led wiring

Ok, in all my years in this business, I've never really had to work with electronics outside of cars. And now I find myself needing to know how to wire 12 LED's up to a single 9v battery. There going to be in 3 rows of 4. Any help is very much appreciated.

what’s the amperage and voltage for the LED's ? you will need resisters for each LED .. also is teh voltage always 9 volts, or can the voltage change like a cars can. (like large jumps, +/- .5 volts)

be default, white, blue, green leds use a 330 OHM resistor for 9 voltsthe led would be a 5mm , 3.3 volt, 20 mA LED (the calculations say it should be a 285 ohm resistor, but i would go with a 330)your voltage shouldn’t change, but some LED's the amperage is 30mA . then it would be a 190 ohm resistor, but i would go to a 200+ one. each LED needs its own resistor (unless u are really cheap, and done want a good result)IF YOUR LED IS RED, OR YELLOW, the voltage could be around 2.2 on those LEDS.  and you would need to change your calculations. you could try to wire up 3 leds in series and see if they are bright, but 9 volts wont be enough to make them bright. if 3 in series IS bright, then u could just wire them up as 3 in series, and the 4 rows in parallel.if you have the yellow LEDS that are only 2.2 volts, you could wire up 4 in a row, and it should be safe..  PLEASE let me know what your normal operating voltage is (not max)  and your amperage of the LED is.  if you dont know, let me know.  i would recemend a 330 ohm resistor unless its yellow.  (and maybe red?)  the place u bought them, or the package they are in should tell you.