Coil Gun
I started working on coilguns near the beginning of my Junior year in high school. I had read about using flash circuits from disposable cameras to power small, hand wound coils. If you have never heard of a coil gun before, it operates on electromagnetism. An inductor - loops of wire - creates a magnetic field when an electrical current is sent through. By placing a small ferrous payload just outside of the coil, the fast energy dump put out by a capacitor creates a powerful magnetic field. The field will pull the ferrous rod into the inductor, then shut off when the rod is half way through. If the rod moves farther than halfway, the field will begin to slow it back down. In reality, there is no triggered control to shut off the magnetic field, so we'll just hope that the capacitor finishes discharging a little before the rod reaches the center. Future improvements will likely implement some further calculation on the capacitor discharge rates. Multiple stages can also be used to increase power, where the consecutive coils are triggered by an optical switch as the payload approaches.
V1
The first version was packaged into a small cardboard box. The capacitor bank was made by connecting several capacitors in parallel (increases capacitance). One flash circuit - powered by a single AA - was used to charge the whole bank, which took some time (~20 seconds).
The discharge was triggered by direct contact between two metal plates. The discharge was very inefficient, as a lot of energy was lost by the large spark in the trigger. The project could have progressed further into incorporating an SCR to trigger the discharge, and a diode to protect it from the large voltage kickback.
V2
This version contained a smaller capacitor bank, but a more entertaining case. Lots of pretty lights and an actuated aiming arm! It was a lot of fun to build, but really didn't perform much better than V1.