### Project 11 What’s the air pressure

**1.Introduction**

In this lesson, we will measure the Temperature, Pressure, Standard Atmosphere and Altitude using a BMP180 Barometric Pressure Sensor. You can also apply the experiment principle to make a vertical velocity indicator, a device that can regulate fan power etc.

**2.Hardware required**

- EASY plug controller Board x1
- USB cable x1
- EASY plug cable x1
- EASY plug BMP180 Barometric Pressure Sensor x1

We will briefly take a look at this BMP180 first.

![](media/image-20251126145339411.png)

BMP180 is a pressure sensor with high precision, compact design and low power consumption. It can be used in mobile devices. Minimum absolute accuracy reaches 0.03hpa with only 3uA power consumption. Below are its specifications:

- Pressure range: 300~1100hPa (Altitude 9000M~-500M)
- Supply voltage: 1.8V~3.6V (VDDA), 1.62V~3.6V(VDDD)
- Low power consumption: 0.5uA, standard mode
- I2C interface
- With temperature output
- Leadless, comply with RoHS
- MSL 1 response time: 7.5ms
- Standby current: 0.1uA
- No need for external clock circuit
- Size: 35*20mm
- Weight: 6g

**3.Connection Diagram**

Now, connect the BMP180 module to the IIC port of the controller board using the EASY plug cable.

![](media/image-20251126145450454.png)

