Why does analogRead(A0) deliver different Values when called in a library file vs calling it in the sketch directly

Question

I wanted to write a library representing a sensor to later on have an array of sensors. I generated a file MoistureSensor.h:

#ifndef MoistureSensor_h
#define MoistureSensor_h
#include <Arduino.h>
class MoistureSensor {
 private:
 uint8_t sensPin;
 int sensValue;
 int sensId;
 public:
 MoistureSensor(uint8_t pin,int id);
 int getValue();
 int getId();
};
#endif

and MoistureSensor.cpp:

 // MoistureSensor.cpp
#include "MoistureSensor.h"
MoistureSensor::MoistureSensor(uint8_t pin, int id) {
 sensPin = pin;
 sensId = id;
}
int MoistureSensor::getValue() {
 sensValue = analogRead(sensPin);
 return sensValue;
}
int MoistureSensor::getId() {
 return sensId;
}

this is the main sketch running on a Arduino Mega:

#include "MoistureSensor.h"
MoistureSensor sens1("A0", 1);
void setup() {
 Serial.begin(9600);
}
void loop() {
 Serial.println("Value from analogread:");
 Serial.println(analogRead(A1));
 Serial.println("Value from library:");
 Serial.println(sens1.getValue());
 Serial.println("End");
 delay(1000);
 
}

So when I run this the serial monitor shows:

Value from analogread:
600
Value from library:
496
End

Why doesn't this print the same value? Does the .cpp file interpret the data from analogRead() different than the sketch?

Also both values where moving up and down depending on the response of the sensor.

Thanks in advance.

• Juraj: it should be MoistureSensor sens1(A0, 1);, not MoistureSensor sens1("A0", 1);. we don't write analogRead("A0")
• Gil: In simple terms it has to use more memory to call the library. When you read it in the sketch it is one line. If in a library function you have to call the library, this causes the system to save its location in RAM then it executes the instruction. You are doing many more instructions with the library.

Answer 1

Trying to compile your sketch, I get the following compiler warning:

warning: invalid conversion from 'const char*' to 'uint8_t {aka unsigned char}'
 MoistureSensor sens1("A0", 1);
 ^
note: initializing argument 1 of 'MoistureSensor::MoistureSensor(uint8_t, int)'

This means that, whereas the MoistureSensor() constructor expected its first argument to be of type uint8_t, you provided a const char* instead. Indeed, a string literal like "A0" decays to a const char* pointer when you pass it as a function argument.

The fix is to pass the actual pin number to your constructor:

MoistureSensor sens1(A0, 1);

But then, there is a second issue. In your test sketch, you have your MoistureSensor object read A0, whereas the direct analogRead() reads pin A1. If you want to get the same value, you should read the same pin.

Side note: the change from uint8_t to byte has absolutely nothing to do with your issue. These two types are synonyms.

Answer 2

I found a solution:

changing the parameter of the constructor MoistureSensor::MoistureSensor(uint8_t pin, int id, int rawMax, int rawMin)`` from uint8_t tobytesolved this issue for me. I also changed the parameterization to MoistureSensor sens1(1, 1);`

• Juraj: there is no difference between byte and uint8_. it just a different name for the same thing.