Low voltage PIR's?

Yes, chat to your local marina, car security shop or caravan showroom if you want to run them off 12-24VDC. Otherwise, most of the ones I've seen have a relay output.

Alarm PIRs run off a nominal 12v DC and have reed relay outputs. What do you want to switch?

hi Dave, Just general hobby stuff "on the bench" - nothing specific just now. Well, I was going to build the "cat water pistol deterrent" - but don't want to involve the mains in that. (Power the lot from a spare car battery - if the project gets that far :-) )

As Dave L said, Alarm PIR sensors (not usually water proof, but could be made to be) run off 12v, however, they normally only have normally closed (N/C) connection, so when they see somthing this opens a circuit, rather than closes it.

You could use a small relay to reverse this, then the second relay can be used to switch on your 12v pump, or a 12v valve connected to the mains water like this.