Interrupt fires multiple times

Question

I have a Photo Interrupter which I am using to count the RPM of a motor.

I am incrementing an integer to show the amount of interrupts there have been.

The issue is that there can be several interrupts fired for each time the "beam" of the Photo Interrupter is broken.

I have a 5mm length of plastic which breaks the "beam". If I make the plastic shorter than that, the Photo Interrupter doesn't pick up the break.

I am using the following, very basic code:

int pin = 13; 
volatile  int tcnt = 0; 
volatile int state = LOW; 

void setup() { 
  Serial.begin(9600);
  pinMode(pin, OUTPUT); 
  attachInterrupt(0, blink, CHANGE); 
} 

void loop() { 
  Serial.println(tcnt); 
  digitalWrite(pin, state); 
} 

void blink() { 
  tcnt = tcnt + 1; 
  state = !state; 
}

Answer 1

I would recommend adding a comparator with hysteresis so as to get a clean digital pulse, pictured below:

^{simulate this circuit – Schematic created using CircuitLab}

This has the correct values.

I recommend the comparator with hysteresis as the phototransistors are prone to cause issues with interrupts even when used with pul-up/pull-down resistors, thus is as they very the voltage more than give a clean 0/1 digitally.

Here is your code, modified with the addition of RISING and of a 10mS delay.

int pin = 13; 
volatile  int tcnt = 0; 
volatile int state = LOW; 

void setup() { 
  Serial.begin(9600);
  pinMode(pin, OUTPUT); 
  attachInterrupt(0, blink, RISING); 
} 

void loop() { 
  Serial.println(tcnt); 
  digitalWrite(pin, state);
   delay(10);
} 

void blink() { 
  tcnt = tcnt + 1; 
  state = !state; 
}

This method produces very reliable results. It is also accurate with objects moving rapidly because of the interrupt.

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I would also say that the problem you have when you use a piece of plastic < 5mm is that the length of the slit in the sensor is 6.0±0.1(seen below). If the plastic is less you probably find that the phototransistor still has enough IR falling on it to allow voltage to flow, but enough to trigger and interrupt. With the comparator you could probably use a smaller piece of plastic and find the voltage swing with a DMM and modify the voltage divider (R1 and R2) to suite the needs.

Answer 2

Two other possible problems, apart from what Jake C has adressed in his answer are:

1. Hardware bounce

If your signal from the sensor isn't a clean transition from high to low but rather "bouncy" (see image below). The microcontroller will register multiple transitions in the meantime.

Solution to this would be to add a small capacitor (wil slow it down though) or add debouncing code... Best would be to get a "clean" sensor.

2. Floating contact

When I even move my finger close to the wire running to PIN2 the interrupt fires.

That kinda seems like it's a floating contact, where it's not really connected to anything.

This is usually solved by using a pullup/pulldown resistor. Or maybe there is something wrong with the connection.

---

Also:

Maybe due to the type of sensor?

You should note the part number of the sensor or link a datasheet, so we can see if it's typical behaviour for this type of sensor.

Answer 3

As far as it firing multiple times, are you aware that you have the interrupt mode set to CHANGE? This means the interrupt will fire once when something enters the beam, and again when something leaves the beam. You will probably want to set it to RISING or FALLING so that it only fires on one of those events, not both.

As to the not detecting issue I can think of a few reasons why this could be:

1. Your motor is moving so fast that the sensor changes again before it has had the chance to finish the interrupt service routine and reset the interrupt on the pin.
2. The photo interrupter you are using is too slow to report the piece of plastic passing through it. Sensors like this usually indicate a rise time and a fall time, and if it moves through the beam faster than these times it will not register the interruption.
3. The piece of plastic isn't large enough to block the beam. Depending on the photo interrupter, the beam used could be laser like and very narrow, or it could be wide and more like a flashlight. If it is more like the latter, you will need to have enough material present to block enough light to set off the sensor.

Answer 4

If you want to count RPM, maybe this link will help you: http://rjbotics.blogspot.in/2015/07/counting-real-time-rpm-with-ir-sensors.html