This circuit is from the GLOLAB web page. The document referenced here includes an excellent design as well as an excellent tutorial on PIR devices. You've probably seen these devices in homes and businesses and maybe wondered what they were. Now you can find out what they are, how they work, and how you can build one. We've seen PIR sensors assemblies featured in some of the low-priced surplus magazines that come through so these systems should be very inexpensive to build.
For more quality designs of this type, visit the GLOLAB web page at: http://www.glolab.com.