MAS3:
You can change the frequency, that is what VR1 does.
It trims the frequency to get what you want.
I can't tell if it can do a 1.25 % increase of the frequency, but you can if you build this and test it with your fireplace.
Yes, i understand that, but i want to change it by sending the signal from an other pin on the arduino, not by a physical button.
pin 3 HIGH = 40kHz
pin 4 HIGH = 40.5 kHz