Build a Smart Voltmeter with Arduino & Smartphone – Easy DIY Project

About this project

Demonstration

If you are new to Arduino, you can get started with Arduino Tutorials for newbie.

How it works

We can use Arduino analog input pin to measure voltage. However, the maximum measurable voltage is 5V.

To increase the measurable voltage, we need to use a voltage divisor

InTheory

We has:

V_measure = (R1 + R2) / R2 * V_in

ratio = (R1 + R2) / R2

=> V_measure = ratio * V_in

If we choose R1 = 10 * R2, we have:

V_measure = 11 * V_in

Since the maximum of the allowable V_in is 5V => The maximum of the measurable voltage is 55V

To limit the current run though Arduino's pin, which may cause damage to Arduino, we need to choose resistor value as big as possible.

I choose R1 = 1 Mohm, R2 = 100Kohm

In Practice

There is some problems in practice

• Value of R1 and R2 has errors. The errors cause the error of V_measure
• Power source of of Arduino may be unstable. It makes the V_REF of analog input is unstable. Therefore, it causes the error in calculation of V_in. => it causes error in calculation of V_measure

These problems are solved on the Calibration part

Calibration

1. Measure the real value of (R1 + R2) / R2

This need to be done only one time to get the ratio

Wiring as bellow image:

ratio = (R1 + R2) / R2 = V_measure / V_in = A1_read_value / A0_read_value

ratio = A1_read_value / A0_read_value

Code for this calibration => see ResisterCalibration.ino in code part

2. Instability of power supply

Instability of power supply causes instability in voltage reference of analog pin. It causes the wrong measure in voltage calculation.

To solve it, we need to measure the voltage reference frequently.

How to measure the voltage reference?

Connect 3.3V to pin A1

The voltage reference is measured indirectly based on value in A1:

V_reference= 1023 * 3.3 / A1_value

How to calculate the V_measure:

V_in = mapFloat(A0_value, 0, 1023, 0, V_reference);V_measure = ratio * V_in;

Main wiring:

How to

• Wiring as "resistor calibration wiring" circuit
• Upload ResistorCalibration.ino
• Open Serial Plotter
• Copy the ratio value
• Replace this value in line 5 of Voltmeter.ino
• Re-wiring as "main wiring"
• Upload Voltmeter.ino
• Upload Web User Interface (voltmeter.php and voltmeter_body.jpg) to PHPoC Shield
• Access Web page http://phpoc_shield_ip_address/voltmeter.php to see the voltage.

You can view the voltage on smart phone as follows:

Wiring Tips:

The Best Arduino Starter Kit for Beginner

See The Best Arduino Kit for Beginners

Code

• ResistorCalibration.ino
• Voltmeter.ino
• voltmeter.php

ResistorCalibration.inoArduino

double ratio;

int analogReadAverage(int pin, int read_time)
{
	unsigned long read_value = 0;
	// read the analog in value read_time times:
	for(int i = 0; i < read_time; i++) {
		read_value += analogRead(pin);
		delay(2);
	}

	// Get average
	read_value /= read_time;

	return read_value;
}

double resistorCalibration()
{
	double _ratio;
	double A0_read_value;
	double A1_read_value; 

	A0_read_value = analogReadAverage(A0, 20);
	A1_read_value = analogReadAverage(A1, 20);

	_ratio = A1_read_value / A0_read_value;

	return _ratio;
}

void setup() {
	// initialize serial communications at 9600 bps:
	Serial.begin(9600);
}

void loop() {
	ratio = resistorCalibration();

	Serial.print("ratio:");
	Serial.println(ratio);
	delay(200);
}

Voltmeter.inoArduino

double V_measure;
double V_in; 
double V_reference;

double ratio = 11.59;

double mapFloat(double x, double in_min, double in_max, double out_min, double out_max) {
  return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
}

int analogReadAverage(int pin, int read_time)
{
	unsigned long read_value = 0;

	for(int i = 0; i < read_time; i++) {
		read_value += analogRead(pin);
		delay(2);
	}

	// get average
	read_value /= read_time;

	return read_value;
}


double getVoltageReference()
{
	double A1_value = analogReadAverage(A1, 20); // A1 is connected to 3.3v
	double V_REF = 1023 * 3.3 / A1_value;

	return V_REF;
}

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

void loop() {
  // update voltage reference
  V_reference = getVoltageReference();

  double A0_value = analogReadAverage(A0, 20);

  // map it to the range of the analog out:
  V_in = mapFloat(A0_value, 0, 1023, 0, V_reference);
  V_measure = ratio * V_in;

