a visual indication of whether a sensed voltage is above or below its nominal value, provides different-color indications if the sensed voltage is above or below the preset value
http://www.edn.com/archives/1996/101096/21di_04.htm
monitors four NiCd rechargeable batteries and causes the LED to flash if the voltage of the batteries goes lower than 4V
http://www.edn.com/archives/1996/112196/24_di01.htm
The circuit draws very low current from a +3v battery and could be housed in a package similar to a small ashtray. The assembly might be placed in a suitable out of the way area to collect dust. It would alert a maid when it was time to dust the room. The
http://www.discovercircuits.com/PDF-FILES/dust2.pdf
When the dust reaches a particular level sufficient light is reflected into the phototransistor to change the logic state of the circuit
http://www.imagineeringezine.com/PDF-FILES/dust2.pdf
http://www.web-ee.com/Schematics/Motion Detector/motion_detect.htm
http://www.plazaearth.com/usr/gasperi/motion.htm