How do I build a trap?

So I'm trying to build a trap on the bridge leading into sanctuary.

I placed a pressure plate and a tesla trap, then I wired them up to power. Once I exited the workshop menu I 'activated' the pressure plate by interacting with it and it disappeared.

What did I do wrong?

Traps work exactly as you'd expect - basically in an identical manner to lights and other electrical devices. If you've had a pressure plate fall through the floor I'd call this a bug. It's likely it didn't agree with you placing it exactly on the floor and some clipping issue occurred when you stood on it.

The other thing I came across is that you can effortlessly break these traps (especially the tesla trap) just by having it on. To repair a broken trap in Workshop mode, press E with the item selected.

The things to notice are only the "metal" section of the pressure plate depresses, when it does the red light goes green. When there is no pressure the green light will go red. While the light is green, power will pass to connected devices. While the light is red, it won't.