Spotlight – Understanding LEDs
Birth of a legend.
The LED is one of those inventions that happened slowly then all at once. The idea of electroluminescence was discovered in 1907. H. J. Round, a British scientist, working at Marconi Labs was one of the pioneers of the technology. While working with cats whisker detectors he used a variety of materials and noticed that some of them lit up when current was passed through them. Now the idea of electric light wasn’t exactly new, but this was the breakthrough moment for LED technology.
The Dark Ages
The earliest LED’s had a few characteristics that separated them from what they are today. They were available in any colour you wanted, as long as you wanted red. As for light output, you could even see them in the dark. But not much more than that. LED’s were useful in situations where low current use was needed as the alternatives were usually much brighter, like Neon and Incandescent bulbs. Why did we bother? We had a much better alternative already made! A cheaper lamp that was brighter and with a filter could be any colour we needed it to be.
Well, as we said above, these lamps usually required much higher voltages and currents to operate. Incandescent bulbs can be found as low as 6 V today but this is still very much over the 2 – 3 volts of your typical LED. Neon’s however wouldn’t do much until 60 or more volts were passed through them, meaning low voltage circuits were all but impossible. That’s not to say these bulbs fell out of fashion instantly. I can guarantee anyone old enough to read this remembers the beautiful warm glow of an incandescent bulb in their house before those horrid CFL things started appearing.
As eluded too before, the LED’s development was not one moment in a lab. In fact, “More experienced” electronics hobbyists will remember a time when blue LED’s were becoming commercially viable for the first time. I remember once having a conversation with a “mature” hobbyist who still considered them witchcraft. I then tried to explain my theory that someday transistors might overtake vacuum tubes to which I was told rather impolitely where to go.
Build it, and They Will Come
LED’s today come in a wide variety of packages. From the mini SMD LED on your arduino, to 10mm “Jumbo’s” packaged with kits and science toys, But lets focus on the most commonly used version among hobbyists. The 5mm THT. On first inspection its just a cap and two legs. Looking at the finer details, some other features start to emerge. The Legs will be different lengths, this is our first clue. The longer leg typically denotes the anode or positive side of the LED, remember these are diodes at the end of the day and therefore will only work if current is passed in one direction. But it’s not uncommon to find that the person working on this before you lopped off the legs, there are some sadistic people out there, and you need to find a way to tell. So if you look around the flared base of the LED package you will find one side has been flattened. This is the cathode, or negative side of the LED.
Wont Get Fooled Again
Failing this, there are still other ways to check. I’m not sure who would grind down the other side just to annoy you but I don’t know your situation. Look inside the lens cap, look for two lumps of metal at the top of the legs. These are called, locally anyway, the anvil and hammer. Although I think anvil and post is a little more correct on this.
The anvil is the larger of the two and typically extends over the post, where as the post has a shorted top and typically extends under the anvil. In this case, the anvil is our Cathode and the Post is our Anode. Ok, so you can now spot the polarity of LED’s in a number of ways.
Technically, It's Just a Technicality
We’ve touched on a few of the more common LED types, including SMD and THT but what does this all mean for the actual person designing a circuit or hoping to get started?
I SMD What You Did There
SMD is an abbreviation for surface mount device. Surface mount devices come in a number of sizes. They are called small, really small, and microscopic. They have a slew of technical names too, and I suppose I should mention them. Generally they come with a 4 digit code, which unlike resistors, actually represent the metric size of the component. So, you’re 8520 is 8.5 mm x 2.0 mm and your 1206 is 1.2 mm x 0.6 mm. 1/10 of a millimeter seems like a good enough resolution.
Those familiar with SMD resistors and capacitors may be familiar with this format but be warned that a 1206 resistor is infact 3 mm x 1.5 mm or 0.12 inches x 0.06 inches. Perfect logical consistency as you can see.
Through the Rabbit Hole
The more traditional option, is the “through hole technology” (THT) LED package. These are the ones most people imagine when they think of LED’s. Most commonly found as 5 mm, like our 25 Pack in the shop, or as a smaller 3 mm version. They are perfect for breadboard and perf board projects. Simple to use, and the 2.54 mm / 0.1 inch spacing between the legs leaves a nice, snug, board hugging fit.
LED’s, as we’ve mentioned more times than we’d care to count, use a tiny amount of current and a low voltage… typically. But for the beginner, getting your LED’s to turn on can still be a bit of a mystery. To even turn on, an LED has to meet it’s forward voltage. So assuming a 5 Volt supply, and a 1.8 Volt forward voltage, we can expect one or two LED’s to turn on, but not three. You’ll also need a resistor to limit the current. Therefore one LED with a 1.8 Volt forward voltage on a 5 Volt rail will need a resistor to burn off the remaining 3.2 Volts.
Mod my Projects
Adding LED’s to your project can be one of the quickest and easiest mods to install. Got a PSU or Controller and want to know when it’s on, or using a certain function? LED installs can be one of the easiest mods you can do. Just tapping the line you need and adding it in, is a novice project that can give you a rewarding, visual feedback.
Grab 3 LED’s (red, yellow and green work well), a raspberry pi or arduino, and you can build a simple traffic light array. Variations of this project include switch and button controlled traffic lights. Digitally controlled versions using python, and more. These types of projects are perfect for beginners to either electronics of coding.
Fancy a challange? Try to build modular traffic lights that have a central controller. Or for the really adventurous, use optical recognition in your car project to see if it knows when to stop. A great experiment in machine learning, and frustration.
A personal favourite of mine. Spark a little wonder with an LED and a 3V battery. Just add a little slip of paper between the battery and one of the legs, pull out and watch the light come to life. Great for bringing a touch of wonder to a young persons eye.
A little fairy costume is easy enough made, and you can release it into the garden on a quiet night. Alternatively, prepare them in advance and have an instant light source. Check how deep the hole your looking into is, or tape on a magnet and throw onto a metal surface.
Got a favoruite of your own? Leave it in the comments or head over to our forums!