IR Laptimer using Arduino

So here’s my problem, I love karting and take part in lots of different races from corporate grand prix to the British 24hr and anywhere in between. I wanted to record and display my lap times but didn’t want the expense of buying an off the shelf unit at over £500 (cheaper ones are available but I needed one which would record 24 hours’ worth of racing and more than 1000 laps). I also like to tinker with electronics so I decided I would see if I could make something instead (and hopefully it would be cheaper too).

Firstly I needed to work out which type of system to make, Infra-Red or magnetic pick up. Most karting tracks have a magnetic loop fitted in the track so in theory that meant using a hall effect sensor to pick it up. The problem is some track don’t have a loop and some have 2 or 3 loops (for intermediate timing) so this wouldn’t fulfil my requirements. I decided on the Infra-Red option as it was already used in professional systems and I could take it to any track, the down side is that I now have to build an Infra-Red transmitter as well.

Having then researched the IR option further I came across a very useful blog where Jim Ele had already built a similar timer which utilised an existing IR transmitter but used an Arduino as the processor. This would form the base of my build.

The Transmitter

The first part was to build the IR transmitter which would sit on the pit wall repeatedly sending out a set code. I simply built a home brew circuit board with the bare minimum to run the Arduino’s Atmeg328p processor as per the diagram below and attached 4 AA batteries for the power. I used 2 IR LED’s purchase from Maplin’s to send the signal out.

The power circuits have been simplified for the diagram

The next problem to overcome was how to avoid false triggers from other IR sources, I.e. other IR transmiters that could be at the circuit and also things like the Sun / strip lights etc. The answer was to use a 38Khz carrier frequency with a matched receiver and send out a specific code that the receiver would recognise (The same way your tv knows that you’ve pressed a certain button on the remote). Because the receiver isn’t in the beam for very long I needed a short code which could be received very quickly. After again looking at what Jim Ele had found out I decided to give 3 ‘On’ pulses each 416µs with a 1200µs ‘Off’ gap between each. After the 3 pulse there is a 6ms gap an then the code repeats itself. I’ve tried to show this visually below.

I housed all this inside a suitably side housing purchased from Maplin and attached an old mini tripod as a stand. Waterproofing is done by covering the whole box is a clear plastic bag with the 2 LED tubes sticking out. The total cost was less than £20 and has a range of about 20 meters. The AA batteries easily last for more than the 24hours I needed (more than 50 hours actually).

Finished Transmitter, The two small plastic tubes sit over the LEDs and are mainly for Aesthetics.

Transmitter Sketch

The Receiver

As the beam was using a 38Khz carrier frequency I needed an IR receiver that was set to that frequency so I opted for the TSOP38238 as it was easily available for a low price and had good performance and will run at 3.3v. The TSOP38238 receives the signal from the transmitter and inverts the signal as shown below.

 I housed this about half way inside a separate small box with a small window so that it could be attached to the front panel of the kart for best view of the transmitter. This also meant it wasn’t in direct sunlight. It was sealed closed to make it waterproof. A 3 core lead then connected it directly to the main unit on the steering wheel.

The Main Unit

Again I started with a basic circuit to support the Atmega328p processor. I then added a 16x2 LCD display module with LED backlighting (be careful here as the backlighting is the main current draw and can easily exceed 100mA with some models. I choose a module that had a single LED for backlighting to reduce the current consumption for optimum battery life). To record the lap times I added a microSD module. The microSD card also contains a text file which can be altered to tell the unit how long to ignore the receiver for (I’ll come back to this later)

The power circuits have been simplified in this diagram

Note: The system runs at 3.3v not 5 as per the diagram. Because of the change to 3.3v I also changed the IR receiver to a TSOP38238.

This was then housed into suitably sized box from Maplin with foam padding around the circuit board to protect it from vibration.

I then used a second identical box to house the batteries which was then mounted onto the back of the first box around the steering wheel. This meant it could be swapped from one kart to another very quickly and even attached to hire karts.

Originally I wanted to use 9v pp3 batteries for the power but an initial test showed these couldn’t stand the vibration and they destroyed themselves within an hour. I swapped these out for an RC car Ni-mh receiver pack battery – 6v 2500mAh and again wrapped it in foam. This works great and lasts over 30 hours per charge.

