Introduction: ArduinoUSB3.3

About: Lazy Old Geek


So this Lazy Old Geek (L.O.G.) has been working on Arduinos for many years. As most people do, I talk to my Arduinos through a USB port on my computer. The ‘standard’ Arduino like the UNO has a USB adapter built in but I make a lot of my own Arduino boards and have used Arduino Pro Minis so I need a USB to serial adapter that works with Arduinos and I wrote an Instructable:

This is using PL2303 ICs.

Now I’ve also been converting/making my Arduinos to 3.3Vdc versions:

Problem: So I ordered a batch of 10 PL2303s and got them in. The problem I found is that these are set for 5Vdc logic, e.g., Tx is 0-5Vdc. I went back and checked some of my other PL2303s. Some are 3.3Vdc and some are 5Vdc. Now I’ve actually never had any problem with Arduino(Atmega328s) working with either voltage level but some of the newer devices like ESP8266s may not work with 5Vdc.

Solution: So I decided to modify my PL2303s so that they will work at 3.3Vdc levels.

As explained in the first Instructable:

a 0.1uFd capacitor needs to be connected to pin 2 of the PL2303 and to a male header pin. This is the DTR signal. Now to make the logical level 3.3Vdc instead of 5Vdc, pin 4 of PL2303 needs to be connected to 3.3Vdc instead of 5Vdc.


WARNING: This is very hard to do. The PL2303 is SMD and the pins are very small. I had to use a magnifying glass and it took me a long time.


PL2303 pin 4 determines the logic level of the TX pin.

Now on the modules I got, pin 4 is connected to 5Vdc underneath the PL2303 IC. So what I did was bent pin 4 up using a soldering iron and an Xacto knife so that it wasn’t touching the pad. By the way, I did this with pin 2 also as it was a little easier to solder the capacitor that way.

Then I soldered a wire to the floating pin 4 and soldered the other end to 3.3Vdc. On these modules, the bottom male header pin is 3.3Vdc and there is a capacitor right next to the pin connected to 3.3Vdc so I soldered the wire there.

My side view picture isn’t very clear but you can sort of tell that pins 2 and 4 are not soldered to the PCB.

The next picture shows that I hot glued every in that area to help keep anything from shorting out or breaking loose. And to anchor the added DTR pin.

Step 1: PL2303 to My 3.3Vdc Pro Mini

PL2303 to my 3.3Vdc Pro Mini

PROBLEM: Now the PL2303 modules have 3.3Vdc output but the problem is that the output comes from the PL2303 IC and probably only has about 50mA of current. This is probably good enough to program an Arduino but not much else.

In this Instructable:

I modified some 5V Pro Minis to work at 3.3Vdc. They also have a 3.3Vdc regulator that is supposed to supply up to 250mA of current. The problem is that the source for the regulator is the ‘RAW’ pin which is not on the serial header connection.

So I made a special cable that connects the modified PL2303 modules to my 3.3Vdc Pro Mini.

NOTE: You might notice that I used red and gray nail polish to mark the ends so that it is less likely that I hook up the cable wrong.

CONCLUSION: Well, it all works but now I have way too many different cables and adapters. I’m labeling them to help me keep track.