Arduino Nano 33 IOT A4 & A5 not returning expected values

Hardware Nano Family Nano 33 IoT
1

I have a capacitive moisture sensor running on A5 and Photoresistor running on A4 but neither are acting as expected.

A5 just gives a constant reading of ~1023 and A4 seems to have a minimum value of ~600.

I have connected the same sensors to A0-A3 and gotten expected results, just A4 and A5 seem to be an issue, I have also tested on multiple Arduino Nano boards and got the same result.

My assumption is that is has something to do with the SDA SCI on those pins.

I don't have easy ability to draw out the schematic so here is my best sketch.

Code snippet:

#define CMS3PIN A4
#define LRD3PIN A5

int m3;
int l3;

void setup() {
  Serial.begin(9600);
}

void loop() {
  delay(2000);
  m3 = analogRead(CMS3PIN);
  l3 = analogRead(LRD3PIN);
  Serial.print(" | Light: ");
  Serial.print(l3);
  Serial.print(" | Moisture: ");
  Serial.println(m3);
}

IMG_9744.jpg
IMG_9744.jpg3024×4032 1.44 MB

2

Please do not use the 5V pin to source your sensors. The Arduino Nano 33 IoT can only handle 3.3V on its input, hence the name Nano 33. Use the +3V3 pin instead.

I suspect the 5V is not enabled by default. So, you might be lucky.

Edit: There are two pads under the Arduino board near the 5V pin marked VUSB. This would allow you to get 5V from the USB connector to the 5V pin. Please only short them if you are sure the voltage delivered to your I/O does not exceed 3.3V.

3

You should use different pin other than A4 and A5 on the Nano 33. Both pins have a hardwired pullup resistor of 4700 ohm which will influence you reading.

4