pulse reading problems

Projects General Guidance
1

Not sure if this is the right section of the forum?
I am trying to monitor the RPM of a shaft on which I have a magnet and reed relay.
Here is the code

//-----------------------------------------------

 volatile byte rpmcount;

 unsigned int rpm;

 unsigned long timeold;
unsigned long timeTemp;
 void setup()
 {
   Serial.begin(9600);
   attachInterrupt(0, rpm_fun, RISING);

   rpmcount = 0;
   rpm = 0;
   timeold = 0;
   timeTemp = 0;
 }

 void loop()
 {
   if (rpmcount >= 10) { 
     
     rpm = 30*1000/(millis() - timeold)*rpmcount;
     
     timeTemp = millis();
     Serial.print("millis ");
     Serial.println(millis());
     
     Serial.print("timeold ");
     Serial.println(timeold);
     
     Serial.print("rpmcount ");
   Serial.println(rpmcount);

     
     timeold = millis();
     rpmcount = 0;
   }
     
 }

 void rpm_fun()
 {
   rpmcount ++;
  
 }

//-----------------------------------------------

I am using the standard set up with a 10K resistor.
My problem is with between 3 to 5 slow turns ie pulses it goes through the if (rpmcount >= 10) so it must be getting noise. I have tried a push button switch and get the same problem. I have also tried a 10ms delay with similar results.

Would it be better to use an analoge pin and use PMW? or what other way to get clear pulse signals?
Thank you

2

Perhaps you should switch from a mechanical reed switch to a Hall-effect sensor. That should help eliminate contact bounce.