LED - 12V indicator light

Hey guys, I know this is a basic question, but I couldn't find the answer searching. All the topics were too complex. I'm hooking up a water injection system on my car. Its activated by a boost switch. When the boost switch sees a certain boost level, it triggers a relay to give power to the pump. What I want is an LED that will light up when the terminals across the pump are 12V. Probably more like 13.5V really, since the car will be running (I'd like it to work form 12V to 14V or something like that). I know I will have to run the LED in parallel, but don't know what size resistor or anything else. The LED I bought is a 2.1V normal, and 2.4V max(maybe 2.8V max). It's from radio shack. P.S. - I tried to hook up the LED to a AA battery, and nothing. Is 1.5V too low to hook up a a 2.1V LED????? Thanks, Kevin

Kevin, The LED should work with the same resistor in a 12-14V range. A 1k ohm resistor should do the trick. Another thing to think about is if you don't want to worry about resistors, use 6 resistors in series, then apply the 12 to 14 volts to the chain... So, (ground)- led +- led +- led + (12-14volt) get the idea, use 6 leds, no resistor needed. Oh, and by the way, your resistor should work with a 1.5V AA battery, check the polarity of the battery when applying voltage, it matters, short side of the LED leg cathode(-)... Hope this helps. WildWiresCarAudio

Kevin, Sorry, use 6 LEDs in series, sorry for the boo-boo. WildWiresCarAudio.

What do you do if there isn't a short leg on the LEDs?  How will I know which one to hook to positive and which to hook to negative?

Thanks

Kevin

If there isn't a short leg on the LED, then hook it up to the (AA) battery. If it lights up, you have the correct polarity. If no light, switch the connections and it should work. just remember which leg you connect to the positive terminal and which leg you connect to the negative terminal (you can cut the negative leg a little shorter if necessary).-Santiago-

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