Introduction: Build a Real Bell-striking Clock for Your PC and a Fire Extinguisher-striking Clock.

A brass bell, a small relay a few more things and a real bell can strike the hours on your desktop.
Though this project runs on Windows and Mac OS X also, I idecided to install Ubuntu Linux on a PC I found in the trash and work on that : I had never worked with Linux before, I learned some Processing language and wrote a sketch to run an analog clock on it.
Then I interfaced the bell to the serial port of the PC and built this bell striking clock.

Read further and learn how to make a fire extinguisher striking clock also.

In the video the clapper strikes a bit too fast for the camera...

Step 1: Processing

Processing Processing is both a programming language and an integrated development environment aimed at electronic arts and visual design. It is based on Java and is open source as most good things are now. It can be downloaded from Processing's home page.
In its basic use it is very simple to learn and gives "instant gratification of visual feedback" as aptly written in Wikipedia.
I'm not much of a programmer but it took a few hours to me to get to the result from scratch thanks to a great deal of examples and tutorial.
Many stunning visual artistic performances are given with the aid of Processing and many of them have their Processing source code available for download. One of my preferred is Substrate : I spent a lot of time looking at the drawing build under my eyes.

The code i wrote is very basic : it creates a canvas, assigns color for the background. Every hand of the clock is drawn the same vertical position with respect of the coordinate system, the trick is to displace and rotate the coordinate system. A common trick I saw on many examples : the angle of rotation is in linear ratio with the seconds, minutes and hours.

At the hours a burst of space characters is sent to the serial port. The number of characters per burst being the number of strikes I want the bell to ring, the hours actually.

You don't really have to learn Processing to replicate this toy. In the attached you'll find the applications ready to run on Linux, Windows and MAC OS X.
For windows just extract the directory and run the exe. That's it.
The program will look for the first COM port available (serial port COM1 if you don't use it) and attach to it. The character to drive the bell will come out of it.

Step 2: The Bell's New Clapper

The support for the bell is made from Meccano. The pictures should tell it all.
The bronze bell I found at a hardware store and sounds pretty nice. I unscrewed the orginal clapper and replaced with a 4mm 120mm long screw with washers and bolt.
The relay is a miniature one. I removed the copper contacts from the armature and glued a thin u-shaped steel wire. A small screw and nut complete the clapper.

Step 3: Electronics, Just a Little Bit - No, Really !

The serial port is interfaced through a simple transistor interface to the relay. Every character sent to the serial port makes the relay click. Larger relay may require two space characters per click, others may require a larger Baud rate than the current 300 for a shorter pulse.

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.

Ding !