Schematics / Security
Pyroelectric Infrared (PIR) Motion Sensor
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....
Ultrasonic Motion Detector 
This circuit from Poptronix uses ultrasound to detect motion. The circuit can be easily modified to trigger an ultrasonic noise generator to drive off rodents and some insects.. Since it's from Poptronix, a circuit board layout is included....
Touch Activated Switch and Doorknob Alarm
Here are two circuits from Discover Circuits - a site created by David Johnson, P.E. I urge you to visit his site and check out his library! The italicized text has been reproduced from his site....