Sorry, i dont understand that formula. Its a bit more complicated than i thought.

Ive been reading a few webpages (and re-reading this thread) and i think im starting to understand how it works. I like the 555 idea but if i need to send it in bursts then ill need 2 555s (1 for the 38.5khz frequency and 1 to control the bursts). Other sites also recommend a variable resistor for fine tuning.

I think its best if i do use the PIC chip. I do still have the MCLR pin which can be used as an input only (for a sensor). Hopefully one of the pins on this chip has hardware PWM and hopefully that works how i think it does. That way i can adjust the frequency to suit and have the burst thing without having to make the PCB bigger and use extra components... and i wouldnt have to figure that equation out

Im going to put an order in soon and get some new emitters/PTs and ill see how it goes. No doubt ill be back for more help when it doesnt work :P