Archive for December, 2010


Having been rather addicted to understanding electronics lately, I’m always looking for something to play with. Last week I was about to throw out a broken wireless Microsoft mouse, but I caught myself and ended up taking it apart.

Rotary Encoder from a Mouse

I dug down to the scroll wheel and found out it was a nice little rotary encoder – there’s an LED pointed at two phototransistors (light detectors) in it, as well as a circle of tiny black posts that pass between the LED and the detectors, as you can see to the left.  When the posts pass in front of the detectors, the Arduino reads them and is able to determine which direction they are going and increments or decrements a variable that I can access.

It took me about 4 hours to figure out how to hook this thing up, so I figured I would share the info in case someone else wants to use a rotary encoder from a Microsoft mouse in their project.

If you are holding the encoder so that the rotating half is facing you and the wires are facing down, the pinout is as follows:

Pin 1: Photo Sensor 1
Pin 2: Vcc
Pin 3: Photo Sensor 2
Pin 4: IR LED +
Pin 5: IR LED –

Unlike most of the examples I found for using a rotary encoder, this one requires +Vcc on Pin 2 – power will not flow the other way.  The LED power is very picky too;  I am powering it from the 5v regulated supply on my Arduino through a 1k resistor to keep the right amount of light in the encoder.  Too much light and both sensors will be high for too long and too little, neither will go high.  The hardest part to figure out was the voltage divider circuit for the sensors.  I could get them working ok on an analog input – swinging between about 80-600, but it wasn’t clean enough to input as digital until I found the right resistor: 10k.  This nicely divides the input between ground and +5v so the encoder can be read.

Here is the Arduino sketch for the project as shown in the video above:

// Arduino Rotary Encoder from Mouse Scroll Wheel
// and SparkFun 4 Digit Serial 7-Segment Display
// by Steve Kamerman 12/18/2010
// http://www.stevekamerman.com/2010/12/understanding-a-mouse-scroll-wheel/

// These MUST be on interrupt pins!
#define encoderPinA 2
#define encoderPinB 3

#define displayTXPin 10
#define displayRXPin 11

// used only for sprintf()
#include <stdio.h>

// If you don't have NewSoftSerial, grab it from:
// http://arduiniana.org/libraries/NewSoftSerial/
#include <NewSoftSerial.h>
NewSoftSerial displaySerial(displayRXPin, displayTXPin);

// the value - this must be volatile because it is modified
// during an interrupt function and used in a normal function
volatile unsigned int encoderPos = 0;

void setup() {
  // setup the encoder
  pinMode(encoderPinA, INPUT);
  pinMode(encoderPinB, INPUT);
  attachInterrupt(0, readEncoderA, CHANGE);
  attachInterrupt(1, readEncoderB, CHANGE);  
 
  // setup the display
  pinMode(displayTXPin, OUTPUT);
  displaySerial.begin(9600);
  // reset display
  displaySerial.print("v");
  // Set display brightness
  displaySerial.print(0x7A, BYTE);
  displaySerial.print(0x05, BYTE);
  // set the display to "   0"
  displaySerial.print("xxx0");
}
void loop(){
  displayNumber(encoderPos);
}

// prints the given number, right justified padded with spaces
// sprtinf() is not a great idea if you want to save space
// but I'm really not too worried about it :)
void displayNumber(int num){
  char buf[4];
  sprintf(buf, "%4d", num);
  displaySerial.print(buf);
}

void readEncoderA(){
  if (digitalRead(encoderPinA) == HIGH){
    encoderPos += (digitalRead(encoderPinB) == LOW)? 1: -1;
  }else{
    encoderPos += (digitalRead(encoderPinB) == HIGH)? 1: -1;
  }
}
void readEncoderB(){
  if (digitalRead(encoderPinB) == HIGH){
    encoderPos += (digitalRead(encoderPinA) == HIGH)? 1: -1;
  }else{
    encoderPos += (digitalRead(encoderPinA) == LOW)? 1: -1;
  }
}

Yes, I’m still on the electronics kick. This is something I’ve been wanting to do as part of a complete home automation project down the road: monitor the temperature of the different rooms / zones in my house. Now that I’ve got a few Arduinos laying around and some thermal probes (thermistors) from some broken Radio Shack temperature displays, I’ve got the hardware necessary to make it happen! I first installed the thermal probes in the ceilings of the master bedroom, my daughter’s bedroom and our living room in locations that I thought were least affected by vents, fans, lights and other sources of heat. Here’s a picture of the probe in my living room:

