How do I code the conversion from Celsius to Fahrenheit?

Projects Programming
1 
/*
 * Arduino weather station with ST7735 color TFT display (128x160 pixel) and
 *   BME280 barometric pressure, temperature & humidity sensor.
 * This is a free software with NO WARRANTY.
 * https://simple-circuit.com/
 */
 
#include <Adafruit_GFX.h>      // include Adafruit graphics library
#include <Adafruit_ST7735.h>   // include Adafruit ST7735 TFT library
#include <Adafruit_BME280.h>   // include Adafruit BME280 sensor library
#include <SPI.h>
#define TFT_RST   7      // TFT RST pin is connected to arduino pin 8
#define TFT_CS    9      // TFT CS  pin is connected to arduino pin 9
#define TFT_DC    8     // TFT DC  pin is connected to arduino pin 10
// initialize ST7735 TFT library
Adafruit_ST7735 tft = Adafruit_ST7735(TFT_CS, TFT_DC, TFT_RST);
 
// define device I2C address: 0x76 or 0x77 (0x77 is library default address)
#define BME280_I2C_ADDRESS  0x76
 
Adafruit_BME280  bme280;  // initialize Adafruit BME280 library
 
void setup(void)
{
  tft.initR(INITR_BLACKTAB);     // initialize a ST7735S chip, black tab
  tft.fillScreen(ST7735_BLACK);  // fill screen with black color
  tft.drawFastHLine(0, 30,  tft.width(), ST7735_WHITE);   // draw horizontal white line at position (0, 30)
 
  tft.setTextColor(ST7735_WHITE, ST7735_BLACK);  // set text color to white and black background
  tft.setTextSize(1);                 // text size = 1
  tft.setCursor(4, 0);               // move cursor to position (4, 0) pixel
  tft.print("ARDUINO + ST7735 TFT");
  tft.setCursor(19, 15);              // move cursor to position (19, 15) pixel
  tft.print("WEATHER STATION");
 
  // initialize the BME280 sensor
  if( bme280.begin(BME280_I2C_ADDRESS) == 0 )
  {  // connection error or device address wrong!
    tft.setTextColor(ST7735_RED, ST7735_BLACK);   // set text color to red and black background
    tft.setTextSize(2);         // text size = 2
    tft.setCursor(5, 76);       // move cursor to position (5, 76) pixel
    tft.print("Connection");
    tft.setCursor(35, 100);     // move cursor to position (35, 100) pixel
    tft.print("Error");
    while(1);  // stay here
  }
 
  tft.drawFastHLine(0, 76,  tft.width(), ST7735_WHITE);  // draw horizontal white line at position (0, 76)
  tft.drawFastHLine(0, 122,  tft.width(), ST7735_WHITE);  // draw horizontal white line at position (0, 122)
  tft.setTextColor(ST7735_RED, ST7735_BLACK);     // set text color to red and black background
  tft.setCursor(25, 39);              // move cursor to position (25, 39) pixel
  tft.print("TEMPERATURE =");
  tft.setTextColor(ST7735_CYAN, ST7735_BLACK);  // set text color to cyan and black background
  tft.setCursor(34, 85);              // move cursor to position (34, 85) pixel
  tft.print("HUMIDITY =");
  tft.setTextColor(ST7735_GREEN, ST7735_BLACK);  // set text color to green and black background
  tft.setCursor(34, 131);              // move cursor to position (34, 131) pixel
  tft.print("PRESSURE =");
  tft.setTextSize(2);                 // text size = 2
 
}
 
// main loop
void loop()
{
  char _buffer[8];
  // read temperature, humidity and pressure from the BME280 sensor
  float temp = bme280.readTemperature();    // get temperature in °C
  float humi = bme280.readHumidity();       // get humidity in rH%
  float pres = bme280.readPressure();       // get pressure in Pa
 
  // print temperature (in °C)
  if(temp < 0)    // if temperature < 0
    sprintf( _buffer, "-%02u.%02u", (int)abs(temp), (int)(abs(temp) * 100) % 100 );
  else            // temperature >= 0
    sprintf( _buffer, " %02u.%02u", (int)temp, (int)(temp * 100) % 100 );
  tft.setTextColor(ST7735_YELLOW, ST7735_BLACK);  // set text color to yellow and black background
  tft.setCursor(11, 54);
  tft.print(_buffer);
  tft.drawCircle(89, 56, 2, ST7735_YELLOW);  // print degree symbol ( ° )
  tft.setCursor(95, 54);
  tft.print("C");
 
