Hey there,
I allready posted another thread regarding this project but the question evolved so I make a new one. Thanks to the people who got me where I am now project wise 
Im using this sensor: TCRT5000 IR Barrier Sensor
My code (simplified, the code is much longer):
unsigned long rpmLastReading = 0;
volatile unsigned int rpmSensorPulses = 0;
unsigned int rpm = 0;
void setup()
{
interruptPin(9);
}
void loop()
{
getRPM();
}
void interruptPin(byte pin)
{
*digitalPinToPCMSK(pin) |= bit (digitalPinToPCMSKbit(pin)); // enable pin
PCIFR |= bit (digitalPinToPCICRbit(pin)); // clear any outstanding interrupt
PCICR |= bit (digitalPinToPCICRbit(pin)); // enable interrupt for the group
}
unsigned int getRPM()
{
if (millis() - rpmLastReading > 1000)
{
unsigned int dTime = millis() - rpmLastReading;
noInterrupts();
unsigned int sPulse = rpmSensorPulses;
rpmSensorPulses = 0;
interrupts();
rpm = sPulse * 30 * dTime/1000.0;
rpmLastReading = millis();
Serial.println(rpm);
}
return rpm;
}
ISR (PCINT0_vect)
{
rpmSensorPulses++;
}
Wiring is simple: D0 (digital out of sensor) -> Pin 9 on Arduino
D0 is LOW when a black stripe is before the sensor otherwise its HIGH.
I need to get the PORT interrupt because I need an encoder simultaniously and its works like a charm on the external pins 2 & 3.
I adjusted the poti on the sensor accordingly and it works like a charm. I checked the value with the help of an anlaog reading of the sensor ( approx. 50 when "white", 600 when "black") -> the sensor has also an analog output besides the digital output. I know the sensor pulses on every change. Not rising or falling edge. Every time the black stripe gets by, there should be 1 pulse and when the black stripe leaves there should be another pulse. So each revolution has 2 pulses.
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Unfortunately the ISR gets called multiple times and it varies between 6 - 10 pulses for each change event. I check via Serial.print(). Is it possible to get a clean reading?
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Furthermore I get sort of RPM readings (approx. 60RPM) on very low speeds. If it goes higher the rpm drops to 0. Any ideas why?
