The battery (CR2450) in my garage door sensor was getting low, so I replaced it. I’ll keep the old one in my electronics kit for LED testing, as shown in this video. Touch the longer leg (anode) of the LED to + and the shorter leg (cathode) to –. Usually + is the top of the battery where the words are. Don’t worry, you won’t hurt the LED if you connect it the wrong way.
Who would have thought it would be so exciting to light up some LEDs? I had a late night playing with a breadboard for the first time. It was also my first time soldering anything. I had to attach some headers to a Raspberry Pi Zero.