I can't find a particular part on SparkFun. Where can I buy it?
SparkFun always maintains the pages of discontinued parts. See if you can find a part number from the original manufacturer, which you can use to order the same part from DigiKey or Mouser. Otherwise, use the description of the part to find something as close as possible.
I have my own resistors / capacitors / potentiometers / buttons / etc. Can I use these instead of the recommended parts?
Sure, as long as the footprint and specs are the same.
I prefer to order circuit boards, laser-cut acrylic, or 3D-printed parts from a manufacturer other than the ones you recommend. Is this OK?
Yes. SilverCircuits, Ponoko, and APProto were chosen for their price and reliability, but they are by no means the only choices. The design files are compatible with a variety of manufacturers.
Can I use a different material for the case?
Definitely. A wood or metal case would look particularly classy.
Building the twister
I'm not sure I did step X correctly. How do I make sure it's right before continuing?
After every step, it's recommended to test your solder connections with a multimeter. Make sure there is electrical contact between the metal leads and the board. You should also check for solder bridges between adjacent leads.
If you've gotten past step 2, you can already plug an Arduino into your circuit board. If you skip ahead to step 9, you can use the buzzer as a built-in sanity check to make sure everything is working OK. Upload the simple sketch located in Code/BuzzerTest. If you can hear a beeping sound after the Arduino is plugged into your board and has power, it means that nothing has gone catastrophically wrong. If you can't hear anything, it's possible that you have too much or too little solder somewhere. If something isn't obviously awry, try checking your connections with a multimeter, looking at the board under a microscope, or washing the board with ethanol. If that still doesn't fix it, perhaps it's time to scrap this board and start again.
The cover doesn't fit properly. Did I do something wrong?
Possibly, but it's not the end of the world. You can always shave away excess material with a Dremel or a file.
After it's finished
My Twister is plugged in, but I don't see anything on the screen. How do I fix it?
First, try turning the contrast knob. It's possible that the screen is simply too dim. If the contrast knob doesn't fix things, try uploading the Arduino sketch again. If it's still not working, you may have a bad solder connection. Disassemble the case and check your connections by hand.
The motor is turning really slowly. How can I speed it up?
The servo speed is controlled by the voltage difference between the red and black leads, which should be approximately 6V. Measure this voltage with a multimeter to make sure it's in the right range. If it's significantly below 6V when using a 9V power supply (NOT a USB cable), double-check that you've used the correct resistor combination for the resistors labeled "220" and "820" on the board.
The motor is turning when the Twister is idle. How can I stop it?
There's a small screw on the motor that can be used to set the resting state. Turn it clockwise or counterclockwise until the motor stops moving.