[flyerrich]

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?

 

[Dennis461]

wall warts almost always have a small picture showin polarity.

 

[wvgca]

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

 

[Severn]

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)

 

[Stan D]

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.

 

[wvgca]

actually LEDs will light on AC, they use half of the sine wave to light up ..

 

[MichaelE]

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

 

[gregc]

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

since the LED is on for half the time, the resistor value should be halved to maintain the same average intensity

 

[wvgca]

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