Another update:

• Added an LED to the breadboard, making the closed circuit visible, or at least, visible in a darkened room.
• Modified microcontroller code to check the length of time that has passed since it saw the magnet, but before it saw the magnet disappear. If this time is greater than 5 seconds (as in, I stopped the magnet over the sensor for more than 5 seconds), the controller will issue a "pause" signal. This lets me, if I want to spend time getting the magnet in the exact right position, pause both the video and the program if I need to stop, or, if I want to alternate to another exercise without losing my place in the video / desired revolutions.

The "pause" mode could potentially be used for cheating, I guess, to rest at the same time your video is paused, but you still lose time / steps trying to get the magnet into position, and time also waiting for it to get into pause mode.