Here's a quick science experiment for a wet Saturday afternoon...
When things are moving to fast to see we can take a picture of them to capture a single moment of the movement. We can do the same thing without the camera, by simply using the camera's flash. If you're in a dark-ish room, the you'll see a single bright frozen instant of a moving subject.
But what if something was spinning - like a power drill, an engine or a wheel. It looks like a blur, so its hard to see whats going on, but if we could freeze the image every time the rotation got to the same place, it would look like it was stationary. We can do that by flashing a bright light at exactly the same speed as the object is spinning, so if an engine is rotating at 100 revs per second then flashing a light at 100Hz would make let us see the motor clearly, as if it was still. If fire it at 99Hz then on each revolution the engine will get a little further around - and it would look like the object is spinning at the difference of the two speeds - once per second!
To make this happen I hooked up a could to LED's to arduino pins 2 and 3, so we could flash them. We can also use a potentiometer connected to A0 to adjust the speed. A few lines of Sniff:
make led1 digital output 2
make led2 digital output 3
make pot analog input A0
make delay number
..set delay to pot*0.2
..wait delay secs
..set led1 to on
..set led2 to on
..wait 0.005 secs
..set led1 to off
..set led2 to off
And we've got a stroboscope!
Here we've got a lego cog held in an electric drill. It's spinning fast enough that it would normally look like a blurred disk, but with the arduino slowing the movement down it looks as if its spinning slowly. Update: Try stepping through the video a frame at a time... The video frame rate is faster than the strobe, so you can see the light flashing on and off, capturing the same part of the rotation each time, while the other parts of the cycle are in darkness!!!
Adjusting the pot to control the delay changes the apparent speed of the motion. You can also play with the 0.005 second delay - making it longer makes everything brighter, but if its too long then it will look blurry.
The code could easily be developed so you could display and/or set the rate of flashing exactly, so you could measure how fast the drill is actually spinning...