SparkFun Inventor's Kit Experiment Guide - v4.0

Contributors: Joel_E_B

Circuit 4B: Temperature Sensor

Want to create a DIY environmental monitor or weather station? You can use a small, low-cost sensor like the TMP36 to make devices that track and respond to temperature. In this activity you will also use the LCD screen to display sensor readings, a common use for LCDs in electronics projects.

Parts Needed

Grab the following quantities of each part listed to build this circuit:

New Components

TMP36 Temperature Sensor

This temperature sensor has three legs. One connects to 5V, one to ground, and the voltage output from the third leg varies proportionally to changes in temperature. By doing some simple math with this voltage we can measure temperature in degrees Celsius or Fahrenheit.

New Concepts

Algorithms

An algorithm is a process used in order to achieve a desired result. Often, the information needed to create an algorithm lives in the part's datasheet. This sketch uses a few formulas to turn a voltage value into a temperature value, making them all part of the larger temperature-retrieving algorithm. The first formula takes the voltage read on analog pin 0 and multiplies it to get a voltage value from 0V--5V:

language:c
voltage = analogRead(A0) * 0.004882813;

The number we are multiplying by comes from dividing 5V by the number of samples the analog pin can read (1024), so we get: 5 / 1024 = 0.004882813.

The second formula takes that 0--5V value and calculates degrees Centigrade:

language:c
degreesC = (voltage - 0.5) * 100.0;

The reason 0.5V is subtracted from the calculated voltage is because there is a 0.5V offset, mentioned on page 8 of the TMP36 datasheet. It's then multiplied by 100 to get a value that matches temperature.

The last formula takes the Centigrade temperature and converts it to a Fahrenheit temperature using the standard conversion formula:

language:c
degreesF = degreesC * (9.0/5.0) + 32.0;

Together, these three formulas make up the algorithm that converts voltage to degrees Fahrenheit.

Hardware Hookup

The temperature sensor is polarized and can only be inserted in one direction. See below for the pin outs of the temperature sensor. Pay very close attention to the markings on each side as you insert it into your circuit.

Ready to start hooking everything up? Check out the circuit diagram and hookup table below to see how everything is connected.

Circuit Diagram

Note for Advanced Users: If you know how to read datasheets and schematics, you can also refer to the schematic below as an alternative.

Having a hard time seeing the circuit? Click on the image for a closer look.

Hookup Table

Open the Sketch

To open the code, go to: File > Examples > SIK_Guide_Code-V_4 > SIK_Circuit_4B-TemperatureSensor

You can also copy and paste the following code into the Arduino IDE. Hit upload, and see what happens!

language:cpp
/*
  SparkFun Inventor’s Kit
  Circuit 4B - Temperature Sensor

  The LCD will display readings from a temperature sensor in degrees Celsius and Fahrenheit.

  This sketch was written by SparkFun Electronics, with lots of help from the Arduino community.
  This code is completely free for any use.

  View circuit diagram and instructions at: https://learn.sparkfun.com/tutorials/sparkfun-inventors-kit-experiment-guide---v40
  Download drawings and code at: https://github.com/sparkfun/SIK-Guide-Code
*/

#include <LiquidCrystal.h>                  //the liquid crystal library contains commands for printing to the display
LiquidCrystal lcd(13, 12, 11, 10, 9, 8);    // tell the RedBoard what pins are connected to the display

float voltage = 0;                          //the voltage measured from the TMP36
float degreesC = 0;                         //the temperature in Celsius, calculated from the voltage
float degreesF = 0;                         //the temperature in Fahrenheit, calculated from the voltage

void setup() {

  lcd.begin(16, 2);                         //tell the lcd library that we are using a display that is 16 characters wide and 2 characters high
  lcd.clear();                              //clear the display
}

void loop() {

  voltage = analogRead(A0) * 0.004882813;   //convert the analog reading, which varies from 0 to 1023, back to a voltage value from 0-5 volts
  degreesC = (voltage - 0.5) * 100.0;       //convert the voltage to a temperature in degrees Celsius
  degreesF = degreesC * (9.0 / 5.0) + 32.0; //convert the voltage to a temperature in degrees Fahrenheit

  lcd.clear();                              //clear the LCD

  lcd.setCursor(0, 0);                      //set the cursor to the top left position
  lcd.print("Degrees C: ");                 //print a label for the data
  lcd.print(degreesC);                      //print the degrees Celsius

  lcd.setCursor(0, 1);                      //set the cursor to the lower left position
  lcd.print("Degrees F: ");                 //Print a label for the data
  lcd.print(degreesF);                      //print the degrees Fahrenheit

  delay(1000);                              //delay for 1 second between each reading (this makes the display less noisy)
}

What You Should See

The LCD will show the temperature in Celsius and Fahrenheit. The temperature readings will update every second. An easy way to see the temperature change is to press your finger to the sensor.

Program Overview

1. Get the analog value from the TMP36 and convert it back to a voltage between 0 and 5V.
2. Calculate the degrees Celsius from this voltage.
3. Calculate the degrees Fahrenheit from this voltage.
4. Clear the LCD.
5. Print the Degrees C with a label on the first row.
6. Print the Degrees F with a label on the second row.
7. Wait for a second before taking the next reading.

Code to Note

Coding Challenges

Troubleshooting