Make my backyard scream??

[GothicCandle]

Extremly annoying pranksters have been messing in our back yard. We have found evidense of it five times, a few nights ago they played "alien" themed music outside our window and a few nights before that they knocked down garden tools, then the night after that they came back and set them upright, a few nights before that they moved my halloween props up in front of the window and another time they moved my pvc pipe from one side of the yard to the other. They have never stolen anything and they seem to just be stupid annoying teens. My mom is going to have motion sensor lights added to try and scare them away, which is good. I want to build something that screams, or shouts, or makes a large noise to scare the $%#$ out of them. How could I do this? and also not spend an insane amount of money?

[The Archivist]

laser pointer mounted on a motor connected to a motion sensor and tracking software. Make them think about what is on the other end...

[The Bloodshed Brothers]

shotguns are loud

[The Bloodshed Brothers]

YouTube- Get off my lawn

[RoxyBlue]

You did report this to the police, yes? They may not mean any harm, but they're on your property uninvited and creating a disturbance.

A motion-sensored light is a good idea.

[Zurgh]

Do Not Be A Victim....Opportunities prevail them selves towards the unwary. Act, do not react. Lock up thing that you can(Lock up your house tooo!) Set up a seurity defence perimeter..(never know when those zombies could strike...) File a report w/ the cops IF they seem to be able to help. I know not of the local laws in your area... reload shotgun shells w/rock-salt & FIRE! ... No, not the best way, But they TRESPASSED, Bear traps, staked WELL to the ground will keep them in place... If you want some other solution.. PM me... I might have some unorthodox ways... But what happens when YOU let them escalate........ Sorry If I find this BAD, Or not...

[Devils Chariot]

Bear Mace comes in a big can, shoots 30-50 feet. It's like big stinging fire extinguisher.

[GOT]

Most of these suggestion are how to make the kids scream, not the yard. There are dozens of threads on how to set up a PIR with a speaker. They are not expensive ($10 + a controller + speakers). You could set up a few near your tools.
You could put a servo on one of your tools and set it up to violently fall over when the speaker gives out a gunshot noise. That might work.

[GOT]

Put a stick pin on another servo and aim it at a balloon covered in flour. Then you get a little puff with you gunshot.

[Haunted Bayou]

Be sure to make a police report of the multiple trespassing occurrences. If they hurt themselves on your property, a dumb parent can sue. Yeah...it is stupid but it has happened before.

Also, laws on defending your property are not always clear.
If you know they are on the property...call the cops. That might be all it takes to keep them away.

[Bone Dancer]

Haunt Batyou's ideas are the way to start. The motion senor light is also good and will always be useful to you later. DO NOT start setting traps that may hurt some one. It may sound neat and cool but you will be the one being sued for damages.
Make a police report so you are on record as having this problem and call them if you see them or see they have just been there (sensor light comes on). If the kids doing this see the cops in the area right after thier actions they may figure the risk is to high and move on.

[fritz42_male]

I definitely wouldn't go with the gunshot effects - the cops would be out at your place in seconds and would be looking to get YOU!

I'd hook up a siren to a PIR - that way when the light comes on, it will scare them silly. Also ring a videocamera so that you can identiify the perps and show the video to the cops.

[GothicCandle]

I'm aware of the (stupid) laws that say if someone is hurt on your property then they can sue you, which is why I would never set up a trap of any kind. All I want to do is make a motion activated sound to scare them. It's really no different then a light. We always lock our doors and windows, in fact some of my family makes fun of us for being "over-protective" because we do lock them, where as other family members always leave their windows and doors open when it's a warm. I find this a stupid thing to do though. Of course, the ones who do this also have 6'5 "big tough" men who know how to use guns, and this gives them a false sense of security. A law here says that you have full right to shoot someone who has trespassed, but you must have a "no trespassing" sign somewhere on your property, BUT THEN if the person can prove they did not see it, or understand it then YOU can still be charged with a crime. Therefore the whole idea is wasted by the "Oh I didn't see it!" plea.
We have a detective who lives up the street, and yet my neighborhood isn't the best, though it isn't the worst ether. The whole area within 5 cities/towns is well known for crime having to do with theft and drugs. Great place to live, as long as you keep everything locked up, which we do. The things which are not locked up are not worth stealing (I would be mad if someone stole unfinished props, however what could they do with newspapers, pop bottles, clothes hangers and wood scrap???) All of my tools and things I deem important are in the house, or the shed.

[Zurgh]

How about getting them wet? The scarecrow motion activated sprinkler.
http://www.planetnatural.com/site/scarecrow-motion-sprinkler.html

I've toyed with the idea of building one, using a PIR light, wall wart & sprinkler valve, but haven't gotten around to it (it would cost around 1/2 what they charge). And disregard any of my dangerous boobie trap ideas made previously, I tend to overreact.:googly: Also PIR lights can be "hacked" to turn on any 120v electrical device, like a radio or a tape-player for instance. With a relay, they can turn off devices instead.
Do file a report with the police, if you haven't already. Photograph any vandalism caused, too. Don't go out & get a gun if you are not familiar & comfortable with them already. Guns are a big responsibility & a liability if misused. Plus, you must be 100% ready to take a life when one is drawn. Even gunfire sounds could cause problems, if it doesn't draw the attention of law enforcement, it could cause a panic among the vandals.

[Demon Dog]

Definitely inform the police, and your mom's idea about the automatic light sounds good. But has it just been your house on the street that they've bothered? If you haven’t already, I’d ask around. Whatever else you do, try to be sure they don't take it as a "game" or "challenge" for them. Trying simple scares may have the opposite effect to what you want. See what the police say, but if it persists after using the automatic light, consider an automatic wildlife camera like this to identify them for the police:
www.amazon.com/Moultrie-Game-D-40-Megapixel-Camera/dp/B0013R8HZE/ref=cm_cmu_pg_t

Just be sure to mount it where they can't easily take it down.
They want darkness, so give them light. They want anonymity, so take their picture.

Good luck getting rid of your human pests - the worst kind of pest there is