What is it?

In microcontroller and Arduino boards, the 6-pin "FTDI" serial connector is commonly used to connect such boards to your computer through an USB-to-serial converter. One caveat is that the voltage on both sides of the connection usually needs to match: Connecting a 5V USB-to-serial adapter to a 3.3V Arduino will overload its pins and potentially break them. This board can be inserted between an USB-to-serial adapter and microcontroller board with mismatching voltages, and it will convert the voltage levels on the RX, TX and DTR lines automatically.

This converter is particularly useful when connecting to projects with non-standard voltages (e.g. an Arduino running off two AA batteries), since it will be pretty much impossible to find a USB-to-serial adapter whose vomltage is near enough.

This listing is for a bare PCB only. See below for the components you will additionally need. If you are interested in a kit (board plus needed components) or fully assembled board, let me know and I'll consider adding an option for that.

Note that FTDI refers to the manufacturer of the chip that was originally often used, by now a lot of these USB-to-serial boards use chips of other manufacturers as well.

Why did I build it?

I was working with a battery-powered microcontroller board, whose voltage could be anywhere between 3.2V (battery full) and 2V (batteries as good as empty). I first created the design on a bit of protoboard, using slightly different components, and then made a cleaner PCB design in KiCad.

I ordered some boards at Seeedstudio, but since the minimum order quantity for the PCB was 10 pieces, I have a few spare boards that I'm happy to sell.

Features

• Bidirectional, so works high-to-low as well as low-to-high voltage.
• Room for a regulator, so the low side can be powered from the high side. The regulator output voltage is fixed, but different version are available (2.5V, 2.8V, 3.0V and 3.3V should all work).
• Regulator can be enabled or disabled using a jumper
• When the regulator is not used, both sides should be separately powered, but their voltages can be anything (as long as the high side has a higher voltage than the low side).
• Connects the DTR pin, which is typically used for auto-reset during the upload process for Arduino boards, so uploading sketches through the converter works.
• The DTR pin can be disconnected using jumpers (jumpers at both sides to completely disconnect the pullup resistors too).

Needed components

The components to completely build this design, are:

• 6x 10k resistors (SMD 0805 package, though 1206 might also fit)
• 3x 2N7000 transistors (Through-hole TO-92 package)
• 1x MCP1700 regulator (SMD SOT89-3 package, exact type depends on desired voltage)
• 2x 1uF capacitor (SMD 0805 package, though 1206 might also fit)
• 2x 6-pin angled pin header
• 3x 2-pin straight pin header
• 3x jumper

Depending on your needs, some parts can be left out:

• If you do not need to connect DTR, you can leave out one transistor, two resistors, two straight pin headers and two jumpers.
• If you do not need a regulator, you can leave it off as well as the two capacitors.
• If you always need DTR connected or the regulator enabled, you can solder in a wire instead of a pin header and a jumper.

Specs

• Dimensions: 43.2mm x 17.8mm x 1.6mm
• Weight: <5g
• Design license: CERN Open Hardware License
• Design files:
• Design files & schematic: v1 board release on github
• Further development: On github