Bicycle speedometer with Arduino
Click on the pictures to enlarge

Circuito (disegnato con Fritzing)

Circuit (designed with Fritzing)

Quando ho deciso di fare un tachimetro con Arduino, ho cercato con Google e quasi subito ho trovato questo progetto di John Boxall (guardate l'Esempio 37.3), che sfortunatamente non comprendeva schemi del circuito e non era stato testato con un vero interruttore magnetico.
Lo ho adattato a un tipo differente di display LCD ed ho cercato senza successo di implementare il debouncing a livello hardware e software - sfortunatamente, molto spesso l'interrupt è chiamato due volte, perché il magnete causa un secondo rialzo del voltaggio mentre si allontana dal sensore - in questo caso, la velocità sarà raddoppiata.
In ogni caso, ritengo il progetto interessante perché mostra come lavorano gli interrupt in Arduino.

When I decided to make a speedometer with Arduino, I googled around and immediately found this project by John Boxall (see Example 37.3), that unfortunately did not include circuit schematics and wasn't tested with an actual reed switch.
I adapted it to a different type of LCD display and tried unsuccessfully to implement hardware and software debouncing - unfortunately, quite often the interrupt is called twice, because the magnet causes a second raise in voltage while leaving the sensor - in that case, the speed will be doubled.
In any case, I deem the project interesting because it demonstrates how interrupts work in Arduino.

Dettaglio della micro-breadboard dove è attestato il sensore

Detail of the micro-breadboard where the sensor is connected

Dettaglio della micro-breadboard dove è attestato lo schermo LCD

Detail of the micro-breadboard where the LCD screen is connected

Source code

  Arduino Speedometer
  by Cesare Brizio | CC by-sa-nc
  this sketch is in the public domain

  Version 1.0 - 26 february 2019
  The implementation is relatively rough: 
  - wheel circumference has to be hard-coded
  - an attempt at debouncing has been made both at physical and software
    level - to no avail. Will be revised soon.
  A few more details here:
 Modified from an original by John Boxall  Example 37.3 - Basic speedometer using millis(); > chapter 37  John Boxall | CC by-sa-nc  Hardware - you will need a sensor. For example - a reed switch and magnet.  Consider the reed switch to be a normally-open button, and connect as usual  with a pull-down resistor.  ==========================  The circuit for LCD board  (HD4770 compatible display)  4-wire dialogue  ========================== * LCD RS pin to digital pin 15 * LCD Enable pin to digital pin 14 * LCD D4 pin to digital pin 5 * LCD D5 pin to digital pin 4 * LCD D6 pin to digital pin 3 * LCD D7 pin to digital pin 2 * LCD R/W pin to ground * LCD VSS pin to ground * LCD VCC pin to 5V * 10K resistor: * ends to +5V and ground * wiper to LCD VO pin (pin 3) */ // include the library code: #include <LiquidCrystal.h> // initialize the library with the numbers of the interface pins const int rs = 13, en = 12, d4 = 11, d5 = 10, d6 = 9, d7 = 8; LiquidCrystal lcd(rs, en, d4, d5, d6, d7); float start, finished; float elapsed, time; float circMetric=2.093; // wheel circumference (in meters) float circImperial; // using 1 kilometer = 0.621371192 miles float speedk, speedm;    // holds calculated speed vales in metric and imperial void setup() {  // convert metric to imperial for MPH calculations  circImperial=circMetric*.62137;  // the syntax with digitalPinToInterrupt should allow portability  //among different Arduino models - see  attachInterrupt(digitalPinToInterrupt(2), speedCalc, RISING); // interrupt called when sensors sends digital 2 high (every wheel rotation)  //attachInterrupt(0, speedCalc, RISING); // interrupt called when sensors sends digital 2 high (every wheel rotation)  //start now (it will be reset by the interrupt after calculating revolution time)  start=millis();  // set up the LCD's number of columns and rows:  lcd.begin(16, 2);  // Print a transitory message to the LCD.  lcd.print("Ardu-Speedometer");  delay(2000); //just to allow you to read the initialization message  // top serial dialogue speed improves precision  Serial.begin(115200); } void speedCalc() {  //Function called by the interrupt  if((millis()-start)>100) // 100 millisec debounce    {    //calculate elapsed    elapsed=millis()-start;    //reset start    start=millis();      //calculate speed in km/h    speedk=(3600*circMetric)/elapsed;    //calculate speed in mph    speedm=(3600*circImperial)/elapsed;    } } void loop() {  // The loop will be interrupted by the sensor each time the  // magnet passes near the sensor, in other words once per revolution  // Top line in the 16 char, 2 lines display - speed data  lcd.setCursor(0,0);  lcd.print("                ");  lcd.setCursor(0,0);  lcd.print(int(speedk));  lcd.print(" km/h ");  lcd.print(int(speedm));  lcd.print(" MPH ");  //bottom line the 16 char, 2 lines display - time data  lcd.setCursor(0,1);  lcd.print("                ");  lcd.setCursor(0,1);  lcd.print(int(elapsed));  lcd.print(" ms/rev      ");  // adjust for personal preference to minimise flicker  //delay(250); }