check the polarity of any source up to 12V DC or AC
http://www.aaroncake.net/circuits/polarity.htm
portable systems usually monitor their battery input to obtain an early warning of a loss of battery voltage
http://www.edn.com/archives/1997/091297/19di_08.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
http://www.solorb.com/elect/solarcirc/lvbeep/index.html
handy when you have to follow the mains wires buried in the wall or even water pipes provided they are not too far away (2-4cm max)
http://www.geocities.com/CapeCanaveral/Lab/5185/circuit.html#ELECTRIC
for Pinball Machines
http://www.zaccaria-pinball.com/misc/vbat/
http://www.uoguelph.ca/~antoon/circ/batmon12.htm
When you hold the sensor close to the wall, the LED will indicate the current flow around where it detects the wire. Works on 9VDC
http://jap.hu/electronic/wirescan.html