This post will give you some code to generate high frequency square waves for IR sensors as well as explain how the Fast PWM mode works for Arduino.
I needed to interface with some IR sensors when designing the high tech mini golf hole and had to generate a 38 kHz square wave to drive the infrared LED so that it could be seen by the TSOP4838 IR receivers.
The basic Arduino analogWrite function is limited to less than 1 kHz (depending on your board), so you need to directly access one of the other on board Timer/PWM modules. The Arduino Uno (ATmega328) has 3 timer/counters:
- Timer/Counter0: 8 bit, already used by Arduino for timing functions like millis() and delay().
- Timer/Counter1: 16 bit, which is more than I need for this, so I'd rather save it for something else.
- Timer/Counter2: 8 bit, fits my needs perfectly.