  // 2: print humidity
  sprintf( _buffer, "%02u.%02u %%", (int)humi, (int)(humi * 100) % 100 );
  tft.setTextColor(ST7735_MAGENTA, ST7735_BLACK);  // set text color to magenta and black background
  tft.setCursor(23, 100);
  tft.print(_buffer);
 
  // 3: print pressure (in hPa)
  sprintf( _buffer, "%04u.%02u", (int)(pres/100), (int)((uint32_t)pres % 100) );
  tft.setTextColor(0xFD00, ST7735_BLACK);  // set text color to orange and black background
  tft.setCursor(3, 146);
  tft.print(_buffer);
  tft.setCursor(91, 146);
  tft.print("hPa");
 
  delay(1000);    // wait a second
 
}
 
// end of code.

This is the conversion formula (Celsius * 9 / 5 + 32)
I'm not very proficient at coding, so I don't know how to code the conversion. Any and all help is greatly appreciated.
Thanks in advace!

2

Try ((Celsius * 9.0 / 5.0) + 32)

3

As formalyAWOL says...

float tempF =  (tempC * 9.0 / 5.0) + 32.0;
4

pcbbc:
As formalyAWOL says...

float tempF =  (tempC * 9.0 / 5.0) + 32.0;

Where do I put the conversion? Do I mark out the original line?
I realize that I need to make other changes to the code such as "C" to "F" and will do so once the conversion works.

Again Thanks to all who replied!

5

It depends on what you want to do with the result. Do you wish to simply replace the Celsius output value with a Fahrenheit output value?

6

I want the results to display in Fahrenheit so yes I want to replace the Celsius output with Fahrenheit!

Where in the code do I put "float tempF = (tempC * 9.0 / 5.0) + 32.0;"?

When I replaced the original line with the one above I get and error that tempC was not defined

7

Malx56:
When I replaced the original line with the one above I get and error that tempC was not defined

Please post your best attempt

8

Malx56:
Where in the code do I put "float tempF = (tempC * 9.0 / 5.0) + 32.0;"?

In between where you obtain tempC, and where you use tempF.

9

The was the first homework assignment when my business partner and I took a "C" class at a community college in the Seattle area.

I am surprised that you can't figure it out.

Paul

10

C * 1.8 + 32
(eliminates one math operation)

1 Like
11

runaway_pancake:
C * 1.8 + 32
(eliminates one math operation)

Don't you think the compiler can do arithmetic?

12

TheMemberFormerlyKnownAsAWOL:
Don't you think the compiler can do arithmetic?

9/5 is a derivation (reduced fraction) of (bpF - fpF)/(bpC - fpC)+32 where bp, fp are boiling point and freezing point of water, (212-32)/(100-0)+32. So to really put compiler math to work, you can start from first principles. :wink:

13

TheMemberFormerlyKnownAsAWOL:
Don't you think the compiler can do arithmetic?

Does it boil it down then or with 9/5 do a multiplication and then a division (two operations)?

Compiling both ways shows 444 bytes are taken up.

float degCin = 50.0;
float Fdeg;
void setup() {
  // put your setup code here, to run once:
}
void loop() {
  Fdeg = 9.0 * degCin / 5.0 + 32.0;
}

float degCin = 50.0;
float Fdeg;
void setup() {
  // put your setup code here, to run once:
}
void loop() {
  Fdeg = 1.8 * degCin + 32.0;
}
14

runaway_pancake:
Does it boil it down then

Yes, at 100C or 212F. :slight_smile:

But seriously, modern compilers optimize all constant expressions by evaluating them at compile time, including floats and doubles.

1 Like
15

To those that replied without reading my question I stated that I have NO formal programming experience! I am teaching myself!! So everything isn't clear to me yet! SO after trying what I think could work and searching Google which can be more confusing. I am still at a lost, but will try the suggestions provided. SO to Paul_KD7HB if your not going to be helpful Don't ass- u-me that it's that it should be that simple to figure out for everyone! Too me your response is just useless!

To those that provided suggestions THANKS! I'll give them a try and report the results!

THANKS!

Malcolm W.