blow off circuit.

The threshold voltage will depend on the components. Using a 47k potentiometer is suggested.

High impedance pre amplifier

Electronic Dice Circuit

ISTE Creative Circuit

PAM8403_Low_Pass

//IR Tachometer 

long int value;
long int t1;
long int t2;
int blank;
void setup() {
  Serial.begin(9600);
  
  blank=analogRead(A0);
  // put your setup code here, to run once:

}
void out(long int a)
{
  float freq;
  freq= (float)1000000/a;
  
 Serial.println(freq);
 delay(1500);
 
}
void loop() {
  value=analogRead(A0);
  if(value-blank>20)//When IR bounces back from bright surface;
  {
    t1=micros();
    while(analogRead(A0)-blank>20);//Loop till the bright surface disappears
    
    while(analogRead(A0)-blank<20);//Loop till the bright surface reappears
    
    t2=micros();
    out(t2-t1);//timeperiod is sent to function out to calculate rotations per second
  }
  //Serial.println(value);
}
  
  
  
  
  // put your main code here, to run repeatedly: