int switchPin = 8; // Hall effect sensor pin
int switchState = LOW; // set switch to LOW
int revolution = 0; // number of revolutions
float circum = 2.16; // tyre circumference
unsigned long time = 0; // time (ms) per revolution
float speed, rpm;
int reading;
void setup()
{ // pull-up resistor on hall
pinMode(switchPin, INPUT_PULLUP); // effect sensor pin
Serial.begin(9600); // set baud rate for Serial Monitor
}
void loop()
{
reading = digitalRead(switchPin); // read Hall switch
if(reading != switchState) // switch state changed
{
if (reading == HIGH && switchState == LOW)
{ // start of new revolution
revolution = revolution +1; // increment number of revolutions
time = millis() - time; // time (ms) since last revolution
speed = 3600.0*circum/time; // speed calculationin km/h
rpm = 60000.0/time; // revolutions per minute
Serial.print(revolution); // print number of revolutions
Serial.print("\t"); // and a tab
Serial.print(speed,1); // print speed to 1DP
Serial.print("km/h\t"); // with " km/h" and tab

Serial.print(rpm,0);Serial.println("rpm"); // print rpm, "rpm"
time = millis(); //update revolution time
}
switchState = reading; // update hall switch state
}
}