April 2013 – Jeon's laboratory

My car doesn’t have a compass which I wish to have one. So I started making one using my JeonLab mini v1.3 (minimalist Arduino compatible board), popular 16×2 LCD panel with a back light LED, and a 3 axis magnetometer. Here is the part list and pictures of the LCD, JeonLab mini v1.3 and the prototyping board.

LCD: 16×2 HD44780 LCD (white text on blue background)
JeonLab mini v1.3
Digital compass: MAG3110 (bought an assembled from ebay)
Temperature sensor: TMP36
Phototransistor: LTR-4206E
7805 regulator
resistors and capacitors (see schematic diagram)
Car battery jack
prototyping board
push button switch
solid copper wire (1mm in diameter) for bracket
cable ties

The JeonLab mini v1.3 is so small that can be attached to the back of the LCD.

First of all, the LCD, JeonLab mini and the magnetometer, MAG3110 have been assembled on a breadboard and tested. The magnetometer has 3 axis sensor, but because, fortunately, the most roads where I live and commute to work are relatively level. So I didn’t bother to use complicated equations, but decided to calculate simply the heading angle using ATAN from X and Y readings. And it really works just good enough. Take a look at the source Arduino code below.
/* JeonLab Car Digital Compass & Thermometer

Functions:
LCD display
Digital compass (MAG3110)
Interior temperature
LCD backlight brightness automatic adjust

MAG3110 read: Original code from Sparkfun example code
(https://www.sparkfun.com/products/10619)
*/

#include
#include #include <avr/eeprom.h>

#define MAG_ADDR 0x0E
int avgX, avgY;

struct settings_t
{
long maxX, minX, maxY, minY;
}
settings;

LiquidCrystal lcd(2,3,6,7,8,9);

void setup()
{
Wire.begin();
config(); //configuration of the magnetometer, MAG3110
pinMode(10,INPUT); //temperature sensor
pinMode(5,OUTPUT); //LCD backlight LED
analogWrite(5, 100);

//read previously stored calibration data from EEPROM
eeprom_read_block((void*)&settings, (void*)0, sizeof(settings));
avgX=(settings.maxX+settings.minX)/2;
avgY=(settings.maxY+settings.minY)/2;

lcd.begin(16, 2);
}

void loop()
{
//read the magnetometer and calculate heading
float heading = (atan2(readx()-avgX, ready()-avgY))*180/PI;

//read and calculate the interior temperature in celcius
float temp = (analogRead(0)/1024.0*5000.0-500)/10;

//read ambient brightness from the photo-transistor
int brightness = map(analogRead(1), 900, 1023, 50, 255);
analogWrite(5,brightness); //LCD backlight LED voltage control by PWM

//diaplay compass bearing
lcd.setCursor(0,0);
lcd.print(” “);
lcd.setCursor(0,0);
if (abs(heading) <= 22.5) lcd.print(“N “); if (abs(heading) >= 157.5) lcd.print(“S “);
if (heading >= 67.5 && heading <= 112.5) lcd.print(“E “);
if (heading <= -67.5 && heading >= -112.5) lcd.print(“W “);
if (heading > 22.5 && heading < 67.5) lcd.print(“NE”);
if (heading < -22.5 && heading > -67.5) lcd.print(“NW”);
if (heading > 112.5 && heading < 157.5) lcd.print(“SE”);
if (heading < -112.5 && heading > -157.5) lcd.print(“SW”);

if (heading < 0) heading += 360.0; //display heading lcd.setCursor(3,0); lcd.print(int(heading)); lcd.setCursor(7,0); lcd.print(int(temp)); lcd.print(“\337C”); lcd.setCursor(0,1); lcd.print(” “); lcd.setCursor(0,1); lcd.print(“Brightness: “); //show PWM value for a reference lcd.print(brightness); if (digitalRead(10) == HIGH) //monitor calibration button status { analogWrite(5,200); lcd.setCursor(0,1); lcd.print(“calibrating…..”); delay(1000); calXY(); } delay(1000); } void calXY() //magnetometer calibration: finding max and min of X, Y axis { int tempXmax, tempXmin, tempYmax, tempYmin, newX, newY; lcd.setCursor(0,1); lcd.print(“Rotate the car “); delay(1000); lcd.setCursor(0,1); lcd.print(“and press button”); delay(1000); lcd.setCursor(0,1); lcd.print(“Now, begin! “); delay(500); tempXmax = tempXmin = readx(); tempYmax = tempYmin = ready(); while(digitalRead(10) == LOW) { newX = readx(); newY = ready(); if (newX > tempXmax) tempXmax = newX;
if (newX < tempXmin) tempXmin = newX; if (newY > tempYmax) tempYmax = newY;
if (newY < tempYmin) tempYmin = newY;
}
settings.maxX = tempXmax;
settings.minX = tempXmin;
settings.maxY = tempYmax;
settings.minY = tempYmin;

//store new X, Y values in the EEPROM
eeprom_write_block((const void*)&settings, (void*)0, sizeof(settings));

avgX=(settings.maxX+settings.minX)/2;
avgY=(settings.maxY+settings.minY)/2;

lcd.setCursor(0,1);
lcd.print(“Calibration done”);
delay(3000);
lcd.setCursor(0,1);
lcd.print(” “);

}

void config(void) //MAG3110 config taken from Sparkfun example
{
Wire.beginTransmission(MAG_ADDR); // transmit to device 0x0E
Wire.write(0x10); // cntrl register1
Wire.write(0); // send 0x00, standby mode
Wire.endTransmission(); // stop transmitting

delay(15);

Wire.beginTransmission(MAG_ADDR); // transmit to device 0x0E
Wire.write(0x11); // cntrl register2
Wire.write(0x80); // send 0x80, enable auto resets
Wire.endTransmission(); // stop transmitting

delay(15);

Wire.beginTransmission(MAG_ADDR); // transmit to device 0x0E
Wire.write(0x10); // cntrl register1
Wire.write(0x19); // send 0x01, active mode
Wire.endTransmission(); // stop transmitting

delay(15);
}

void print_values(void)
{
}

int readx(void) //MAG3110 read X value taken from Sparkfun example
{
int xl, xh; //define the MSB and LSB

Wire.beginTransmission(MAG_ADDR); // transmit to device 0x0E
Wire.write(0x01); // x MSB reg
Wire.endTransmission(); // stop transmitting