Ammeter Project

Measuring Current.

PARTS USED.

Arduino Pro Mini. I chose the Pro over the Nano because I do not expect to be re-programming it and the USB interface could not be justified. I have positioned it so that I can access the serial input pins if required.
ACS712 current sensor module.  Any of the available variations of maximum rated amperage will do, but note that the resolution is less at higher rated amps. The code provided assumes the 5A version.
128x64 OLED display with I2C interface.
On/Off button. Salvaged from an old PC.
Momentary push button. Salvaged from an old PC.
Hookup wire and a scrap of perfboard.
9v Battery with clip and leads.
3D printed case with 3 3mm screws and knurled inserts.

DISPLAY.

THE CIRCUIT

THE PROTOTYPE

Note that a prototyping board was not used for this project because the components sit at different heights relative to the top surface, so it would have been difficult to get everything at the correct level if they were soldered to a single board.

The cover for the OLED was sourced from Thingiverse and scaled to suit.  There are many different sizes of OLED, so some experimenting will probably be required to get just the right size. When adjusted correctly the frame clamps the mounting surfaces between the front and rear sections, without imposing any pressure against either the OLED surface or the module substrate.

THE CASE

With the layout confirmed I designed a case to suit.  Only the ACS module and the perfboard strip have mounting holes, so everything else sits on an offset on the bottom of the case, and is positioned by dividing pieces or by the cutout in the lid.

There is a cutout for the Pro programming pins connections at the back, with a cover plate that can be pressed into place to conceal them.

ASSEMBLED

COMPLETED

CODE

The code for the ammeter is not complex, but it does include a calculation to allow for calibration. This calculation occurs at startup, so there is an assumption that no current is attached the the ACS terminals when the Arduino initialises.

The calibration code also executes whenever the user presses the calibration button.  This should only be done when there is no load across the ACS terminals. This is not really required, as it is almost as easy to turn the unit off and turn it on again, but it is available as a quick check that the battery voltage has not changed significantly.

The code uses the Ascii and AsciiWire versions of the SSD1306 OLED libraries.  These libraries provide all the functionality that the application needs with much less overhead than the standard libraries. The standard libraries could be substituted with a few simple and obvious changes to the code (although some juggling might be required to get the display laid out just right).

Confirm that the I2C address for your OLED is correct.  Adjust the number of samples if required (above about 100 it makes very little difference).

#include <Wire.h>
#include <SSD1306Ascii.h>
#include <SSD1306AsciiWire.h>

// OLED I2C address
#define I2C_ADDRESS 0x3C
SSD1306AsciiWire display;

boolean startup = true; // Flag to indicate auto calibrate
const int acsIn = A0;   // Analog input for ACS reading
const int btnIn = 9;    // Digital input for button press (active low).

void setup() {
  Wire.begin();
  Wire.setClock(400000L); // Adjust to suit OLED, if required.
  
  pinMode(btnIn, INPUT_PULLUP);
  pinMode(acsIn, INPUT);
  //1 Serial.begin(9600); //Start Serial Monitor to display current read value on Serial monitor
  display.begin(&Adafruit128x64, I2C_ADDRESS);

  display.setFont(fixed_bold10x15);
  display.clear();
  display.println();
  display.setLetterSpacing(2);
  display.println(F("DIGITAL"));
  display.print(F("   AMMETER"));
  display.setFont(Adafruit5x7);
  delay(1000); // Pause for 2 seconds
}

float cNominal = 1024.0;  // ADC count max
float cCentre = 512.0;    // ADC count centre (zero amps)
float vpc = 0;            // Volts per count

void loop() {

  unsigned int x = 0;
  float vInput = 0.0, nSamples = 0.0, vAverage = 0.0, vAdjusted = 0.0, aCalculated; 

  for (int x = 0; x < 150; x++) {   //Get 150 samples.
    vInput = analogRead(acsIn);     //Read current sensor values.
    nSamples = nSamples + vInput;   //Accumulate sample values.
    delay (5);                      // Required ADC settling time.
  }
  vAverage = nSamples / 150.0;      // Calculate average value.

  //vpc is the assumed supply Vcc.  The ADC value is referenced to this voltage.
  //You must change the offset according to the input voltage
  //0.185 (185mV) is output voltage change for 1A current ACS712-05 ( 5 Amp).
  //0.100 (100mV) is output voltage change for 1A current ACS712-20 (20 Amp).
  //0.066 (66mV) is output voltage change for 1A current ACS712-30 (30 Amp).

  vpc = 5.00 / cNominal;    // millivolts per ADC count. Adjust if Vcc is not 5v.
  vAdjusted = (vAverage - cCentre) * +vpc; // Volts = Count * Volts per count.
  aCalculated = vAdjusted / 0.185;         // Amps = Volts / Volts per Amp.   
  
  // Re-calculate zero amps reading and max count at startup and at button press. 
  if ((digitalRead(btnIn) == LOW) || startup) {
    cCentre = vAverage;
    cNominal = cCentre * 2.0;
    startup = false;
  }

  display.clear();
  display.setFont(System5x7);
  display.println(String(cNominal, 3) +  ' ' + String(vAverage, 2));
  //display.println();
  display.setRow(2);
  display.setFont(ZevvPeep8x16);
  display.set2X();  // Double-size font
  if (aCalculated >= 0.0) {
    display.print("  " + String(aCalculated, 3));
  }
  else {
    display.print(" " + String(aCalculated, 3));
  }
  display.set1X();  // Standard-size font.
  display.setCursor(98,6);
  display.println("Amps");

  delay(50);
}

This site was last updated 27th September 2022.