Worked mostly as expected. Loose USB cable caused minor problems. Received possibly interference? from other radios communicating as we would occasionally receive input neither of us had sent, despite having set each other as addressees.
I wanted to explore working with a servo for this. Plugging the servo in, and seeing the regular beat of its movement reminded me of the dull rhythm of working out. At first, i was just using the servo arm to push the cutout up and down, but found I could give it more life by tying a string to the wrist and attaching it to the servo arm. This way, as the arm moves away to push the cutout up, it pulls the arm in. The string relaxes as the arm resets, and lets the cutout “relax” its own arm.
The metronome itself was easy enough to coordinate with, except that I was too far to receive its signal the first time the attempt was made. There is also an obvious delay between the transmitting radio and the reception of its data on the Arduino, noticeable when it takes a few beats for the speed to change after adjusting the potentiometer.