LED polarity

On the positive side of LED: a thin wire inside of the bulb, a bump on the side of the bulb and a long pin. 

On the negative side of LED: a thick wire inside of the bulb and a short pin.

I reuse LEDs from different gadgets. Once you cut out LEDs from devices the size of the contact pins may not be useful to identify the polarity of LED.

However, the size of the wires inside of the bulb remains the same! That is the most reliable visual guide to identify polarity of LED.

We can use a pair of AAA 1.5V or a pair of AA 1.5V or any other pair of 1.5V or one 3V battery for this purpose. Any 3V source of electricity with known polarity will do.

I personally like to use one 3V battery as a tester.

Wrongly connected LED does not light.

Have fun and happy DIY :)