Measure 4V from 3.3V pin on Arduino mega 2560

Projects General Guidance
1

Hello!

I have a couple of Plantower PMS5003 and HPMA115C0-004 particle sensors. Each sensor is connected to its own Arduino Mega 2560 rev3 board as well as an SD card reader and RTC (D3132). From the data sheet I can see the TX/RX pins use 3.3V logic while the sensors need 5V. I purchased some bi directional level shifters to adjust the voltage for the RX/TX pins. However, only one of the Arduino boards measures the expected 3.3V output; the rest measure 4V. I have checked my wires and they look identical. Could my boards be broken? I have attached a drawing of my setup(I know it is not a good drawing sorry).
It's worth mentioning that the sensors work with the Arduino IDE, but I'm concerned about potentially damaging them or the boards if there's an issue with too much voltage.

færdigsetup
færdigsetup2040×1530 169 KB

The rest of the SD card reader pins and D3132 pins are plugged directly in the arduino, but I have not drawn them on.

2

I am confused, Mega has 5V I/O, not 3.3V. I don't think you need any level conversion. What datasheet were you looking at?

3

How are you powering the 2560, USB or Vin?

4

I have tried via the USB connector and the barrel jack. Same result

5

If you trust your voltmeter/DMM, I would say that those boards are damaged.

6

For the HPMA: https://prod-edam.honeywell.com/content/dam/honeywell-edam/sps/siot/en-us/products/sensors/particulate-matter-sensors-hpm-series/documents/sps-siot-particulate-hpm-series-datasheet-32322550-ciid-165855.pdf
For the PMS5003: PMS5003 Series Manual Datasheet by Adafruit Industries LLC | Digi-Key Electronics
I can see many people dont use level conversion for the PMS5003 but understand the datasheet as I should do it

7

Without trying to decipher your drawing, the first thing i’d look at is ensure the 0V/ground lines are common across all your devices.

8

That was what I suspected. My voltmeter measures 4.9-5V on the 5V pin so I trust it. If I power it anyway will I destroy my sensor?

9

Wait
You have the level translators backwards.

10

So I should reverste is so RX and Tx pin from the sensor go into LV3 and LV4 instead?

11

No, the HV side connects to the Arduino, the LV side connects to the sensor.

12

If the sensor requires 3.3V, then you have the level translator is reversed as @jim-p says.

13

Sorry what do you mean? I am new to arduino

14

Both sensors requires 5V. The RX/TX pins uses 3.3V logic.

15

Since you had the level translators backwards, you may have damaged them, the arduino or the sensors.
I suggest you disconnect everything and start from scratch.

16

Okay just to be sure I understand.
RX(sensor) ----> LV4 HV4 <----- TX (arduino)
TX(sensor) ----> LV3 HV3 <----- RX (arduino)
GND (arduino) --> GND GND <-- GND(arduino)
3.3V (arduino) ---> LV HV <---- 5V (arduino)

17

Correct but also make sure the sensor ground is connected to the Adruino GND

Before you go any further, I would dicconnect everything and again check the 3.3V on the 2560 output. If you still get 4V, I would not use those boards.

18

Okay so this will make the RX/TX pin run on 3.3V logic and the sensor on 5V?
And thank you for quick response!

19

Yes

20

Thank you so much!