Please help me understand this code (lines with double stars) for digital capacitance meter

Projects Programming
1 
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x3f,20,4);  

const int OUT_PIN = A2;
const int IN_PIN = A0;
const float IN_STRAY_CAP_TO_GND = 24.48;
const float IN_CAP_TO_GND  = IN_STRAY_CAP_TO_GND;
const float R_PULLUP = 34.8;  
const int MAX_ADC_VALUE = 1023;

void setup()
{
  pinMode(OUT_PIN, OUTPUT);
  pinMode(IN_PIN, OUTPUT);
  lcd.init();
  lcd.backlight();
  }

void loop()
{
    pinMode(IN_PIN, INPUT);
    digitalWrite(OUT_PIN, HIGH);
    int val = analogRead(IN_PIN);
    digitalWrite(OUT_PIN, LOW);

    **if (val < 1000)**
**    {**
**      pinMode(IN_PIN, OUTPUT);**

**      float capacitance = (float)val * IN_CAP_TO_GND / (float)(MAX_ADC_VALUE - **

**val);**

      lcd.setCursor(0,0);
      lcd.print("Scale:   1pF-1nF");
      lcd.setCursor(0,1);
      lcd.print(capacitance,3);
      lcd.setCursor(14,1);
      lcd.print("pF");
    }
    
    **else**
**    {**
**      pinMode(IN_PIN, OUTPUT);**
**      delay(1);**
**      pinMode(OUT_PIN, INPUT_PULLUP);**
**      unsigned long u1 = micros();**
**      unsigned long t;**
**      int digVal;**

**      do**
**      {**
**        digVal = digitalRead(OUT_PIN);**
**        unsigned long u2 = micros();**
**        t = u2 > u1 ? u2 - u1 : u1 - u2;**
**      } **
**      **
**      while ((digVal < 1) && (t < 400000L));**

**      pinMode(OUT_PIN, INPUT);  **
**      val = analogRead(OUT_PIN);**
**      digitalWrite(IN_PIN, HIGH);**
**      int dischargeTime = (int)(t / 1000L) * 5;**
**      delay(dischargeTime);   **
**      pinMode(OUT_PIN, OUTPUT);  **
**      digitalWrite(OUT_PIN, LOW);**
**      digitalWrite(IN_PIN, LOW);**

**      float capacitance = -(float)t / R_PULLUP / log(1.0 - (float)val / (float)MAX_ADC_VALUE);**

      lcd.setCursor(0,0);
      lcd.print("Scale:   1pF-1nF");
      
      if (capacitance > 1000.0)
      {
        lcd.setCursor(0,1);
        lcd.print(capacitance / 1000.0, 2);
        lcd.setCursor(14,1);
        lcd.print("uF ");
        
      }
        
      else
      {
        lcd.setCursor(0,1);
        lcd.print(capacitance);
        lcd.setCursor(14,1);
        lcd.print("nF");
        
      }
  }
    while (micros() % 1000 != 0);
}
2

This is quite basic programming so it looks like you are just beginning with Arduino.

Just to start it off:

The first line is an 'if' statement. If the variable 'val' has a value of less than 1000 the code between the braces '{' '}' is executed.

The pinMode function can set a pin as INPUT, OUTPUT etc. In this case. the rather oddly named IN_PIN is defined as an output pin and can be set in the program (sketch) by the digitalWrite() function.

3

The question was about the use of "double stars".

4

what about these lines?

else
        {
          pinMode(IN_PIN, OUTPUT);
          delay(1);
          pinMode(OUT_PIN, INPUT_PULLUP);
          unsigned long u1 = micros();
          unsigned long t;
          int digVal;
    
          do
          {
            digVal = digitalRead(OUT_PIN);
            unsigned long u2 = micros();
            t = u2 > u1 ? u2 - u1 : u1 - u2;
          } 
          
          while ((digVal < 1) && (t < 400000L));
    
          pinMode(OUT_PIN, INPUT);  
          val = analogRead(OUT_PIN);
          digitalWrite(IN_PIN, HIGH);
          int dischargeTime = (int)(t / 1000L) * 5;
          delay(dischargeTime);   
          pinMode(OUT_PIN, OUTPUT);  
          digitalWrite(OUT_PIN, LOW);
          digitalWrite(IN_PIN, LOW);
    
          float capacitance = -(float)t / R_PULLUP / log(1.0 - (float)val / (float)MAX_ADC_VALUE);
5

No, sir. It's for the line with double star. I was trying to bold the lines but that didn't happen instead double star appeared.

6

Its a bit impolite to post code without saying where it came from. (Ideally with a link, like this)

https://www.circuitbasics.com/how-to-make-an-arduino-capacitance-meter/

Perhaps you are trying to understand HOW the capacitance is meassured and calculated, rather than what the code instructions (eg if(val<1000) do?

If so I'd suggest you refer back to the excellent and detailed information provided.

7

Sorry, I did not know about that. Thank you for letting me know.
I was just trying to understand how the mentioned part of the code works.

8

It looks like you really need to start with some elementary Arduino tutorials to get a basis for understanding that program.

Some of the statements and functions are pure C++ and you will find these by a Google type search. For example, the if statement as in if (val < 1000)

Other statements or functions are specific to "Arduino", for example digitalWrite()`` as in digitalWrite(IN_PIN, HIGH);`. These are in the Arduino reference manual:

1 Like
9

Measuring how fast a capacitor charges or discharges through a known resistor will allow you to calculate the capacitance. It takes longer to charge/discharge a larger capavitor.

10

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