Gas, Air, Liquid, Moisture, Smoke etc.

Electronics Infoline Web

Water-level sensor

http://www.uoguelph.ca/~antoon/circ/sensor1.htm

based on PIC16F876

http://hawthorn.csse.monash.edu.au/~njh/electronics/watersensor/

http://www.mitedu.freeserve.co.uk/Circuits/Alarm/walarm.htm

http://www.mitedu.freeserve.co.uk/Circuits/Alarm/water_lev_alm.htm

http://www.web-ee.com/Schematics/pH Meter/ph.htm

http://www.aaroncake.net/circuits/raindet.htm

http://www.zen22142.zen.co.uk/Circuits/Alarm/walarm.htm

http://circuitos.tripod.cl/schem/r1.gif