Simple Polarity Tester

 

This tester can be used to check the polarity of any power source, and is therefore very useful when installing automotive equipment, alarm systems or anything else you can think of. Because this circuit is so simple and cheap, even frying one with an over voltage is not a big deal. 

R1           1              1K 1/4W Resistor            

D1           1              Green LED          

D2           1              Red LED               

D4, D5, D6, D7    4              1N4001 Silicon Diode     1N4004, 1N4005, 1N4007

MISC     1              Board, Wire, Case, Probes           

1. To use the circuit, just connect your probes to the source under test. If D1 lights up, the left most probe (on the schematic) is connected to positive. The opposite is true if the left probe is negative. If both LEDs are on, the source being probed is AC.
2. Be careful when using this tester not to probe a source greater than about 12V.

  ref: aaroncake.net/Circuits/polarity.asp