I could not have constructed my barn door without the information I found on the web. Here are the most useful sites.
The Double Arm Barn Door Drive It all begins here. This is Dave Trott's original article describing the double arm barn door, published in the February 1988 issue of Sky & Telescope magazine, along with a followup from the April, 1989 issue. Read this material to learn how barn doors work and to understand why the double arm design is superior to the single arm type.
How to Build a Double-Arm Barn Door Tracker Steve Gagnon's page provided the dimensions I used in constructing my barn door. You'll also want to read about his experiences in building a very large, mostly-metal version.
A Quartz Controlled Scotch Mount This site from Ray Grover features a wealth of valuable information and ideas, including the schematic diagram for a stepper-motor drive circuit upon which I based my own circuit. There are excellent discussions of polar alignment, calculation of dimensions, and sources of tracking error.
555 Timer Tutorial The operation of the 555 timer chip is explained at this site. When used as a frequency source, the 555 is configured in "astable" mode, so refer to those sections that discuss astable operation.
555 Timer This is another site that explains the operation of the 555. Unique at this site is a 555 timer component selection program that you can download and run on your Windows computer.
Stepper Motor Driver Circuit Rob Paisley has designed a simple circuit to drive unipolar stepper motors. You can purchase printed circuit boards, complete parts kits, and even fully assembled boards from Rob.
A Hand-Driven Scotch Mount This page explains how to construct an ultra-simple hand-operated barn door, suitable for exposures of up to 10 minutes with the "normal" lens on your 35mm SLR.
"Scotch Mount" Camera-Tracking Platform This excerpt from the first edition of Phil Harrington's Star Ware book includes plans for building a very simple single-arm barn door.
Barn Door Tracker The author of this page made a manually-operated single-arm barn door, then added a motor salvaged from a floppy disk drive. He also shows how to make an inexpensive adjustable mount for the camera.
Barn Door Clock If you have constructed a manually-operated single-arm barn door tracker, then you know tracking errors quickly accumulate, limiting the length of exposures before star-trailing occurs. To address this limitation, Agnes Clark has developed Barn Door Clock, an app for Android mobile devices. Barn Door Clock tells you when to manually turn the drive screw on a single-arm barn door to make possible error-free long exposures.
Motorize Your Telescope This section of Mel Bartels' excellent site describes his computerized telescope drive system. In contains much useful information about stepper motors and driver circuits.
Heavy Duty Tangent Arm Drive Not a barn door, but based on the same principles, this beautiful camera mount built by Kevin J Kilburn is a work of art.