Getting 5V from the RAW pin on an Arduino Pro Mini 3.3V when plugged into USB

Using Arduino Project Guidance
1

I have been planning to build an air quality measuring device using the Honeywell HPMA115S0 and an Arduino.

There are a few tutorials out there on how to do it, such as this one Arduino: measuring PM2.5 and PM10 with Honeywell HPMA115S0 - Electronza

The thing that makes things slightly complicated is that the Honeywell HPMA115S0 needs a 5V power supply, but its UART interface only accepts 3.3 volts.

So the interface cannot be connected directly to an Arduino UNO, because it has a 3.3V logic level interface

The author of the above tutorial solves this by using an Arduino Due, which has a 3.3V logic level interface.
But that's a fairly expensive solution, so I want to see if there is a cheaper way to do it.

My idea was to use an Arduino Pro Mini 3.3V with an integrated USB connector.

As far as I can figure out, the RAW pin on the Arduino supplies 5V when the board is plugged into USB, since there seems to be a direct connection between the 5V on the USB connector and the RAW pin. At least, that's what I gather from this post: RAW power and USB at the same time? - SparkFun Electronics

I suggested the following solution to the author of the above tutorial:

I use an Arduino Pro Mini 3.3V version,
Pin 2 (Vcc) goes to the RAW pin on the Arduino board (which is at 5V, since the Arduino is powered via USB).
Pin 6 (UART TX) goes to the RX pin of the Arduino board. This pin uses 3.3V logic level (since this is the Arduino Pro Mini 3.3V version).
Pin 7 (UART RX) goes to the TX pin of the Arduino board.
PIN 8 (GND) goes to GND pin of the Arduino Board.

The author replied that this won't work, but I don't see why.
Can anybody help me verify his conclusion?

2

What Pro Mini has USB? The whole point of the Pro Mini is that it doesn't have a USB connector or any power-wasting electronics related to USB. Maybe you have a Micro or Pro Micro?

The Pro Micro Schematic says that the 3.3V version will have 4.3V available on the Raw pin when powered from USB. It's not 5V because of the diode there. You could bridge over the diode with a blob of solder if you really need that last 0.7V - there doesn't seem to be much downside to that.

However a HIGH signal on a 3.3V device is also perceived as HIGH by a 5V device. You can plug the UART TX into a UNO's RX pin with no problem. If you actually need to transmit data to the sensor then a voltage divider on the UNO TX pin can drop the voltage to 3.3V.

3

sikjar:
The author replied that this won't work, but I don't see why.

Its simple; the Pro Mini has no USB connection, as you will see when you check out the schematic.

The RAW pin does not output anything, its only connected to the input of the 3.3V regulator.

Again this is clear if you check the schematic.

4

Thank you for the replies, it should indeed have been an Arduino Pro Micro 3.3V.
Will give it a try and ley you know if it works.

5

I finally made it work with an Arduino UNO and a level translator. Wasn’t as difficult as I imagined…