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If I use an old transformer, example 3V size and I cut the round plug off the end does it matter which side I hook the legs of the leds to?
If it does how do I determine on the wall wart which wire is positive and which is negative?
 

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wall warts almost always have a small picture showin polarity.
 

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If I use an old transformer, example 3V size and I cut the round plug off the end does it matter which side I hook the legs of the leds to?
If it does how do I determine on the wall wart which wire is positive and which is negative?

a multimeter is the easiest way ....
if you don't have one, a regular white led can be substituted, if it's right, it will light .. on a led the longer lead is positive
 

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I believe the simple answer is no but you need a resistor in there or you're likely to burn it out. Because I'm not an expert all I can really do is point you to this for some good info:

https://learn.adafruit.com/all-about-leds

(generally speaking I've found sites like adafruit to have excellent beginner guides on these kind of things -- or at least I think)
 

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Something else to keep in mind, a small percentage of wall warts are AC, and therefore not usable without a rectifier to make it DC.
 

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And if you're not sensitive to 60 Hz flicker you'll never know the difference.
 

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I am only sensitive to the AC flicker on -some- LEDs, and as I get older, the tendancy is less flicker ... I normally run up to 1k on resistor value, because I prefer a dimmer LED
 
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