Use any Arduino pin for transmission with the IRremote libary by soft PWM

I really love Ken Shiriffs IRremote libary for Arduino but it had a dealbreaking limitation for my current project: it makes use of the PWM functionality and is therefore bound to pin 3. As I really needed all PWM pins for other stuff and didn’t want to upgrade to a Mega board, I worked out this hack.

A IR signal consists of on- and off states and that is literally switching the IR LED on and off in special periods. While a off state is actually switching the LED off at all, a on state is more like blinking with a really high frequency of 38kHz. This is a typical task for PWM but after all it is again nothing else than just switching it on and off.

First at all we have to find out how fast we have to switch on/off with this frequency.

36000 / 1s = 1 / n * s
n = 1/36000s =  27,7 μs

So we have to switch it on and off every 27,7μs (fortunately the delayMicroseconds function works accurately for values above 3μs) and it’s simple as this:

while (OnStateDuration) {
  digitalWrite( sendPin, HIGH);
  delayMicroseconds(14);
  digitalWrite( sendPin, LOW);
  delayMicroseconds(14);
}

=== Instruction ===
You can download the modified libary here or go through these steps:

Replace the following methods in IRremote.cpp:

void IRsend::mark(int time) {
  unsigned long n = micros();
  while (micros()-n < time) {
    digitalWrite(irparams.sendpin, HIGH);
    delayMicroseconds(irparams.pulselength);
    digitalWrite(irparams.sendpin, LOW);
    delayMicroseconds(irparams.pulselength);
  }
}

void IRsend::space(int time) {
  digitalWrite(irparams.sendpin, LOW);
  delayMicroseconds(time);
}

void IRsend::enableIROut(unsigned int khz) {
  irparams.pulselength = 500 / khz;
  pinMode(irparams.sendpin, OUTPUT);
  digitalWrite(irparams.sendpin, LOW); // When not sending PWM, we want it low
}

IRrecv::IRrecv(int recvpin)
{
  irparams.recvpin = recvpin;
  irparams.blinkflag = 0;
}

Add the sendpin parameter to the constructor signature in IRremote.h:

class IRsend
{
public:
  IRsend(int sendpin);

Define the sendpin and period variables in IRremoteInt.h:

typedef struct {
  uint8_t sendpin;
  unsigned int pulselength;

When using in a sketch, you have to specify the pin for transmission:

IRsend irsend;  // deprecated
IRsend irsend(10);  // send on pin 10

======

PS: If you’re experimenting with IR you might find this useful. You can use an ordinary red LED instead of a IR LED on short a short distance of let’s say about 2 or 3 cm. I accident found this and I’m very happy about not needing to use my mobile phones camera to check the IR light any more.