**4.Sample Code**

Connect the board to your PC using the USB cable; copy below code into Arduino IDE, and click upload to upload it to your board.

```c
#include <Wire.h>  // place file “Wire.h” under the directory “library” of Arduino
#define BMP085_ADDRESS 0x77  // I2C address of BMP085
const unsigned char OSS = 0;  // Oversampling Setting
// Calibration values
int ac1;
int ac2;
int ac3;
unsigned int ac4;
unsigned int ac5;
unsigned int ac6;
int b1;
int b2;
int mb;
int mc;
int md;
// b5 is calculated in bmp085GetTemperature(...), this variable is also used in bmp085GetPressure(...)
// so ...Temperature(...) must be called before ...Pressure(...).
long b5; 
void setup(){
  Serial.begin(9600);
  Wire.begin();
  bmp085Calibration();
}

void loop()
{
  float temperature = bmp085GetTemperature(bmp085ReadUT()); //MUST be called first
  float pressure = bmp085GetPressure(bmp085ReadUP());
  float atm = pressure / 101325; // "standard atmosphere"
  float altitude = calcAltitude(pressure); //Uncompensated caculation - in Meters 
  Serial.print("Temperature: ");
  Serial.print(temperature, 2); //display 2 decimal places
  Serial.println("deg C");
  Serial.print("Pressure: ");
  Serial.print(pressure, 0); //whole number only.
  Serial.println(" Pa");
  Serial.print("Standard Atmosphere: ");
  Serial.println(atm, 4); //display 4 decimal places
  Serial.print("Altitude: ");
  Serial.print(altitude, 2); //display 2 decimal places
  Serial.println(" M");
  Serial.println();//line break
  delay(1000); //wait a second and get values again.
}

// Stores all of the bmp085's calibration values into global variables
// Calibration values are required to calculate temp and pressure
// This function should be called at the beginning of the program
void bmp085Calibration()
{
  ac1 = bmp085ReadInt(0xAA);
  ac2 = bmp085ReadInt(0xAC);
  ac3 = bmp085ReadInt(0xAE);
  ac4 = bmp085ReadInt(0xB0);
  ac5 = bmp085ReadInt(0xB2);
  ac6 = bmp085ReadInt(0xB4);
  b1 = bmp085ReadInt(0xB6);
  b2 = bmp085ReadInt(0xB8);
  mb = bmp085ReadInt(0xBA);
  mc = bmp085ReadInt(0xBC);
  md = bmp085ReadInt(0xBE);
}

// Calculate temperature in deg C
float bmp085GetTemperature(unsigned int ut){
  long x1, x2;
  x1 = (((long)ut - (long)ac6)*(long)ac5) >> 15;
  x2 = ((long)mc << 11)/(x1 + md);
  b5 = x1 + x2;
  float temp = ((b5 + 8)>>4);
  temp = temp /10;
  return temp;
}
// Calculate pressure given up
// calibration values must be known
// b5 is also required so bmp085GetTemperature(...) must be called first.
// Value returned will be pressure in units of Pa.
long bmp085GetPressure(unsigned long up){
  long x1, x2, x3, b3, b6, p;
  unsigned long b4, b7;
  b6 = b5 - 4000;
  // Calculate B3
  x1 = (b2 * (b6 * b6)>>12)>>11;
  x2 = (ac2 * b6)>>11;
  x3 = x1 + x2;
  b3 = (((((long)ac1)*4 + x3)<<OSS) + 2)>>2;
  // Calculate B4
  x1 = (ac3 * b6)>>13;
  x2 = (b1 * ((b6 * b6)>>12))>>16;
  x3 = ((x1 + x2) + 2)>>2;
  b4 = (ac4 * (unsigned long)(x3 + 32768))>>15;
  b7 = ((unsigned long)(up - b3) * (50000>>OSS));
  if (b7 < 0x80000000)
    p = (b7<<1)/b4;
  else
    p = (b7/b4)<<1;
  x1 = (p>>8) * (p>>8);
  x1 = (x1 * 3038)>>16;
  x2 = (-7357 * p)>>16;
  p += (x1 + x2 + 3791)>>4;
  long temp = p;
  return temp;
}
// Read 1 byte from the BMP085 at 'address'
char bmp085Read(unsigned char address)
{
  unsigned char data;
  Wire.beginTransmission(BMP085_ADDRESS);
  Wire.write(address);
  Wire.endTransmission();
  Wire.requestFrom(BMP085_ADDRESS, 1);
  while(!Wire.available())
    ;
  return Wire.read();
}

// Read 2 bytes from the BMP085
// First byte will be from 'address'
// Second byte will be from 'address'+1
int bmp085ReadInt(unsigned char address)
{
  unsigned char msb, lsb;

  Wire.beginTransmission(BMP085_ADDRESS);
  Wire.write(address);
  Wire.endTransmission();
  Wire.requestFrom(BMP085_ADDRESS, 2);
  while(Wire.available()<2)
    ;
  msb = Wire.read();
  lsb = Wire.read();
  return (int) msb<<8 | lsb;
}
// Read the uncompensated temperature value
unsigned int bmp085ReadUT(){
  unsigned int ut;
  // Write 0x2E into Register 0xF4
  // This requests a temperature reading
  Wire.beginTransmission(BMP085_ADDRESS);
  Wire.write(0xF4);
  Wire.write(0x2E);
  Wire.endTransmission();
  // Wait at least 4.5ms
  delay(5);
  // Read two bytes from registers 0xF6 and 0xF7
  ut = bmp085ReadInt(0xF6);
  return ut;
}
// Read the uncompensated pressure value
unsigned long bmp085ReadUP(){
  unsigned char msb, lsb, xlsb;
  unsigned long up = 0;
  // Write 0x34+(OSS<<6) into register 0xF4
  // Request a pressure reading w/ oversampling setting
  Wire.beginTransmission(BMP085_ADDRESS);
  Wire.write(0xF4);
  Wire.write(0x34 + (OSS<<6));
  Wire.endTransmission();
  // Wait for conversion, delay time dependent on OSS
  delay(2 + (3<<OSS));
  // Read register 0xF6 (MSB), 0xF7 (LSB), and 0xF8 (XLSB)
  msb = bmp085Read(0xF6);
  lsb = bmp085Read(0xF7);
  xlsb = bmp085Read(0xF8);
  up = (((unsigned long) msb << 16) | ((unsigned long) lsb << 8) | (unsigned long) xlsb) >> (8-OSS);
  return up;
}

void writeRegister(int deviceAddress, byte address, byte val) {
  Wire.beginTransmission(deviceAddress); // start transmission to device 
  Wire.write(address);       // send register address
  Wire.write(val);         // send value to write
  Wire.endTransmission();     // end transmission
}
int readRegister(int deviceAddress, byte address){
  int v;
  Wire.beginTransmission(deviceAddress);
  Wire.write(address); // register to read
  Wire.endTransmission();

  Wire.requestFrom(deviceAddress, 1); // read a byte

  while(!Wire.available()) {
    // waiting
  }
  v = Wire.read();
  return v;
}
float calcAltitude(float pressure){
  float A = pressure/101325;
  float B = 1/5.25588;
  float C = pow(A,B);
  C = 1 - C;
  C = C /0.0000225577;
  return C;
}
```

**5.Result**

Now, open serial monitor, set baud rate to “9600”, serial monitor displays content as pic 1; when you blow air into the sensor with your month, you can see the pressure changes as pic 2 shows.

![](media/4.png)

![](media/5.png)