Published January 10, 2025 © GPL3+

Portable thermometer - NFC/ QT PY/ SHT40

Check the ambient temperature and humidity wherever you go with a compact battery-less design!

BeginnerFull instructions provided10 hours454

Portable thermometer - NFC/ QT PY/ SHT40

Things used in this project

Hardware components

Adafruit QT Py

Sensirion SHT40

STMicroelectronics ST25DV-I2C

Adafruit Qwiic JST 4 pin cables

Software apps and online services

Arduino IDE

Hand tools and fabrication machines

3D Printer (generic)

Story

Smartphones, while versatile and powerful, are not ideal thermometers due to their design and primary purpose. Their internal temperature sensors are intended to monitor device performance, not ambient conditions, and they are affected by heat generated by the phone itself and its surroundings, leading to inaccurate readings. Yet, understanding ambient temperature and humidity is essential in many scenarios, from ensuring comfort in living spaces to optimizing storage conditions for sensitive materials. For outdoor enthusiasts, knowing these parameters can improve safety and preparedness, while homeowners and hobbyists benefit from monitoring their environments to maintain optimal conditions for health or specific projects. This is where a portable thermometer, powered by a smartphone's USB-C port and using NFC, becomes an invaluable tool for obtaining accurate, real-time environmental data.

Touch and tap: Measure and read

How it works?

To power the device, it needs to be plugged with a USB-C cable to a smartphone. A green LED will turn on.

Then, just touch the female pin header connected to the capacitive touch of the QT PY to trigger a measurement. The SHT40 measures the temperature and humidity, and the readings are stored in the NFC tag memory. A green LED will toggle to indicate a new reading is available.

To retrieve the latest measurement, enable the NFC feature on your smartphone and touch the device.

Full demo video

Technology used

One of the advantages of the QT Py SAMD21 is that it has 6 pins that can be capacitive touch pins without the need for a separate driver pin. It allowed me to add a button feature to trigger a measurement when needed without adding another component. A capacitive touch pin is a sensor used to detect touch by measuring changes in capacitance. It works by generating a small electric field, which has a baseline capacitance when untouched. When a finger or conductive object comes near or touches the pin, the capacitance increases due to the added conductive surface. This change is detected by the QT Py microcontroller, which processes it as a touch input.

An NFC dual tag is a device with two interfaces: an NFC interface for wireless communication and a wired interface like I²C or SPI for connecting to a host system. It allows NFC-enabled devices, like smartphones, to wirelessly read or write data stored in the tag, while the host device simultaneously accesses the same memory via the wired connection. In this case, the QT Py microcontroller writes the data from the SHT40 sensor into the tag via I2C, and the smartphone retrieves it wirelessly.

Results and conclusion

It is a nice device to build under 25$ USD. I brought it and snapped pictures in different locations where we typically don't have access to the current ambient temperature and humidity: an airport, a supermarket, a Starbucks and a shopping center.

1 / 4 • Portable thermometer in the airport

This portable thermometer could also help with checking temperatures in offices or buildings to support the property management team regulating the HVAC system without needing a monitoring device in each room.

Thanks for checking my project!

Schematics

Code

Code

#include <Adafruit_NeoPixel.h>
#include <SensirionI2cSht4x.h>
#include <Wire.h>
#include "Adafruit_FreeTouch.h"
#include "ST25DVSensor.h"

Adafruit_FreeTouch qt_1 = Adafruit_FreeTouch(A0, OVERSAMPLE_4, RESISTOR_50K, FREQ_MODE_NONE);

// create a pixel strand with 1 pixel on PIN_NEOPIXEL
Adafruit_NeoPixel pixels(1, PIN_NEOPIXEL);

// create a SHT40 sensor object
SensirionI2cSht4x sensor;
float aTemperatureC, aTemperatureF = 0.0;
float aHumidity = 0.0;

static int16_t error;

int base_touch;

//NFC dual tag
ST25DV st25dv(1, 2, &Wire);

void measure(){
    error = sensor.measureHighPrecision(aTemperatureC, aHumidity);
    if (error != 0) {
        // set the first pixel #0 to red
        pixels.setPixelColor(0, pixels.Color(255, 0, 0));
    }else{
      //Temperature conversion to Fahrenheit
      aTemperatureF = aTemperatureC*9/5+32;
      pixels.setPixelColor(0, pixels.Color(0, 255, 0));
    }
    //turn on led pixel
    pixels.show(); 
    delay(1000);
    // turn off the led pixel
    pixels.clear();
    pixels.show();
}

void write_NFC(){
  char buffer[50];
  sprintf(buffer, "The temperature and humidity are:\n %.2fC  %.2fF %.2f%%RH", aTemperatureC,aTemperatureF,aHumidity);
  if(st25dv.writeUnabridgedURI(buffer, "")) {
    // Write NFC failed
    while(1);
  }
}

void setup() {
    // initialize the pixel led
    pixels.begin();  

    //NFC init
    st25dv.begin();

    //SHT40 init
    sensor.begin(Wire, SHT40_I2C_ADDR_44);
    sensor.softReset();
    delay(10);

    //Capacitive touch init
    if (! qt_1.begin())  
      {
        // Failed to begin qt on pin A0, turn on red led
        pixels.setPixelColor(0, pixels.Color(0, 255, 0));
      }
    
    base_touch = qt_1.measure();

}

void loop() {
    int result = 0;
    result = qt_1.measure(); 

    if(result > (base_touch+10))
    {
        measure();
        write_NFC();
    }
    
    delay(500);

}

Credits

Maureen Rakotondraibe

3 projects • 6 followers

Embedded systems/software engineer / Passionate about interfacing technologies to create new solutions

Contact

Comments

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Portable thermometer - NFC/ QT PY/ SHT40

Things used in this project

Hardware components

Software apps and online services

Hand tools and fabrication machines

Story

How it works?

Technology used

Results and conclusion

Custom parts and enclosures

Enclosure

Components in enclosure

Schematics

Components assembly

Code

Code

Credits

Maureen Rakotondraibe

Comments

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Portable thermometer - NFC/ QT PY/ SHT40

Portable thermometer - NFC/ QT PY/ SHT40

Things used in this project

Hardware components

Software apps and online services

Hand tools and fabrication machines

Story

How it works?

Technology used

Results and conclusion

Custom parts and enclosures

Enclosure

Components in enclosure

Schematics

Components assembly

Code

Code

Credits

Maureen Rakotondraibe

Comments

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