Best sensor to trigger prop when someone walks by?

[Triode]

I'm thinking about a Halloween project that deploys a prop when a person walks by, and I'm looking for suggestions on the most practical way to trigger it. I know it could be done with an ultrasonic detector, a light detector of some kind, infrared, sound or many other ways, and I have a good idea how to do some of those things, but I don't know what the most tried and true method for a simple thing like this is.

I'd like for it to be something that doesn't have to be extended from the prop deployer, like step pads or laser trip wires because that just makes the whole system so much larger.

My experience is with pic chips and I have a collection of basic parts, but I'm willing to buy parts to make this, or even a premade sensor setup. I've got the other aspects of the prop deployments figured out, such as popping up, dropping or sliding out, I just need to figure out a reasonably reliable sensor to trigger them.

 

[3v0]

The easy way would be to use a PIR sensor with the PIC.

PIR Sensor Module - RadioShack.com

 

[Triode]

That seems like it would be easy to implement, and it's well within the budget. I'll order at least one to try it out. Are they fairly reliable? Thanks for the suggestion.

 

[crutschow]

They're very reliable. It's what's used in the motion sensors for burglar alarms and lighting.

 

[Triode]

I ordered one, if it works out it should be fairly budget friendly at $10 a pop. Even after adding pic chips, or other ICs to control the action, motor driver chips to power it, and the servos and motors to make it work these should come to about $30-60 a unit. Off the shelf haunted house props that do anything automatically are a few hundred at least, so this'll be cool.