There is another way (depending on available Power Supplies)... set Vss to a minus Voltage level (eg -1v) and set Vdd correspondingly 5v away from Vss.

I have a favourite AC switch called a ACS108 that I use... it's a dream for interfacing directly to a PIC... however, it needs a negaitive current on its trigger. Simple solution... connect the PIC's Vdd to 0v and have Vss set to -5v.

Never be afraid of the unconventional... the PIC might need 5v to work, but that 5v can be referenced anywhere in the universe.

Now, without knowing a lot more about your application, I can't say that this is the ideal solution here, it may not be... but what I am suggesting is that you can break out of conventional thinking to design a solution.

You may also want to do an internet search on "Level Shifters" which is what you basically need. Op-Amps and Opto-Isolators are the obvious choices... but an Op-amp may need a split Power Supply to handle negative going signals, and if you're going to do that, you might as well use a cheap PIC with ADC as "Electronic Glue" as I mentioned at the start of this posting.

Melanie