  // print the results to the Serial Monitor:
  Serial.println(V_measure);

  delay(200);
}

voltmeter.phpPHP

<!DOCTYPE html>
<html>
<head>
<title>PHPoC Shield - VOLTMETER</title>
<meta name="viewport" content="width=device-width, initial-scale=0.7">
<link rel="stylesheet" media="screen" href="https://fontlibrary.org/face/segment7" type="text/css"/>
<style>
html {height: 100%;}
body {width: 100%; height: 100%;; text-align: center;}
.container {width: 100%; min-height: 99%; max-height: 100%;}
canvas {
	background: url(voltmeter_body.jpg);
	background-size:contain;
	font-family: 'Segment7Standard'; 
	font-weight: bold; 
	font-style: italic;
}

</style>
<script>
var COLOR_LCD_TEXT		= "#000000";
var COLOR_LCD_BGR		= "#00FFFF";
var IMG_WIDTH = 583;
var IMG_HEIGHT = 1235;
var METER_WIDTH, METER_HEIGHT;

var ws;
var canvas;
var ctx;
var buffer = "";
var voltage = 0;

function init()
{
	canvas = document.getElementById("graph");
	ctx = canvas.getContext("2d");
	canvas_resize();
	update_view(voltage);

	var ws_host_addr = "<?php echo _SERVER("HTTP_HOST")?>";

	if((navigator.platform.indexOf("Win") != -1) && (ws_host_addr.charAt(0) == "["))
	{
		// network resource identifier to UNC path name conversion
		ws_host_addr = ws_host_addr.replace(/[\[\]]/g, '');
		ws_host_addr = ws_host_addr.replace(/:/g, "-");
		ws_host_addr += ".ipv6-literal.net";
	}

	ws = new WebSocket("ws://" + ws_host_addr + "/serial_monitor", "uint8.phpoc");
	ws.onopen = ws_onopen;
	ws.onclose = ws_onclose;
	ws.onmessage = ws_onmessage;
	ws.binaryType = "arraybuffer";
}

function ws_onopen()
{
	
}
function ws_onclose()
{
	alert("CANNOT connect to Arduino!");
	ws.onopen = null;
	ws.onclose = null;
	ws.onmessage = null;
	ws = null;
}
function ws_onmessage(e_msg)
{
	e_msg = e_msg || window.event; // MessageEvent

	var u8view = new Uint8Array(e_msg.data);
	buffer += String.fromCharCode.apply(null, u8view);
	buffer = buffer.replace(/\r\n/g, "\n");
	buffer = buffer.replace(/\r/g, "\n");

	while(buffer.indexOf("\n") == 0)
		buffer = buffer.substr(1);

	if(buffer.indexOf("\n") <= 0)
		return;

	var pos = buffer.lastIndexOf("\n");
	var str = buffer.substr(0, pos);
	var arr = str.split("\n");
	buffer = buffer.substr(pos + 1);

	if(arr.length)
	{
		voltage = parseFloat(arr[arr.length - 1]);
		update_view(voltage);
	}
}

function update_view(voltage)
{
	ctx.clearRect(0, 0, METER_WIDTH, METER_HEIGHT);

	var top = METER_HEIGHT * 0.125; // 12.5%
	var left = METER_WIDTH * 0.2; // 2%
	var width = METER_WIDTH * 0.6; // 6%
	var height = METER_HEIGHT * 0.12; // 12%

	ctx.fillStyle = COLOR_LCD_BGR;
	ctx.rect(left, top, width, height); 
	ctx.fill();

	ctx.font = "italic 75px Segment7Standard";
	ctx.textBaseline = "middle";
	ctx.textAlign = "end";
	ctx.fillStyle = COLOR_LCD_TEXT;
	ctx.fillText(voltage.toFixed(2), left + width - 50, top + height / 1.8);

	ctx.font = "18px Arial";
	ctx.textBaseline = "middle";
	ctx.textAlign = "center";
	ctx.fillText("VDC", left + width - 25, top + height / 2);
	
	ctx.font = "bold 32px Arial";
	ctx.shadowBlur = 5;
	ctx.shadowColor = "white";
	ctx.fillStyle = "#00FFFF";
	ctx.fillText("ARDUINO", METER_WIDTH / 2, METER_HEIGHT * 0.5);
	ctx.fillText("WEB VOLTMETER", METER_WIDTH / 2, METER_HEIGHT * 0.55);
}

function canvas_resize()
{
	canvas.width = 0; // to avoid wrong screen size
	canvas.height = 0;

	var container = document.getElementById("container");

	var height = container.clientHeight;
	var ratio = height / IMG_HEIGHT;
	var width = ratio * IMG_WIDTH;

	METER_WIDTH = width;
	METER_HEIGHT = height;

	canvas.width = METER_WIDTH;
	canvas.height = METER_HEIGHT;
	update_view(voltage);
}

window.onload = init;

</script>
</head>
<body onresize="canvas_resize()">
	<div class="container" id="container">
		<canvas id="graph"></canvas>
	</div>
</body>
</html>

Schematics