Introduction of the keyer was presented on youtube, as follows.

This is the version 1, which did not support minus keying, nor SD card.

This is the version 2, which supports minus keying. This function is quite important for vacuum tube final rig.

And this is the final and ultimate (for me) version 3. Supports SD card message sending and of course minus-keying.

And here is the whole schematic.

Actually nothing special. It consists of ATmega328P with CP2102N.  I guess two channel (dot-dash) touch pad can be made just by Mega's A/D but one specific IC (Microchip's MTCH102) was used for simplicity. Some pinouts are not general one, because SPI for SD card occupies Dxx and sound out and one input is pushed-out to A/D pins of Arduino (I got to know they has a sequential name for digital I/O, for example D16 for A2...)