How the system works

As the receiver picks up the beam from the transmitter, the sketch checks the length of each pulse and counts them if they are the correct length. Once its counted to 3 it knows a ‘good’ code was received. If a pulse doesn’t fit the right time frame or no further pulses are received then the count is reset. After receiving a good code the sketch then works out the lap time (the time between the last ‘good’ code and this one) to the nearest 1000^th of a second, increments the lap counter, works out the delta time to the fastest lap and adds it to the total time expired since the start of the race in hours and minutes. This is then displayed on the screen and also stored onto the micrSD card as a .csv file type and can be opened in excel. The system then ignores any signals from the IR receiver for a period of time, this period of time was set by the text file on the SD card as mentioned earlier. This time should be set to the best possible lap time minus 3 to 4 seconds. This way the IR cannot receive any false signals from other transmitters. About 4 seconds before you pass through the beam again the system starts looking for a beam again.

Receiver Sketch

A Note on using microSD cards: When reading and writing SD card can use quite a high current (200mA or more) so make sure your voltage regulator can handle it! Also they run at 3.3v not 5v so I run the whole system at 3.3v which means you need an LCD module capable of running at that voltage. I also used the SDFat library for this project not the standard Arduino one as the SDFat library (available from GitHub) puts the microSD card to sleep between read/write events which saves a lot of current and prolongs the battery life. The standard SD library keeps the card awake by default and it makes quite a difference.

Finally to make the unit rainproof I printed the front panel onto OHP transparency in reverse. The text was left blank and spray painted in silver afterwards. Turning the transparency over then reveals a nice shiny front cover with silver text and a clear window for the screen. This was double sided taped to the front and worked great.

The total cost for the unit was around £40

Further improvements coming.

I also tested a OLED 16x2 screen and this worked better than the standard LCD as no backlighting is required and its clearer in bright and dark conditions. I used blue text on a black background.

I’ve also started work on a unit with a 2.9inch graphical touch screen but its yet to make it onto a Kart. More on this one to come.

Laptimer Arduino Sketch

Sketch for the laptimer used here

Simply copy below into the Arduino sketch program.

/* Lap Timer for Team Barnato karting races by Matt Wall
Use at own risk, Sketch is in the public domain but no profit shall
be gained from using it.

 Pin allocation.
 d0 - No Connection
 d1 - display RS
 d2 - IR signal from IR receiver
 d3 - PWM signal for display backlight
 d4 - display data pin - d4
 d5 - display data pin - d5
 d6 - display data pin - d6
 d7 - display data pin - d7
 d8 - display Enable
 d9 - no Connection
 d10 - Sd Card CS
 d11 - Sd Card MOSI
 D12 - Sd Card MISO
 D13 - Sd Card CLK / SCK
 */
#include <LiquidCrystal.h>// include the LCD library
#include <SdFat.h> // include the new sd card library
SdFat sd;
float lapdelay = 20000;// set the default lap delay to 20 seconds unless updated from SD card file.
SdFile myFile; // file to read SD card lap delay data
float decade;// to be used in reading sd card
float temp;// to be used in reading sd card
float number;// to be used in reading sd card
int backlight = 3;//PWM for display backlight to be connected to pin 3
int lapcount = 0;// start with a lap count of 0
long start = 500;//set values (system will random asign if left blank) will be changed later
long finish = 0;//set values (system will random asign if left blank) will be changed later
long laptime = 0;//set values (system will random asign if left blank) will be changed later
long elapsed1 = 0;//set values (system will random asign if left blank) will be changed later
long elapsed2 = 0;//set values (system will random asign if left blank) will be changed later
long oldlaptime = 0;//set values (system will random asign if left blank) will be changed later
long delta = 0;//set values (system will random asign if left blank) will be changed later
#define IRpin_PIN      PIND // assign pin 2 raw mapping to IRpin 2
#define IRpin          2

#define MAXhighPULSE 2000 // Time counts to reset tokens to zero if no beam detected incase of false trigger, 2000 * RESOLUTION = 40ms
#define MAXlowPULSE 28 // the maximum pulse count we need. 560us / RESOLUTION = 28
#define MINlowPULSE 18 // the minimum pulse count we need. 360us / RESOLUTION = 18

#define RESOLUTION 20 // Multiplier - will be used to take reading of IRpin every 20ish microseconds
int lowpulse = 0; // start with 0 for the pulse length.
int highpulse = 0; // start with 0 for the pulse length.
int token = 0; // start with 0 count for tokens. We'll need 3 tokens to trigger the timer.

