SparkFun Inventor's Kit Experiment Guide - v4.0

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Contributors: Joel_E_B
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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.

Project4_Circuit4B

Parts Needed

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

parts

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.

TMP36 Temperature sensor

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) * 0.004882814;

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 = 0.004882814.

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

Pay special attention to the component’s markings indicating how to place it on the breadboard. Polarized components can only be connected to a circuit in one direction.

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.

Temperature Sensor

Heads up! Double check the polarity of the TMP36 temperature sensor before powering the RedBoard. It can become very hot if it is inserted backward!

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

Circuit Diagram

Circuit_4B

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

Hookup Table

Component RedBoard Breadboard Breadboard Breadboard
Jumper Wire 5V 5V Rail ( + )
Jumper Wire GND GND Rail ( - )
A15-A30 (Pin 1 on A15)
Jumper Wire E30 GND Rail ( - )
Jumper Wire E29 5V Rail ( + )
Jumper Wire Digital Pin 8 E28
Jumper Wire Digital Pin 9 E27
Jumper Wire Digital Pin 10 E26
Jumper Wire Digital Pin 11 E25
Jumper Wire Digital Pin 12 E20
Jumper Wire E19 GND Rail ( - )
Jumper Wire Digital Pin 13 E18
Jumper Wire E16 5V Rail ( + )
Jumper Wire E15 GND Rail ( - )
Potentiometer A8 A9 A10
Jumper Wire E9 E17
Jumper Wire E8 GND Rail ( - )
Jumper Wire E10 5V Rail ( + )
A1 (GND) A2 (Singnal) A3 (V+)
Jumper Wire E1 GND Rail ( - )
Jumper Wire Analog Pin 0 (A0) E2
Jumper Wire E3 5V Rail ( + )

In the table, polarized components are shown with a warning triangle and the whole row highlighted yellow.

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 libarry 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, calcuated from the voltage
float degreesF = 0;                         //the temperature in Farenheit, calcuated 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) * 0.004882814;   //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 dgrees 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 Fahreheit

  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.

Project4_Circuit4B_Action

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

CodeDescription
Voltage Conversion Algorithms Many of the sensors that you will use with your microcontroller work by changing a voltage in some predictable way in response to a property of the world (like temperature, light or magnetic fields). Often, you will need to build an algorithm that converts these voltages to the desired value and units. The temperature sensor is a great example of this code. We use three equations to convert a voltage value into degrees in C and F.

voltage = analogRead(A0) * 0.004882814;
degreesC = (voltage - 0.5) * 100.0;
degreesF = degreesC * (9.0/5.0) + 32.0;

Coding Challenges

ChallengeDescription
Display the temperature in degrees KelvinTry adding an equation so that the temperature is displayed in degrees Kelvin (you will have to look up the formula for converting from degrees Celsius or Fahrenheit to Kelvin)
Display a bar graphBy changing the code you can display the temperature as a bar graph instead of a number.
Display values from another sensorYou can swap out the TMP36 for a potentiometer, photoresistor or other sensor and display the new set of values.
Add an RGB LEDAdd an RGB LED that changes color based on the temperature.

Troubleshooting

ProblemSolution
Sensor is warm or hot to the touchMake sure that you wired the temperature sensor correctly. The temperature sensor can get warm to the touch if it is wired incorrectly. Disconnect your microcontroller, rewire the circuit, and connect it back to your computer.
Temperature value is unchangingTry pinching the sensor with your fingers to heat it up or pressing a bag of ice against it to cool it down. Also, make sure that the wires are connected properly to the temperature sensor.
Values not printing to screenIf you see text but no temperature values, there could be an error in your code. If you see no text at all, adjust the LCD contrast.