Here is a copy and paste code for arduino uno kit to make a speed regulated tachometer for DC motors. Algorithm of program is like this, we connect a variable resistor with a ADC pin of the arduino UNO and set it as input pin through coding. This is converted into digital value in the form of an integer. Using IR proximity sensor, we read the speed of DC motor. DC motor is again controlled by a power transistor (BD139 for 1 ampere current). Speed is controlled by PWM signal and PWM is generated by the help of the integer value that we get from variable resistor.

#include <LiquidCrystal.h>
//Assigning pins
int ledPin = 13;
int in_ir = 7; 
int motor = 6; 
int adc_input=A0;
LiquidCrystal lcd(8, 9, 10, 11, 12, 13); /// REGISTER SELECT PIN,ENABLE PIN,D4 PIN,D5 PIN, D6 PIN, D7 PIN

//variables
int timer1_counter;
unsigned int s=0;

unsigned int pwm_timing=0;
unsigned int pwm_duty=0;
unsigned int rpm_set=0;
unsigned int rpm_read=0;
char rpm_read_status = 0;
//timer

void setup()
{
  pinMode(ledPin,OUTPUT);
  pinMode(motor,OUTPUT);
  pinMode(in_ir,INPUT);
  pinMode(adc_input,INPUT);
  

  lcd.begin(16, 2);         // initializes the 16x2 LCD
  lcd.setCursor(2,0);       // sets the cursor at column 2 row 0
  lcd.print("IR TACHOMERTER  "); // prints temperature
  

  // initialize timer1 
  noInterrupts();           // disable all interrupts
  TCCR1A = 0;
  TCCR1B = 0;

  // Set timer1_counter to the correct value for our interrupt interval
  //timer1_counter = 64911;   // preload timer 65536-16MHz/256/100Hz
  //timer1_counter = 64286;   // preload timer 65536-16MHz/256/50Hz
  timer1_counter = 34286;   // preload timer 65536-16MHz/256/2Hz
  
  TCNT1 = timer1_counter;   // preload timer
  TCCR1B |= (1 << CS12);    // 256 prescaler 
  TIMSK1 |= (1 << TOIE1);   // enable timer overflow interrupt
  interrupts();             // enable all interrupts
}

ISR(TIMER1_OVF_vect)        // interrupt service routine 
{
  TCNT1 = timer1_counter;   // preload timer
  s++;
  //digitalWrite(ledPin, digitalRead(ledPin) ^ 1);
}

void loop()
{
 
 if(digitalRead(in_ir) == LOW) {//read rpm senssor
   rpm_read++;
   while(digitalRead(in_ir) == LOW){;}
 }

  if((rpm_read * 12) > rpm_set){
       pwm_duty++;
       if(pwm_duty > 65000){pwm_duty=65000;}
  }
  else {
       pwm_duty--;
       if(pwm_duty < 1){pwm_duty=1;}
  }

 if(pwm_timing < 10){
      digitalWrite(ledPin, LOW);
      digitalWrite(motor, LOW);
 }
 if((pwm_timing > 10) && (pwm_timing > pwm_duty)){
      digitalWrite(ledPin, HIGH);
      digitalWrite(motor, HIGH);
      pwm_timing=0;
      delay(10);
 }
 pwm_timing++;
 
    if(s%5==0){
      if(rpm_read_status == 0){
        lcd.setCursor(8,1);
        lcd.print("                ");
        lcd.setCursor(8,1);
        lcd.print("R=");
        lcd.print((rpm_read * 12)); 
        delay(10);
        rpm_read=0;
      }
      rpm_read_status=1;
    }
    else {rpm_read_status=0;}
    
    if(s%2==1){
      lcd.setCursor(0,1);
        lcd.print("        ");
       lcd.setCursor(0,1);       // sets the cursor at column 2 row 0
        lcd.print("S=");
        rpm_set=analogRead(adc_input); // reads the sensor output
        lcd.print(rpm_set); 
        delay(10); 
    }
   
}