Build AD8495 K-Type Thermocouple ESP32 WebServer

Overview

In this tutorial, we will be interfacing AD8495 Analog Output K-Type Thermocouple Amplifier with ESP32 and displaying the temperature readings on an AJAX-based dynamic web server. Earlier, we used AD8495 with Arduino and Raspberry Pi Pico.

The AD8495 is an analog output thermocouple amplifier. The AD8495 thermocouple amplifiers provide a simple, low-cost solution for measuring thermocouple temperatures. This guide will help you connect the AD8495 K-Type Thermocouple Amplifier with ESP32. Using the datasheet and the mathematical equation for temperature measurement, we will develop code to read the temperature values and display them dynamically on a web server using AJAX for real-time updates.

Bill of Materials

We need the following components for making Digital Thermometer using AD8495 Thermocouple Amplifier & ESP32.

S.N.	Components Name	Quantity	Purchase Links
1	ESP32 Board	1	Amazon \| AliExpress
2	AD8495 K-Type Thermocouple Amplifier	1	Amazon \| AliExpress
3	Connecting Wires	5	Amazon \| AliExpress
4	Breadboard	1	Amazon \| AliExpress

AD8495 K-Type Thermocouple Amplifier

The AD8495 K-Type Thermocouple Amplifier, developed by Analog Devices, is particularly suited for applications requiring high precision. As part of the AD8494/AD8495/AD8496/AD8497 family, it integrates precision instrumentation amplifiers with a cold junction compensator to directly produce a 5 mV/°C output from a K-type thermocouple signal.

This device simplifies the process of temperature measurement by combining an ice point reference with a precalibrated amplifier. This facilitates its use either as a standalone thermometer or as part of a switched output setpoint controller. Specifically calibrated for K-type (chromel-alumel) thermocouples through laser wafer trimming, the AD8495 ensures high accuracy across its optimized ambient temperature range. It supports a wide variety of supply voltages, from 3 V for direct interfacing with low-supply ADCs to up to 36 V for industrial systems requiring broad common-mode input ranges.

The temperature readings can be easily obtained by measuring the output voltage and applying the equation:

Temperature = (Vout – 1.25) / 0.005 V

For instance, a voltage reading of 1.5VDC translates to a temperature of 50°C. The device’s design minimizes self-heating with a total supply current of 180 μA when unloaded, although it is capable of delivering over ±5 mA to a load. This amplifier comes with a fully assembled PCB, which includes the AD8495 and a TLVH431 1.25V precision voltage reference, a 2-pin terminal block for easy connection to a thermocouple, and a pin header for breadboard integration. Refer to the AD8495 Datasheet for more information.

AD8495 Features & Specifications

Works with any K-type thermocouple
Will not work with any other kind of thermocouple other than K type.
Easy to use analog output.
Temp range with 5V power: -250°C to +750°C output(0 to 5VDC)
Temp range with 3.3V power: -250°C to +410°C output(0 to 3.3VDC)
Sensing Accuracy Range: ± 1°C around room temperature, ± 2°C for −25°C to +400°C
Sensing Temperature Max: 400°C
Sensing Temperature Min: -25°C
Supply Voltage: 3-18V DC

AD8495 Breakout Board Pinout

Power & Output Pins

V+ – This is the power pin. This board works with 3.3V and 5V power.
GND – Common ground pins
OUT – This is the output voltage pin. Read using an analog input.

Thermocouple Terminal

Red- – Connect the red or – wire on your thermocouple to this side of the screw terminal.
Ylw+ – Connect the yellow or + wire on your thermocouple to this side of the screw terminal.

Interfacing AD8495 K-Type Thermocouple Amplifier with ESP32

Let us interface the AD8495 K-Type Thermocouple Amplifier with ESP32 WiF-Fi Microcontroller. Here is the connection diagram.

Connect the VCC of the AD8495 to the 3.3V of ESP32.
Connect the GND of the AD8495 to a GND pin on the ESP32.
Finally, connect the Vout (output) of the AD8495 to GPIO33 of ESP32.

You may use a breadboard to place the AD8495 Module. Then, install the thermocouple on the thermocouple terminal of AD8495.

Source Code/Program for Reading Temperature

This code reads a raw ADC value from a thermocouple connected to pin 33 on an ESP32 Then it converts the raw ADC reading to a voltage, and then calculates the temperature from that voltage. The temperature is then printed to the Serial Monitor every 500 milliseconds.

#include "esp_adc_cal.h"

#define THERMOCOUPLE_PIN 33

#define ANALOG_REFERENCE_VOLTAGE 3.3 // ESP32 typically uses 3.3V for AREF

#define ADC_BIT_RESOLUTION 12 // ESP32 uses 12-bit ADC resolution by default

float calculatedVoltage, measuredTemperature;

// Function to convert a raw ADC reading to voltage. ESP32's ADC readings can be 12-bit by default,

// and we're using 3.3V as the reference voltage.

float convertRawAdcToVoltage(uint32_t rawAdcValue) {

// Characterize the ADC

esp_adc_cal_characteristics_t adcCharacteristics;

esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 1100, &adcCharacteristics);

// Convert the raw ADC value to calibrated voltage in millivolts

uint32_t voltageInMilliVolts = esp_adc_cal_raw_to_voltage(rawAdcValue, &adcCharacteristics);

// Convert millivolts to volts

return voltageInMilliVolts / 1000.0;

}

float calculateTemperatureFromVoltage(float voltage) {

return (voltage - 1.25) / 0.005;

}

void setup() {

Serial.begin(115200);

// Note: Additional configuration for the ADC may be necessary depending on your specific setup.

}

void loop() {

// Read the raw ADC value

uint32_t rawAdcReading = analogRead(THERMOCOUPLE_PIN);

// Convert the raw ADC reading to voltage

calculatedVoltage = convertRawAdcToVoltage(rawAdcReading);

// Calculate the temperature in Celsius from the voltage

measuredTemperature = calculateTemperatureFromVoltage(calculatedVoltage);

// Print the measured temperature

Serial.print("Temperature = ");

Serial.print(measuredTemperature);

Serial.println(" °C");

// Delay for a short period before reading the temperature again

delay(500);

}

Upload the code to the ESP32 Wi-Fi Module. Once the uploading is done, open the Serial Monitor.

Source Code/Program for ESP32 AD8495 WebServer

The following code connects an ESP32 to a Wi-Fi network, reads the temperature from a thermocouple connected to pin 33, and serves a web page to display the temperature.

It reads the raw ADC value, converts it to voltage, calculates the temperature, and provides a JSON endpoint to get the temperature in both Celsius and Fahrenheit. The temperature data is updated and served to clients via a web server running on the ESP32.

html.h

First save this html file with the name html.h in the folder where main sketch is located.

const char html_page[] PROGMEM = R"rawString(

<!DOCTYPE html>

<head>

<title>Temperature Monitor</title>

<style>

body {

margin: 0;

padding: 20px;

font-family: 'Roboto', sans-serif;

background: #FFFFFF; /* Background color set to white */

color: #333;

display: flex;

justify-content: center;

align-items: center;

height: 100vh;

overflow: hidden;

}

.container {

text-align: center;

padding: 2em;

background: rgba(255, 255, 255, 0.95); /* Slightly transparent white for depth */

border-radius: 15px;

box-shadow: 0 4px 6px rgba(0, 0, 0, 0.1); /* Soft shadow for a subtle depth effect */

}

h1 {

color: #333; /* Dark grey text for contrast */

margin-bottom: 1em;

}

.temperature {

display: inline-block;

background: rgba(0, 0, 0, 0.05); /* Very light background for the temperature boxes */

color: #333; /* Consistent text color */

padding: 0.5em 1em;

border-radius: 8px;

margin: 0.5em 0;

font-size: 2em;

}

.temperature span {

font-size: 2.5em;

font-weight: 500;

}

.degree-symbol {

font-size: 0.5em;

vertical-align: super;

}

@media (max-width: 600px) {

.temperature span {

font-size: 2em;

}

</style>

</head>

<body>

<h1>ESP32 Temperature Monitor</h1>

</div>

</div>

setInterval(function() {

fetch("readTemp")

.then(response => response.json())

.then(data => {

document.getElementById("TempValueCelsius").innerHTML = data.celsius;

document.getElementById("TempValueFahrenheit").innerHTML = data.fahrenheit;

})

.catch(error => console.error("Error fetching temperature data:", error));

}, 1000);

</script>

</body>

</html>

)rawString";

main.ino

Open the new Arduino sketch and save the file in the folder where html.h file is located.
Change the ssid and password and replace with your credentials.

#include <WiFi.h>

#include <WebServer.h>

#include <esp_adc_cal.h>

#include "html.h"

// Replace with your network credentials

const char* ssid = "***********";

const char* password = "***********";

WebServer server(80);

#define THERMOCOUPLE_PIN 33

#define ANALOG_REFERENCE_VOLTAGE 3.3

#define ADC_BIT_RESOLUTION 12

float calculatedVoltage, measuredTemperature;

float convertRawAdcToVoltage(uint32_t rawAdcValue)

{

esp_adc_cal_characteristics_t adcCharacteristics;

esp_adc_cal_characterize(ADC_UNIT_1, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12, 1100, &adcCharacteristics);

uint32_t voltageInMilliVolts = esp_adc_cal_raw_to_voltage(rawAdcValue, &adcCharacteristics);

return voltageInMilliVolts / 1000.0;

}

float calculateTemperatureFromVoltage(float voltage)

{

return (voltage - 1.25) / 0.005;

}

void setup()

{

Serial.begin(115200);

// Connect to Wi-Fi

WiFi.begin(ssid, password);

while (WiFi.status() != WL_CONNECTED)

{

delay(2000);

Serial.println("Connecting to WiFi...");

}

// Print the IP address

Serial.print("IP Address: ");

Serial.println(WiFi.localIP());

server.on("/", []()

{

server.send_P(200, "text/html", html_page);

});

server.on("/readTemp", HTTP_GET, []() {

float celsius = measuredTemperature;

float fahrenheit = celsius * 1.8 + 32;

char json[70];

snprintf(json, 70, "{\"celsius\": \"%.2f\", \"fahrenheit\": \"%.2f\"}", celsius, fahrenheit);

server.send(200, "application/json", json);

});

server.begin();

}

void loop()

{

uint32_t rawAdcReading = analogRead(THERMOCOUPLE_PIN);

calculatedVoltage = convertRawAdcToVoltage(rawAdcReading);

measuredTemperature = calculateTemperatureFromVoltage(calculatedVoltage);

server.handleClient();

}

After uploading the code, open the Serial Monitor, the ESP32 local IP address will be displayed. Enter the IP address in the Web browser and hit enter.