This is a delightful little benchtop experiment that combines analog sensing, oscilloscope interpretation, and a real-world measurement problem. Rather than buying a dedicated tachometer, the creator builds a simple photoresistor circuit, shines a light up at a spinning ceiling fan, and captures the reflected pulses on a ZOYI ZT-703S scope. Each blade passing overhead produces a periodic dip in the waveform, and from that period you can derive the fan's RPM.

What makes this worth watching is that it teaches several transferable skills at once. You see how to bias an LDR into a voltage divider so the signal swings usefully around the scope's input range, how to interpret a noisy real-world waveform (the blades aren't perfectly identical, so you get amplitude variation between pulses), and how to do the simple math converting period-between-pulses into RPM while accounting for blade count.

It's also a nice demonstration of a principle that comes up constantly in embedded work: you rarely need an expensive purpose-built sensor when a photoresistor, a resistor, and a little signal interpretation will do. The same technique scales to measuring motor speeds, propellers, or anything else with reflective moving surfaces — and could easily be ported to an MCU with an ADC and a bit of edge-detection code.