Step 1: Gather the Parts
-Neodymium magnets (they don't have to be large; mine are pretty small)
-A LOT of 22 gauge magnet wire (just keep the whole spool with you)
-2N3055 NPN Power Transistor or similar
-A battery and matching connector
-An LED (any color)
-A Reed Switch
-A Zip Tie
-Tape or Glue (or both)
-Bolt that fits in the hole of the Lego piece shown in the photo, and 2 nuts to mount it.
-A resistor (see note).
NOTE: I used a 22 ohm resistor with a 9 volt battery and a green LED. I also used a 1,000 ohm resistor with the 12 volt battery in the picture and a green LED. An exact fixed value for the resistor can not be given because when you run the motor on different voltages and use different LEDs, you have to change the resistor to compensate for the changes. Maybe just using a 5,000 ohm potentiometer would be better. For testing, start with 1K ohms.
One more thing. You may be wondering what the LED is used for. It actually acts as a crude speed indicator. The faster it blinks, the faster your motor is spinning. It is also used for testing, but that is detailed in step 6, so don't worry about that for now.
Step 2: Winding the Coil
Step 3: Assembly Part 1: The Base and the Armature
Step 4: Assembly Part 2: The Reed Switch and the Coil
Step 5: The Circuit
This is it! Not only is this the last thing you must BUILD, but it brings all the parts together (the coil, armature, and switch). The schematic is actually very simple. Follow the image notes. For the resistor, I said at the beginning that you would need to experiment. For testing purposes, start with 12 volts (12V) and a 1K ohm 1/4 watt resistor, then adjust resistor value and/or voltage after the motor runs if it runs too slow or the transistor gets too hot. Once you are done building the circuit, your motor is almost ready to run. More info in the next step.
Step 6: Pre-Startup
Now you will test the coil polarity to see which of the wires is positive and which is negative. Use a low voltage and low amperage battery for the test (i.e. AA, 9V). When polarity is determined, plug the positive of the coil to the positive power rail and the negative of the coil to the Collector.
This is where the LED comes in handy as well. Now test the reed switch position to make sure it is in a position where the motor will at least run. You can fine tune the position later. You can still do this test with the 1K resistor I said to plug in and 12V for testing. Before shooting this video, I was running the motor on a 9V battery and didn't bother to switch over. That's why in the video, I used a 22 ohm resistor and 9V battery.