to PC won't work, you need one of those with classic 5-pin DIN connectors:
The SMD elements
If you ordered loose SMD components for adding the MIDI by yourself, you should find this included in your DIY kit.
Soldering the MIDI circuitry
On Gecho board there is a (normally unassembled) area to host a simple MIDI interface. You need to assemble 6 resistors (of 0603 size), one 6-leaded optocoupler (or opto-isolator) and one diode (please note the orientation).
Important: apart from elements, do not forget about the soldering bridge, in blueprint denoted by "IN(-) sel", it is used to select which opto-coupler's lead the negative MIDI IN signal goes to (because various opto-couplers are supported). In our case, we need to connect middle and lower pads of this soldering bridge.
The rest - voltages and blue dashed Rx/Tx signals are shown only for reference and test/debug purposes, and are wired internally in the PCB, no need to connect anything there.
After soldering, it should look like this.
Since DIN5 connectors are bulky, it's a good idea to leave them on a break-out cable and only include smaller connectors in Gecho, for example these standard wire-mounted 3.5mm female Jack sockets. They will fit into the battery compartment when not used.
I got these from eBay and although they were advertised as MIDI cables, internally the connection was wrong. MIDI needs signal to be connected to pins 4 and 5, here the stereo jack was wired simply to pins 1,3,5. Some rework was needed - notice that one of them is now isolated by shrink tube instead of original connector. Perhaps you can come with your own solution, put your cables together the way you want (either with two 3-pole jacks or one 4-pole jack). Here is a good tutorial that explains correct wiring. Otherwise, you can wait until proper cables are available (those will use 4-pole connector).