How to create a 38 Khz pulse with arduino using timer or PWM?

Using Arduino Programming Questions
44

I think I got it this time. You want a standard 38 KHz pulse, which itself turns on and off at 500 Hz with a duty cycle controlled by a pot? This seems to do it:

// Example of modulating a 38 KHz carrier frequency at 500 Hz with a variable duty cycle
// Author: Nick Gammon
// Date: 24 September 2012


const byte POTENTIOMETER = A0;
const byte LED = 9;  // Timer 1 "A" output: OC1A

// 16 MHz clock divided by 500 Hz frequency desired (allowing for prescaler of 128)
const long timer2_OCR2A_Setting = 16000000L / 500L / 128L;

ISR (PCINT2_vect)
   {
    
   // if pin 3 now high, turn on toggling of OC1A on compare
   if (PIND & _BV (3))
     {
     TCCR1A |= _BV (COM1A0) ;  // Toggle OC1A on Compare Match
     }
   else
     {
     TCCR1A &= ~_BV (COM1A0) ;  // DO NOT Toggle OC1A on Compare Match
     digitalWrite (LED, LOW);  // ensure off
     }  // end of if
     
   }  // end of PCINT2_vect


void setup() {
  pinMode (LED, OUTPUT);
  pinMode (3, OUTPUT);  // OC2B
  
  // set up Timer 1 - gives us 38.095 KHz
  TCCR1A = 0; 
  TCCR1B = _BV(WGM12) | _BV (CS10);   // CTC, No prescaler
  OCR1A =  (16000000L / 38000L / 2) - 1;  // zero relative
  
  // Timer 2 - gives us our 1 mS counting interval
  // 16 MHz clock (62.5 nS per tick) - prescaled by 128
  //  counter increments every 8 uS. 
  // So we count 250 of them, giving exactly 2000 uS (2 mS period = 500 Hz frequency)
  TCCR2A = _BV (WGM20) | _BV (WGM21) | _BV (COM2B1);   // Fast PWM mode
  TCCR2B = _BV (WGM22) | _BV (CS20) | _BV (CS22) ;  // prescaler of 128
  OCR2A  = timer2_OCR2A_Setting - 1;                // count up to 250  (zero relative!!!!)
  
  // pin change interrupt
  PCMSK2 |= _BV (PCINT19);  // want pin 3
  PCIFR  |= _BV (PCIF2);    // clear any outstanding interrupts
  PCICR  |= _BV (PCIE2);    // enable pin change interrupts for D0 to D7
  
}  // end of setup

void loop()
  {
  // alter Timer 2 duty cycle in accordance with pot reading
  OCR2B = (((long) (analogRead (POTENTIOMETER) + 1) * timer2_OCR2A_Setting) / 1024L) - 1;

  // other stuff here
  }  // end of loop

Results:

796×248 28.2 KB

I am outputting 38 Khz on pin 9 (output of timer 1 "A" side) and turning that on and off with a pin-change interrupt. The interrupt is generated by timer 2 "B" side which is the output of a fast PWM 500 Hz timer where the duty cycle is variable by the potentiometer input.

You can see from the image when pin 3 (timer 2) goes on and off (in this case with a 24.8% duty cycle) and that the 38 KHz signal on pin 9 is switched on and off corresponding to that.