It's fun to check how fast various things--fans, drills and the like--are rotating. And it's easy to do with the following ingredients that many of us will have at home:
- phone (I am using Android, but I assume iPhone will work, too)
- red LED with clear plastic (red LED with red plastic will probably not work)
- four-connector cable from an old headset
- oscilloscope app (my instructions will be aimed at OsciPrime; source code and free APK are here; I have no connection with the authors)
- fan / drill / drill press / other rotating object
Disclaimer: This works on my phone. It might not work with your LED or your phone. It might damage your phone, etc. All the responsibility is yours.
Background: An LED converts electricity to light but also can be used in reverse as a photodiode that converts light to electricity. The resulting current is low but sufficient to be captured by the microphone input of a phone (at least my Galaxy S3). An oscilloscope app can then capture the output and you can get the rotational speed.
I've used three different methods depending on what the rotating object is. The through-light method is for fans and other objects that you can shine light through. The matte object method is for objects like drills with plastic chucks that are not reflective. The metallic object method is for objects like drills with shiny metal chucks.
One tricky thing is that you might think that the light the LED will most efficiently convert to electricity will be of the same wavelength as the light it emits, but that does not seem to be true for the red LEDs I have. The red LED that I've tested seems most efficient in generating electricity from light in the green range (I get about 1.5 volts by shining a green laser). It seemed to have no measurable output from my white LED flashlight with a red filter (maybe the red component of the flashlight was too narrow and missed the sensitivity window of the red LED). This is important, because it means that if the LED were encased in red plastic, the photodiode function would be effectively killed. So if you use a red LED, make sure it's got clear plastic. Interestingly, the infrared LED that I tested was quite responsive in the red and infrared ranges, so if you want to work with a red laser or infrared light as your light source, use an infrared LED. I am assuming other LEDs will be like mine, but if not you may need to rummage around in your scrap electronics box.
The LED makes a super-simple light detector for your phone. While your phone may have its own light detector, this doesn't work with the oscilloscope apps I've tried, and has a much lower sampling rate than the microphone. The microphone input has a decent sampling rate, though unfortunately it filters out constant voltages on my phone and hence is only sensitive to changes.
I was originally going to explain how to do all this with a stand-alone oscilloscope, and indeed one can do it this way, but one of my kids pointed out that few people have oscilloscopes, so I switched to using a phone. Which is anyway easier than dragging my vintage 30 lb oscilloscope from room to room.