What we came out with was this: a spider with eight LED eyes that flash on and off depending on the position of a couple of tilt sensors. It's perfect for looking innocuous, and then surprising passersby who brush by it, only to have it flash on at them.
We hung it above our dorm room door to scare visitors.
Step 1: Gather materials
-2 Tilt sensors (pictured below)
-a hot glue gun
-solder and soldering iron
-a ping pong ball
-some kind of cord
-batteries for your LEDs & a battery pack
Step 2: Solder the circuit
In ours, the first tilt sensor is before any LEDs are powered, so that there is a way for the spider to be on, but dormant. This is great for surprising people.
CAREFUL: make sure one end of each LED has a connection to ground, and the other to power. You can do this by wiring the ground side of all of the LEDs together and wiring them directly to the ground on your battery pack (bypassing the tilt sensors) and then connecting the other ends to power via the tilt sensors. We might have broken a couple of LEDs soldering this, so check your circuit before you connect power.
Add a slide switch between power and the first LED or tilt sensor.
Step 3: Bundle your circuit
Then bundle them together.
Then bundle the extra wire so it's compact.
Then tape together the tilt sensors so that they are perpendicular to each other.
Then tape that, and the batteries, and the switch all together (make sure the switch is on the outside.