Step 4: Prototype version - Introduction and parts list
The purpose of this prototype is to get something working as quickly as possible. Building the prototype will give me some good ideas about the complexity of the build and the length of time required. This in turn gives me some pointers about how many different versions I have time to test.
This is how I made my prototype, and the things you'll need to cobble this together. I used this set of pieces because it's what I came across first. Other kinds will certainly work too.
- I used foam core board (sometimes called kappaboard) to make my prototype. It's light, glues easily, and is stiff and strong. It's also easy to cut and not messy. Corrugated cardboard will also work handsomely.
The running gear:
- 2x Curtain rails - I got a PVC one from John Lewis' for £7 ($11). You need two pieces, each as long as you want your clock to be. My prototype was 62.5cm long because I used a 125cm curtain rail sawn in two.
- 4x sewing machine bobbins for the driving wheels - plastic ones are easier to work with, and cheaper.
- 2x Pulleys for the non-driving end of the clock - I used these little pulley blocks I had, but sewing machine bobbins would be just as good probably.
- Thread that is thick enough to be controllable, some thin elastic cord, some bits of balsa sheet or other light, thin, strong sheet that will slide OK in the channel of the curtain rail.
- Arduino with Adafruit Motorshield - This is definitely overkill, and I used this simply because I already had it and it works admirably. The stepping circuit on it's own is not complicated and if you have an Arduino compatible board that isn't otherwise gainfully employed, then it's quite easy to whip up a circuit using L293 ICs on breadboard that will drive a couple of stepper motors. Look at Tom Igoe's stepper pages, for circuits for driving steppers from an arduino. However, what the Motorshield gives is a good quality library that allows you to turn the motors off entirely. The simpler circuit does not do this so easily.
- The logic/timing part of the code would be the same for all versions, but the interfacing code will be a little different.
- Any Arduino compatible board will work fine as far as I know.
- 2x Stepper motors. I tested this will some dead cheap little steppers (referred to here) that have almost no torque and only 20 steps per revolution. I paid next to nothing for them. I also made a version with some good quality NEMA17 steppers with 400 steps per revolution. I paid about £14 for each of those and they weighed a pound each. I ended up using the cheap small steppers.
- In the Arduino code, modify the value "stepsPerRevolution" to reflect the kind of motor you have.
- You'll also need a 6-9v DC power supply to run your microprocessor (arduino) when it's not tethered to the PC.
- Knives mostly. Foamcore has a tendency to "crumb" if you don't have a really sharp knife, so use a fresh, long blade and always make sure the blade is moving through the foamcore using a sawing, or slicing action rather than cleaving or chopping.
- Adhesives - I've used anything I had around. Superglue, hot melt glue, contact adhesive and gummed brown paper tape.
- Soldering stuff, or screw terminals, and wire.