This is the air horn office chair prank!
Pneumatic pistons under the seat of most office chairs compresses when weight is applied, an air horn is zip-tied to one side of the piston with the other end braced against something solid. I used a cheap ABS plumbing cap fitted around the piston. When the target sits in the chair the nozzle of the air horn is triggered, releasing a loud honk (and hopefully scaring anyone nearby)
Here's what I used to make mine:
The air horn nozzle is positioned touching the underside of the seat, folded scrap cardboard acts as a spacer between the air horn and the piston. Ensuring the nozzle and air horn body aren't obstructed by chair frame they can be zip-tied in place. With ABS plug zip-tied to piston and air horn positioned all zip-ties are tightened and the leads cut off.
Did you prank someone with your version of this project? Share your pictures or video of your air horn office chair prank in the comments below!