Ok, the photodiode that I have is tuned for the wavelength of the laser (or close enough).

When I hook the diode up to a multi-meter, the resistance is sitting about 2-2.4 M Ohms. If flash a regular light on it, it doesn't really change. As soon as I flash it with my laser, it jumps up to 4.5-5.2 M Ohms.

Should I be able to make this trigger the PIC input if I use say a 3M Ohm resistor? I have tried this with 3 x 1 M Ohm resistors in series, and I can't seem to get the PIC to recognise the change.