Thermal Probe in my Living Room

Next, I connected the three probes to my Arduino’s Analog Inputs using the thermistors as voltage dividers between +5VDC and Ground.  In order to detect the temperatures and make them accessible via the Arduino’s serial interface, I used a few examples on the Arduino site as the basis for this program, which waits for a character to be sent to the serial port, then outputs the temperatures:

#include <math.h>
//Schematic:
// [Ground] ---- [10k-Resister] -------|------- [Thermistor] ---- [+5v]
//                                     |
//                                Analog Pin 0

double Thermistor(int RawADC) {
 // Inputs ADC Value from Thermistor and outputs Temperature in Celsius
 //  requires: include <math.h>
 // Utilizes the Steinhart-Hart Thermistor Equation:
 //    Temperature in Kelvin = 1 / {A + B[ln(R)] + C[ln(R)]^3}
 //    where A = 0.001129148, B = 0.000234125 and C = 8.76741E-08
 long Resistance;  double Temp;
 Resistance=((10240000/RawADC) - 10000);
 Temp = log(Resistance);
 Temp = 1 / (0.001129148 + (0.000234125 * Temp) + (0.0000000876741 * Temp * Temp * Temp));
 // Convert Kelvin to Celsius
 Temp = Temp - 273.15;
 // Convert to Fahrenheit
 Temp = (Temp * 9.0)/ 5.0 + 32.0;
 return Temp;
}

void printDouble(double val, byte precision) {
  Serial.print (int(val));  //prints the int part
  if( precision > 0) {
    Serial.print("."); // print the decimal point
    unsigned long frac, mult = 1;
    byte padding = precision -1;
    while(precision--) mult *=10;
    if(val >= 0) frac = (val - int(val)) * mult; else frac = (int(val) - val) * mult;
    unsigned long frac1 = frac;
    while(frac1 /= 10) padding--;
    while(padding--) Serial.print("0");
    Serial.print(frac,DEC) ;
  }
}

void setup() {
  Serial.begin(115200);
}

#define ZoneOne 1   // Analog Pin 0
#define ZoneTwo 2   // Analog Pin 1
#define ZoneThree 3   // Analog Pin 2

double temp;
int inByte = 0;
int sensorDelay = 10;

void loop() {
  if (Serial.available() > 0) {
    inByte = Serial.read();
    Serial.print("Zone1:");
    printDouble(Thermistor(analogRead(ZoneOne)),3);
    Serial.print(" ");
    delay(sensorDelay);
    Serial.print("Zone2:");
    printDouble(Thermistor(analogRead(ZoneTwo)),3);
    Serial.print(" ");
    delay(sensorDelay);
    Serial.print("Zone3:");
    printDouble(Thermistor(analogRead(ZoneThree)),3);
    Serial.print(" ");

    Serial.println("");
  }
}

I had an old laptop with a dead battery that I wasn’t using, so I loaded it with Ubuntu 10.10 x64, Apache, MySQL, PHP and Cacti and put it up in the attic, connected to the Arduino via USB.  I went through the painfully dry Cacti Documentation and wrote a custom Perl script that Cacti uses to poll the Arduino for temperatures:

#!/usr/bin/perl

use Device::SerialPort;
use Time::HiRes qw( usleep );

# Serial Settings
$PORT = "/dev/ttyUSB0";
$ob = Device::SerialPort->new ($PORT) || die "Can't Open $PORT: $!";
$ob->baudrate(115200)   || die "failed setting baudrate";
$ob->parity("none")    || die "failed setting parity";
$ob->databits(8)       || die "failed setting databits";
$ob->handshake("none") || die "failed setting handshake";
$ob->write_settings    || die "no settings";
$ob->read_const_time(50);

while(1){
        $ob->write("x");
        if($_ = $ob->read(255)){
                chomp;
                print "$_\n";
                undef $ob;
                exit;
        }
        usleep(1000 * 10);
}

undef $ob;

Cacti Graph of my Home's Temperature

Lastly, I put a nice graph together that shows a comparison of the three temperatures over time, with a composite average of all three.  Now I can see that my daughter’s room spikes to nearly 90 deg when the heat comes on – ouch!  Using this data I am better able to adjust the vents in each room to keep the temperature consistent throughout the house.  Next I want to take one of those cheap LCD picture displays and tie it to the Cacti graph and put it near the thermostat so I can think twice about turning on the heat or AC!