Help with code for DHT11 temp sensor

Projects Programming
1

I am a relative noob and in need of some guidance. I am working from the book "Arduino Workshop" by John Boxall. I am working on project 12 which is to make a temp sensor display to the serial monitor using functions.

So far I am able to get the program to run but the temps don't seem right. Instead of Celsius and Fahrenheit I get some weird numbers like 449.51 deg C and 841.12 deg F. I suspect the conversion part of the code (posted below) is off. I am using a DHT11 3 pin temp/humidity sensor. Some guidance on where to look in my code or what should I looking at to do a proper converson.

thanks
John G.

Code:

//Project 12 - Display temp in serial monitor
#include <dht.h>
dht DHT;

float celsius = 0;
float fahrenheit = 0;

void setup() 
{
  Serial.begin(9600);
}

void findTemps()
{
  float voltage = 0;
  float sensor = 0;
  // read the temp sensor and convert the result to degrees C & F
  sensor = analogRead(0);
  voltage = (sensor * 5000) / 1024; //covert the raw data to millivolts
  voltage = voltage - 500; //remove the voltage offset
  celsius = voltage  /10; //convert millivots to celsius
  fahrenheit = (1.8 * celsius )+ 32; //convert celsius to fahrenheit
}

void displayTemps()
{
  Serial.print("Temp is ");
  Serial.print(celsius, 2);
  Serial.print(" deg. C/");
  Serial.print(fahrenheit, 2);
  Serial.println(" deg. F/");
  //use prinln here so the next reading starts on a new line
}

void loop() 
{
  findTemps();
  displayTemps();
  delay(1000);
}
2

There are examples that come with the DHT library. They look NOTHING like your code.

3

@OP

Are you using this sensor?
dht11Pic.png

This is a digital type sensor, and it produces digital data for Temperature and Humidity; but, your codes are processing analog signals.

Try to run the following sketch (tested on UNO):

#include <dht.h>
dht DHT;

#define DHT11_PIN 2   

void setup()
{
  Serial.begin(9600);
}

void loop()
{
  delay(2000); //Delay 2 sec.
  //Read data and store it to variables hum and temp
  DHT.read11(DHT11_PIN);
  int hum = DHT.humidity;
  int temp = DHT.temperature;

  Serial.print("Humidity is = ");
  Serial.println(hum);

  Serial.print("Temperature is = ");
  Serial.println(temp);
  Serial.println("===========================");
}

dht11Pic.png

1 Like
4

Thank you GolamMostafa. Still learning the in's and out's. Now to figure out how to do the conversions to Fahrenheit.

John

5

Now to figure out how to do the conversions to Fahrenheit.

Hint: It's REALLY simple math.

I'll bet the class even has a method to do that.

6

jgarrett129:
Still learning the in's and out's. Now to figure out how to do the conversions to Fahrenheit.

1. I hope you know this formula:
C/5 = (F-32)/9
==> F = ((9/5)*C) + 32 //F = Fahrenheit; C = Celsius

2. Now, write code to evaluate F of Step-2 and include in the main codes of Post#2. Upload and check the result.

3. If working, it will be fine; else, post your codes here; we will help to correct your codes.

7

One would like to think that the code in a book would match the sensor they cover?

OP's sketch looks like it's for an LM35.

8

not_a_noob:
One would like to think that the code in a book would match the sensor they cover?

OP's sketch looks like it's for an LM35.

Or that OP would realize that not all temperature sensors are the same.

9

Looks to be a TMP36 according to this site, assuming it's the same as his book:
https://tronixstuff.com/2010/04/15/getting-started-with-arduino-chapter-two/

Code looks very similar.

/*
example 2.1 - digital thermometer
Created 14/04/2010 ---  By John Boxall --- http://tronixstuff.com ---  CC by-sa v3.0
Uses an Analog Devices TMP36 temperature sensor to measure temperature and output values to the serial connection
Pin 1 of TMP36 to Arduino 5V power socket
Pin 2 of TMP36 to Arduino analog 0 socket
Pin 3 of TMP36 to Arduino GND socket
*/
void setup()
{
Serial.begin(9600);   // activate the serial output connection
}
float voltage = 0; // setup some variables
float sensor = 0;
float celsius = 0;
float fahrenheit = 0;
void loop()
{              // let's get measurin'
sensor = analogRead(0);
voltage = (sensor*5000)/1024; // convert raw sensor value to millivolts
voltage = voltage-400;        // remove voltage offset
celsius = voltage/10;         // convert millivolts to Celsius
fahrenheit = ((celsius * 1.8)+32); // convert celsius to fahrenheit