diy_fan_speed_display.jpg
diy_fan_speed_display_schematic.jpg

Spotted in the MAKE Flickr pool:

Flick user Arms22 built this 7 Segment LED FAN Revolution Display to monitor the speed of a PC case fan. It’s a pretty simple project, but a good introduction to sensing pulse durations and driving multiple seven segment displays. The write-up is in Japanese, or you can find a machine translated version here.