// initialize the library with the numbers of the interface pins
LiquidCrystal lcd(1, 8, 4, 5, 6, 7);
const int chipSelect = 10; //choose the chip select pin for the SD card
SdFile myfile;

void setup() {
  // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  pinMode(backlight, OUTPUT);// Output for the PWM on the backlight
 // pinMode(10, OUTPUT);// must be set fot the SD Card to work
  analogWrite(backlight,80);
  lcd.print("  Team Barnato"); // Print a message to the LCD.
  delay(2000);// Wait 2 seconds before clearing the LCD.

  if (!sd.begin(chipSelect, SPI_HALF_SPEED)){// start up the sd card
    lcd.clear();
    lcd.print("SD Card failed!");// Incase no SD card or problem with SD Card
    delay(10000);
  }
  else{ //SD Card is ok
    lcd.clear();
    lcd.print("   SD Card OK");// tell the world the sd card is ok
    delay(2000);//give people time to read the message
  }
  if (sd.exists("times.csv")){//check if the file to record the lap times in already exists?
    lcd.clear();
    lcd.print("    File OK");//tell the world it exists
    delay(2000);//give people time to read the message
  }
  else{ //it doesn't exist
    lcd.clear();
    lcd.print(" Creating File");//tell the world your creating the file
    myfile.open("times.csv", O_RDWR | O_CREAT | O_APPEND);// create the file
    myfile.close();// save the file
    delay(2000);//give people time to read the message
    if (sd.exists("times.csv")){// re check that the file exists after creating
    lcd.clear();
    lcd.print("    File OK");//tell the world it exists
    delay(2000);//give people time to read the message
   }
    else{// file still not available
      lcd.clear();
      lcd.print("File Error");// tell the world the is a problem
      delay(5000);// give people time to read the message
    }
  }

  if (myFile.open("lapdelay.txt",O_READ)) // Open SD Card and retreive lap delay information.
    {
      lapdelay = 0; // if the file is ok on the sd card then set lapdelay at zero ready for reading the correct delay
      decade = pow(10, (myFile.available() - 1));
      number = 0;
      while(myFile.available())
      {
       temp = (myFile.read() - '0');
       number = temp*decade+number;
       decade = decade/10;
      }
      lapdelay = number*1000;
      myFile.close();
    }
  lcd.clear();
  lcd.print("   Target Lap");
  lcd.setCursor(4, 1);
  lcd.print(lapdelay/1000);
  lcd.print(" secs");
  delay(2000);
  lcd.clear();
  lcd.print(" Ready To Race");
  lcd.setCursor(0, 1);
  lcd.print(" -- Out  Lap --");
}

void displayResult(){
  /*This procedure will calculate the laptime, delta time, display the result to the LCD screen, record the lap time to SD card (not yet working)
   and finally set a system delay to allow you to pass the start / finish straight without re triggering from other beams*/
  laptime=finish-start;// Calculate lap time
  if (lapcount>0){// will display times after completing the first flying lap.
    //This section works out the actual lap time
    float m1,s1,ms1;// create variables for hours,minutes,seconds and milliseconds
    unsigned long over1;// create variable for the calculation over spill
    elapsed1=laptime;
    m1=int(elapsed1/60000); //Calculate the minutes
    over1=elapsed1%60000; // Work out the remainder
    s1=int(over1/1000);// Calculate the seconds
    ms1=over1%1000;//work out the milliseconds
    lcd.clear();
    lcd.setCursor(0, 0);// Set cursor to top left position.
    lcd.print("LAP");//Print text to LCD
    lcd.setCursor(9, 0);
    if (oldlaptime<laptime)// If the last lap was less than the new lap then the delta needs to be a plus value
    {
      delta=laptime-oldlaptime;
      lcd.print("+");//shows these values on screen if new lap was slower then previous lap
    }
    else// If the last lap was more than the new lap then the delta needs to be a minus value
    {
      delta=oldlaptime-laptime;
      lcd.print("-");// shows these if your new lap was quicker then the last
    }
    float s2,ms2;// create variables for seconds and milliseconds
    unsigned long over2;// create variable for the calculation over spill
    s2=int(delta/1000); // work out the delta time
    ms2=delta%1000; //work out the milliseconds
    lcd.print(s2,0);//print delta seconds
    lcd.print(".");
    if (ms2<100){lcd.print("0");}// if milliseconds is below 100 we need to add a zero
    if (ms2<10){lcd.print("0");} // if milliseconds is below 10 we need to add another zero
    lcd.print(ms2,0);
    lcd.setCursor(0, 1);// move cursor to second line first character
    lcd.print(lapcount);
    lcd.setCursor(8, 1);// move cursor to second line 8th character
    lcd.print(m1,0);
    lcd.print(":");
    lcd.print(s1,0);
    lcd.print(".");
   if (ms1<100){lcd.print("0");}// if milliseconds is below 100 we need to add a zero
    if (ms1<10){lcd.print("0");} // if milliseconds is below 10 we need to add another zero
    lcd.print(ms1,0);
    if (myfile.open("times.csv", O_RDWR | O_CREAT | O_APPEND)){// Open SD Card File
      myfile.print("Lap,");//Start line with Lap
      myfile.print(lapcount);//add the lap count to the file
      myfile.print(",");//add a comma for use in excel later
      myfile.println(laptime);//save laptime and start new line
      myfile.close();// close and save the file
    }
  }
  else { // this section is only used during the first flying lap.
    lcd.clear();
    lcd.print("   First  Lap");
    lcd.setCursor(0,1);
    lcd.print(" Timer  Running");
  }
  // This line reserved for entering the save to SD card routine
  start=finish; // set the new lap start time to the same value as the last lap finish time
  oldlaptime=laptime;// store new lap time as old lap time for next lap
  lapcount++;// increase lap count by 1
  token = 0; // reset token counter ready for next lap
  delay(lapdelay);// delay to prevent re triggering if other beams are present.
}

