Esp32 and JsonArduino library reading analog Values in a Array

Question

I'm trying to understand how to use Json with the ESP32 or Arduino. In the following code example the idea is to read the values from a potentiometer and display it on the Serial Monitor. I was expecting to see something like this when I am turning the potentiometer.

"Reading: 0,54,140,175,480,782"

"Reading: 600,523,320,175,48,2"

But I get this

"Reading: 54,54,54,54,54,54"

"Reading: 140,140,140,140,140,140"

#include <ArduinoJson.h>


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

void loop() {

  StaticJsonDocument<500> doc;

  JsonArray analogValues = doc.createNestedArray("analog");
  for (int pin = 0; pin < 6; pin++) {
  
    int value = analogRead(35);

    analogValues.add(value);
  }

 
  Serial.print(F("Reading: "));
  serializeJson(doc, Serial);
  Serial.println();

}

romkey · Accepted Answer · 2022-05-07 21:59:32Z

Your code will take 7 samples from the input pin very quickly - faster than it's likely you'll be able to change the potentiometer. You need to add a delay between the samples to give the potentiometer time to change. So:

  for (int pin = 0; pin < 6; pin++) {
  
    int value = analogRead(35);

    analogValues.add(value);
    delay(200);
  }

would wait 2 tenths of a second between taking samples.

To do some very basic debugging on this you could also confirm that the issues is the samples themselves and not the way you're handling JSON by outputting the sample values as you take them. In your original code this would be:

  for (int pin = 0; pin < 6; pin++) {
  
    int value = analogRead(35);

    Serial.println(value);
    analogValues.add(value);
  }

It's also possible that the act of outputting the samples might slow things down enough that you might start to see variation.

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