From the Spirit prop motion sensor thread:
I basically hacked the PIR to a 12v and then inserted the two appropriate wires into the foot pad slot. First attempt and I was following some directions I was sent. Very possible I messed up something, but the spider works with the current connection. Just incorrectly. I want to prevent this from happening to some of my more expensive or harder to replace props. Thus I need to understand what I have done or why.
First, what kind of PIR is it? Do you have a link from where you bought it, or a model number? Second, the foot pad used by the jumping spiders works because it has a little piezoelectric disk inside, which produces a very quick spike of voltage when it's disturbed. I don't know exactly how the trigger sense circuitry on the jumping spiders works, but I do know that it expects to see a quick, low-current voltage spike, rather than a solid 12V, so that could definitely be causing you problems.
In general, the two things you need to be aware of when looking to trigger a prop are what kind of trigger the prop is looking for, and what voltage (if applicable) the prop is looking for. There are typically two kinds of electrical triggers: a completion-type trigger (try me buttons, older step pads), or an externally-generated trigger (newer Spirit (Tekky) step pads). With completion-type triggers, you're looking to close a connection - basically to flip a switch so two wires are connected to each other. You don't have to worry about voltage for these types of props, because the prop generates the electrical signal. To use your PIR with this kind of prop, you'll need to get a relay. You connect the output from the PIR to the coil of the relay, and the wires from the prop to the switched contacts.
For an externally-generated trigger, like with the jumping spider, the prop is looking for an expected signal from something outside the prop - in this case, the piezoelectric disk. One wire serves as a voltage reference, like a ground, and the other provides your signal. This is where you have to find out what voltage it's looking for. This can be hard to determine for the piezo, since you can't easily just measure the output with a multimeter. I saw
this post, where the poster figured out a simple circuit with an AA battery to emulate this kind of pad, but I haven't tested it, so I can't speak to how well it works. Maybe if you drop the voltage on your PIR output with a zener diode that might work. I don't know, though. I've heard reports that a few people have successfully triggered their spiders just by bridging the two contacts, but I suspect that only works because the circuit is already looking for such a small output from the piezo that the electrical noise created by the device bridging the contacts is enough to set it off.