void loop() {
  highpulse = lowpulse = 0; // start out with no pulse length
  while (IRpin_PIN & (1 << IRpin)) { // while irpin is high - no beam is beam is being received.

    highpulse++;
    delayMicroseconds(RESOLUTION);// wait some time, the multiplier.

    if (highpulse >= MAXhighPULSE) { /* check to see if no signal has been received for over 40ms (we should
     recieve 3 tokens in 4ms, so if we only receive one or two tokens in 40ms it must be a false signal),
     then reset the token count if true as no/false signal is being received.
     */
      token = 0;
      highpulse = 0;// reset high pulse counter
    }
  }
  while (! (IRpin_PIN & _BV(IRpin))) {// while irpin is low, signal being received
    lowpulse++;
    delayMicroseconds(RESOLUTION);
  }
  if ((lowpulse >= MINlowPULSE)  && (lowpulse <= MAXlowPULSE)) {// was the pulse length what we expected?
    token ++; // if so then count 1 token, we need 3 tokens to trigger
  }
  if (token == 3) { // If we receive 3 correct pulses then trigger the lap timer.
    finish=millis();// Record the time
    displayResult();// run through the display procedure
  }
}

IR Transmitter Sketch

Sketch for the IR transmitter used here

Simply paste below into the arduino sketch window.

/* This is the beam pattern for the Team Barnato timing beacon. Use at own risk and no warranties are given.
in Micro Seconds
On - 468 uS
Off - 1200 uS
On - 468 uS
Off - 1200 uS
On - 468 uS
Off - 6000 uS */

int IRledPin =  3;    // LED connected to digital pin 3
int powerLed = 13;    //Power LED is connected to pin 13

void setup()   {              
  // initialize the IR digital pin as an output:
  pinMode(IRledPin, OUTPUT);
  pinMode(powerLed, OUTPUT);
  digitalWrite(powerLed, HIGH);
  delay(2000);
  digitalWrite(powerLed, LOW);
}

void loop()                  
{
  pulseIR(468);
  delayMicroseconds(1200);
  pulseIR(468);
  delayMicroseconds(1200);
  pulseIR(468);
  delayMicroseconds(6000);
}

// This procedure sends a 38KHz pulse to the IRledPin
// for a certain # of microseconds.
void pulseIR(long microsecs) {
  // we'll count down from the number of microseconds we are told to wait

  cli();  // this turns off any background interrupts

  while (microsecs > 0) {
    // 38 kHz is about 26 microseconds - to give 50% duty cycle 13 microseconds high and 13 microseconds low
   digitalWrite(IRledPin, HIGH);  // this takes about 3 microseconds to happen
   delayMicroseconds(10);         // hang out for 10 microseconds
   digitalWrite(IRledPin, LOW);   // this also takes about 3 microseconds
   delayMicroseconds(10);         // hang out for 10 microseconds

   // so 26 microseconds altogether
   microsecs -= 26;
  }

  sei();  // this turns them back on
}