File content as of revision 1:034f79c00c98:

// Scalextric Timer for Ada Lovelace Day
//
// Uses Nucleo board F072 with LCD Keypad shield
// Connect D3 to Trig & D2 to Echo
// Connect VCC to 5V (pin CN7 18)
// Connect GND to 0V (pin CN7 19)

#include "mbed.h"
#include "hcsr04.h"     // Ultrasonic sensor
#include "TextLCD.h"    // LCD1602

// serial PC for debug
Serial pc(SERIAL_TX, SERIAL_RX);
// Ultrasonic sensor
HCSR04  usensor(D3,D2);  // Trig, Echo

// LCD Shield
TextLCD lcd(D8, D9, D4, D5, D6, D7);   // used to drive display
AnalogIn keys(A0);                     // used to sample buttons

Timer lap_timer;
Timer wait_timer;

// display text on LCD
void textLCD(char *text, int line) {
    char tmpBuf[16];
    for (int i = 0; i < 16; i++) tmpBuf[i] = 0x20;
    for (int i = 0; i < strlen(text); i++) {
        if (i < 16) tmpBuf[i] = text[i];
        lcd.locate(i, line);
        lcd.putc(tmpBuf[i]);
    }
}


int main()
{
    // Variables used
    int dist = 0;
    float current_time = 0;
    int lap_count = 0;
    float wait_time = 2.0;
    pc.baud(9600);
    char lap_string[16];
    char laptime_string[16];
    
    // Values to tune
    float min_laptime = 1.0; // Minimum laptime - time to wait after seeing car before measuring again in seconds
    int sample_time = 100;   // Sample time - time between sampling value from sensor in milliseconds
    int trigger_distance = 4;  // Trigger distance - distance below which the car registers as detected

    // Intro Screen
    pc.printf("ARM Sheffield Scalextric Challenge \n");
    textLCD("ARM Scalextric ", 0);

    // Main loop
    while(1) {
       usensor.start();
       wait_ms(sample_time);
       dist=usensor.get_dist_cm();
       pc.printf("Distance %d\n\r", dist);  // Debug to Serial port/USB
       if (lap_count > 0) { 
         wait_time = wait_timer.read();
       }
       if ((dist < trigger_distance) and (wait_time > min_laptime)) { // spotted car at least a minimum laptime later
         wait_timer.reset();
         wait_timer.start();
         if (lap_count == 0) {   // First Lap - start timer
           lap_timer.reset();
           lap_timer.start();
           pc.printf("Timing... \r");        // Debug
           textLCD("Timing..........", 1);   // 
           lap_count++; 
         } else {                // Completed a lap
           current_time = lap_timer.read();
           lap_timer.reset();
           pc.printf("Lap : %d, Lap time : %f s \r", lap_count, current_time);
           sprintf(lap_string, "Lap : %d ", (char*)lap_count);
           sprintf(laptime_string, "Lap time : %f s, ", current_time);
           textLCD("                ", 0);
           textLCD(lap_string, 0);
           textLCD("                ", 1);
           textLCD(laptime_string, 1);
           lap_count++; 
        }
      }  
    }   
}