Step 4. A blinking light, with a physical source for the oscillation. A photocell under the fan acts as a variable resistor. The potentiometer varies the speed of the fan. As the blades turn, the photocell receives more or less ambient light, causing the LED to flicker. It was hard to get the fan to turn slowly enough for this effect to be apparent, so I removed many of the blades on one side. This helped quite a bit.