Matt Mets made this “Light Record:”

I am using some photocells to read the intensity of light that shines through a sheet of paper, upon which I have encoded an image using shades of grey. By spinning this paper around and shining a light through it, the image can be viewed on an oscilloscope. The circuit consists of two resistor dividers, one for each channel, made up of a photocell and a fixed resistor. When there is a large amount of light on the photocell, its resistance drops dramatically and the output voltage rises; when there is little light on the photocell, its resistance goes down and the voltage goes down as well. That’s it!

It’s the first of Matt’s ambitious Thing-a-Day efforts. – Link.