Using IR sensor and IR receiver to measure the speed of the shaft

Other Hardware Science and Measurement
1

Hi,
my code is

float value=0;
float rev=0;   
int rpm;
int oldtime=0;
int time;

#include<SD.h>

void isr() //interrupt service routine
{
rev++; //Each rotation, this interrupt function is run twice, so 
//take this into consideration 
//when calculating ROC Disk Speed
//update count
}
void setup()
{
SD.begin(); //initialise the SD
Serial.begin(9600); //initialise the serial communication
attachInterrupt(0,isr,FALLING);  //attaching the interrupt
//Interrupt 0 is digital pin 2, so that is where the IR detector is connected
//Triggers on FALLING (change from HIGH to LOW)
}

void loop()
{
  delay (1000);                 //Do not process interruptions during calculations

detachInterrupt(0);           //detaches the interrupt
millis();
time=millis()-oldtime;        //finds the time 
rpm=(rev/time)*60000;      //calculates rpm
oldtime=millis();             //saves the current time
rev=0;                        //reset the revolutions counted
Serial.print("ROC BD    " ); 
Serial.print(rpm);
Serial.println(" RPM");
attachInterrupt(0,isr,FALLING); //restart the interrupt processing
}

The circuit diagram is attached. Two variations from the attached circuit are:
(a) Instead of using the OLED screen, I am trying to log the data in the SD card.
(b) I have no switch on my circuit.

I am only getting values around 3000rpm even for a speed that should be for less than 100rpm.

Please advice, I am a novice in this area. I need the tachometer for my postgraduate project.

Thanks.

circuit diagram for making measuring RPM with IR LED and IR receiver.pdf (82.6 KB)

2

Please do an autoformat before posting, makes your code a lot more readable. Formatting is a mess.

You don't need to detach and then attach the interrupt. That's very bad practice. The normal way is to disable interrupts, copy the value to another variable, and re-enable interrupts:

noInterrupts();
int revCount = rev;
rev = 0;
interrupts();

Then the rpm calculation is probably going wrong as you're using 16-bit integers, and 60,000 is awfully close to the 65,535 limit of an unsigned int. As you're doing a division you probably best make the whole thing floating point:

rpm=(rev/time)*60000.0;      //calculates rpm

Otherwise, making that 60,000 an unsigned long should also do the job forcing the compiler to use a big enough variable:

rpm=(rev/time)*60000ul;      //calculates rpm
3

Oh, and rev of course has to be declared volatile. Forgot that one. That may be another source of problems.

4

Thank you for the response. I will incorporate your comments in my code and see what happens.