PIC Tutorial - Infrared Board
This is the Infrared Board, we need two of these, so that we can communicate between two main boards, it consists of two distinct parts. Firstly the IR receiver, comprising R1, R2, C1, and the IR receiver I/C itself (feeding port pin 2), and secondly the IR transmitter, comprising Q1, R3, R4, R5,C2, IR1, and IR2 (fed from port pin 1). If you only want to do one way communication you could build just the transmitter on one board, and just the receiver on the other, but building both on both boards gives the possibility of two way communication.
The receiver I/C detects IR signals modulated with a 38KHz signal, R2 and C1 are to provide decoupling for the supply (to avoid instability problems), and R1 is just a pull-up resistor as the I/C has an open-collector output (just like RA4 on the PIC).
The transmitter is a simple single transistor digital switch, when pin RB1 goes high this turns the transistor on, passing current through the IR LED's, with the current limited by R5 between the LED's. This passes quite a high current through the LED's and it's important that they are pulsed and not left on permanently or damage will probably occur - C2 is fitted to provide the required high current pulses without upsetting the main 5V rail. By pulsing the LED's with high current we increase the range and lower the current requirements - this is standard practice in IR remote controls, R5 limits the current through the LED's. As the receiver detects 38KHz modulation, we need to pulse the LED's at 38KHz, this can be done by feeding the LED's with a 13uS pulse followed by a 13uS space - in actual fact I decrease the pulse length, and increase the space length (keeping the total length at 26uS) - this reduces the power consumption.
Although it's labelled as connecting to PortB, as with most of the boards, it can also be connected to PortA if required.