The circuit detects a sudden shadow falling on the light-sensor and sounds the bleeper when this happens. The circuit will not respond to gradual changes in brightness to avoid false alarms. The bleeper sounds for only a short time to prevent the battery running flat. Normal lighting can be used, but the circuit will work best if a beam of light is arranged to fall on the light-sensor. Breaking this beam will then cause the bleeper to sound. source

Here is a simple way to track the location of a vehicle and identify whether it has an authorized driver based on 1-Wire® technology. This has many uses such as finding stolen vehicles, creating automatic vehicle logs, and managing a fleet of trucks and buses. An example of how to do this is described. It uses a Garmin GPS 35 GPS receiver, a Siemens TC35 GSM module, a DS1990A iButton®, a Parallax BASIC Stamp® (BS2P) microcontroller, and, of course, sample source code.source

The Sharp GP2D12 is an infrared ranging sensor that uses triangulation to determine distance with a range of 10 to 80cm. The sensor comes with a JST 3 pin connector. The output of the GP2D12 is not linear. Therefore, a little math is required to determine the distance. source

These circuits use a CdS photocell to simulate motion detection, by sensing shadows and other small disturbances in ambient lighting caused by nearby people.Reverse engineered circuit diagram of the sensor circuitry out of a very sensitive children’s photoelectric Room Gaurd toy. The original toy would talk and make alarm sounds, whenever an intruder walked by or cast any kind of shadow onto the photocell. This circuit has immunity to battery voltage transients. source

Piezo sounder elements are often used to sense shock and vibration.This is the reverse engineered circuit diagram of the vibration sensing circuit out of a toy dog that barks when he senses a thump nearby.The circuit is very simple to build.source