[Jared] sent in this optical tachometer by [Mike Freeman]. The write-up and code requires a free registration, but if you’ve got a use for it, it’s probably worth the hassle. The zip archive includes a nice writeup in PDF format, and the code needed for the pic controller. The tach was built specifically for model helicopters. The user looks through the viewfinder and adjusts the pot on the side until the spinning helicopter blade visually stops turning.