Voltmeter can measure voltage up to 50VDC. It automatically calibrates when voltage of power supply is unstable.

Hardware components
 Arduino UNO & Genuino UNO
× 1
 PHPoC WiFi Shield for Arduino
× 1
 Resistor 100k ohm
× 1
 Resistor 1M ohm
× 1
 Jumper wires (generic)
× 1

Demonstration

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

In Theory

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 M ohm, R2 = 100K ohm

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:
resistor calibration wiring

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

voltage reference calibration

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
• Open Serial Plotter
• Copy the ratio value
• Replace this value in line 5 of Voltmeter.ino
• Re-wiring as "main wiring"
• 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:

Code

1. ResistorCalibration.ino
Code:
```double ratio;

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

// get average

}

double resistorCalibration()
{
double _ratio;

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);
}```

2. Voltmeter.ino
Code:
```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;
}

{
unsigned long read_value = 0;

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

// get average

}

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);
}```

3. voltmeter.php
PHP Code:
``` <!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> ```

4. background_body.jpg

Wiring Tips: