Step 3: Electronics, just a little bit - no, really !
I modified the relay removing the contacts and gluing a sort of clapper made with steel wire and a nut and bolt. Simple but effective.
The tricky part is to find the best position of the relay-clapper to make it hit the bell without damping the oscillations.
I put everything on Meccano for faster modification and alignment.
As I said the schematic is very crude : almost any low / medium power NPN transistor will do.
The diode in parallel with the relay prevents the back EMF from relay coil to destroy the transistor.
The power source is provided by a USB port of the PC, 5Vdc are available at pins 1 and 4 of the connector. The PCB mating connector I took from a dead printer. In case an USB port is not available, an external DC voltage wall adapter is fine. The adapter voltage must match the relay and not exceed the transistor rating. Any regular Radio-Shack wall adapter should do, but the USB is more attractive to me.
Larger relay provide larger clappers to hit larger bells. A stand alone version I built and based on Arduino-compatible hardware is described in my blog. In this case any good sounding thing is fine to be used as a bell : in my case I used a CO2 fire extinguisher.
Not being designed as an Instructable I did not post it here though.
In my blog you can find some directions, schematic, code and a video.