Step 3: Wiring the transformer
I used a variable voltage (8-16-24 volt) doorbell transformer made by Heath/Zenith. It cost about $15 at my local home center. You need a transformer that produces 24 volts.
I direct wired the transformer to a grounded, 3 prong plug and mounted it directly above a power strip near my workbench. The 4.7k-Ohm resistor was soldered in place and then secured with a piece of heat shrink tubing I had on hand; it's directly beneath the transformer in this picture.
If you wish, you could cover the wiring with plastic channel for a neater appearance; I simply chose to use insulated staples.
One other thing I did later on was to plug the power strip into a lamp timer, and set the timer so the strip is on only between 8pm and midnight. There's no reason to power the transformer 24/7, as the only critical time is at night when we're going to bed and (presumably) have forgotten to close the garage door. Which leads us to the final step...