I2C-tiny-usb is an open source USB to i2c (master) interface, designed by Till Harbaum. It has driver in the mainline Linux kernel, but there are drivers for Windows as well. The design is based in Atmel AVR ATtiny45 microcontroller, with bit-banged USB implementation. I use this neat device since years for developing/debugging i2c drivers and reading/writing i2c eeproms directly from my laptop, and after using a version built on universal PCB for years, I’ve decided that it deserves a proper PCB and an enclosure and some more features, so I gave it a redesign, and contributed back to the original project. It’s might be a bit overdone, but this was also a practice project for PCB layout and manufacturing. The core of this version’s schematic pretty much follows the original design, but it has a 3.3V regulator and a level shifter, so it can directly work with 3.3V devices, and it has a LED on the i2c clock line:

The operation is straightforward, the USB interface and the microcontroller circuit is the original, the level shifter is the classic Philips/NXP application note solution (see: https://www.nxp.com/docs/en/application-note/AN10441.pdf, and I added a switching FET for the LED on the SCL line, so the i2c line activity is indicated.
The PCB is designed in Kicad, although it’s double-sided, as the SMD parts are 1206, and there is only one via, it’s possible to build it on a DIY PCB as well. I got my PCB fabricated at Aisler, but as you can see on the cover image, I also made a prototype myself.The fabricated, populated board looks like this:

The board is designed to fit into the HM-1551GBK Hammond box, but I’ve also designed a 3D printable enclosure, see in the “enclosure” directory in the git repo and on Thingiverse: https://www.thingiverse.com/thing:4261556. This is how the boards look like in these enclosures:

As a bonus, I also made a simple label with the pinout to stick it on the
enclosure:

The Kicad project available in the original i2c-tiny-usb repo at https://github.com/harbaum/I2C-Tiny-USB/tree/master/pcb/i2c-tiny-usb integrated with this pull request, so feel free to build it as it is, or redesign the layout based on the Kicad project.

This a simple CH340G breakout board design, for using (mainly) on breadboards. There are a dozen of good designs out there, but this one tries to differentiate itself by being breadboard-friendly (meaning it’s possible to plug it into your board without sticking out vertically), provides (fused) power supply directly to the power rails of the breadboard (selectable 5V or 3V3), and it exposes all of the modem control lines.

 I decided to design this when I was starting prototyping an other project, and wished I had something similar. After a quick, unsuccessful query on Tindie (this is something similar, but not designed for breadboard, and not OSHW), the next thing I noticed was that I have Kicad open and I’m laying out this board. It’s nothing groundbreaking, but as you can see on the pictures, it’s directly pluggable into breadboard, and it can also power your breadboard circuit from the USB port, so I’m sure it can come handy sometimes to everyone on Earth. The schematic is pretty much the default design for the CH340G chip, plus there is some ESD protection on the USB line (USBLC6-2SC6), a 3.3V regulator with jumpers to select the output VCC and the UART line voltage and of course some LEDs for power and the TX/RX lines, as blinking LEDs are mandatory for any reasonable gadget.

The PCB is designed with Kicad, it’s double sided, but possible to build it on single-sided board as well. You can find all of the CAD files and documentation over at Gitlab (Github mirror), so here I just show a teaser:

The usage it’s pretty straightforward: just plug it into your breadboard, connect the UART lines to your circuit, select the operating voltage with the jumper, and connect the USB cable to your PC. If you are using any {r,d}ecent Linux distribution, the CH340G driver most likely is installed on your machine. If you are on Windows, you have to click around a bit, see for example the tutorial at Sparkfun