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LED Brightness Standardization – Information


Now not will sun shades be required to have a look at LEDs!





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Howdy people! I am Dryw, a brand new engineer right here at SparkFun! I joined in March of this 12 months and have already launched a handful of merchandise, corresponding to these Bosch strain sensors and the IoT Redboard – ESP32. I’ve additionally been engaged on some extra thrilling merchandise that might be popping out quickly, keep tuned for these!

As I have been working right here, I’ve observed quite a lot of our merchandise use totally different resistor values for LEDs. Constant LED brightness hasn’t been a crucial design requirement for us, which has resulted in several brightnesses throughout our catalog. Probably the most extreme instance I noticed was on a prototype, the place the inexperienced and yellow LEDs had been barely seen, and the crimson and blue LEDs felt like staring into the solar! So I made it my mission to search out the right resistor for every of our LEDs, with a purpose to give a constant brightness throughout all our merchandise, and to by no means want sun shades ever once more!

It is a pretty easy downside to unravel, so that you is perhaps pondering “simply take a look at the LED’s datasheet and compute the best resistor with Ohm’s Regulation.” You are completely proper, and that will be a completely legitimate method. Nevertheless there are some nuanced human elements that make this extra difficult than it first seems:

  • Completely different colours will not be all perceived equally. For instance, human eyes are most delicate to inexperienced mild, so they’ll seem to have a distinct brightness than crimson or blue LEDs regardless of having the identical absolute brightness.
  • The dimensions of the LED impacts the perceived brightness. For a similar absolute brightness, bigger LEDs seem dimmer as a result of mild being produced over extra floor space.
  • Completely different cone angles change the quantity of sunshine that really hits your eye. Some LEDs produce mild in a really slim cone, whereas others unfold the sunshine extra evenly in all instructions.
  • Most LED datasheets charge their brightness in millicandela (mcd), and I’ve obtained no instinct for the way vivid a millicandela is!
  • Completely different folks need totally different brightnesses. In my testing (see beneath), there was a variety of brightnesses that individuals most well-liked.
  • Completely different ambient lighting situations require totally different LED brightnesses.

On prime of that, there are some extra technical issues:

  • Every coloration requires a distinct voltage. Shorter wavelength photons are extra energetic, so blue LEDs sometimes require a better voltage than crimson LEDs to activate. That is often known as the “ahead voltage drop” of the LED.
  • Completely different colours require totally different quantities of present to supply the identical brightness. In my testing, inexperienced LEDs normally required way more present than crimson or blue to attain the identical brightness.
  • The identical LED produced by totally different producers have totally different brightnesses for a similar currents. Probably the most excessive instance I discovered was between the inexperienced LEDs we use; at 20mA, one has a rated brightness between 5-12mcd, and the opposite is rated between 18-71mcd!

In precept, all these elements might be thought-about and accounted for. However in my view, it is significantly better to only play with the LEDs themselves so there isn’t any doubt in regards to the perceived brightness. So I set to work determining one of the simplest ways to check every of our LEDs.

All of the LEDs we have a tendency to make use of are floor mount, so I wanted to create a customized PCB to check them. Actually, we truly use 3 totally different sizes of LEDs, together with 0402, 0603, and 1206. So reasonably than making totally different PCBs for every, I figured I may embrace every of these footprints in parallel on a single board. And heck, whereas I am at it, why not add an 0805 footprint and a few plated through-holes for LEDs with legs! Then we may take a look at any LED!

In collection with these LED footprints, I included a potentiometer as an adjustable resistor. I knew we would not must exceed 10k, so I used our blue 10k trimpot. It goes all the way in which to 0 Ohms on the low finish, so I additionally included a 100 Ohm resistor in collection to forestall any LEDs from blowing up. Here is what the schematic seems like:

LED Tester Schematic

To be clear, just one LED is added to every board. There are simply 5 totally different footprints to accommodate any LEDs.

Subsequent step was laying out the board. As I performed with it, I used to be deciding on a skinny PCB that was a bit below 2″ lengthy. That obtained me pondering… pulls out ruler and a breadboard… Aha! Solderless breadboards have energy rails which are slightly below 2″ aside from one another! That might allow a person to have a number of boards plugged into and powered by a single breadboard’s energy rails! I actually love solderless breadboards, so this was excellent!

Nevertheless, this result in a query that is surprisingly onerous to reply: What is the spacing between these energy rails? You’d in all probability assume it is on a 0.1″ grid, however it’s truly not! A fast Google search yields only a few helpful outcomes, a few of which did not agree with one another. They’re normally throughout the vary of 1.85″ to 1.9″ between the outer rails, so I measured my very own breadboard:

Power rail spacing

I measured a number of different breadboards across the workplace, all had been very near 1.85″, so that is what I settled on. There could also be some breadboards which are barely totally different, however the PCB makes use of header pins to fit into the facility rails, which may bend barely if wanted. I additionally selected to make use of a row of 5 header pins on every finish, since that is the usual grouping measurement of the facility rails, and extra headers make the boards extra steady. That result in this ultimate structure:

LED Tester Layout

I additionally added some silkscreen across the potentiometer to point the approximate resistance because the knob is rotated; the arrow on these blue potentiometers is a good function for pointing to these marks! Nevertheless mechanical tolerances and parallax make the exact resistance onerous to learn, so I additionally added some take a look at factors (“R+” and “R-“) for measuring the precise resistance with a multimeter.

Be aware – you will see a number of variations of the board on this submit. Some are of the preliminary prototypes, some are of the ultimate product. Extra particulars are on the finish of the submit!

As soon as the PCBs arrived, I grabbed our mostly used LEDs, inlcuding a 1206 and 0603 of every crimson, yellow, inexperienced, and blue, plus a crimson 0402. That makes 9 boards, which I soldered collectively and caught right into a breaboard. I set the enter voltage to three.3V, and every potentiometer to 1k for comparability:

Assembled LED Testers

Identical Resistance (1k) – Uneven Brightness

First off, I really like how this seems! I’ve not seen a breadboard utility that completely makes use of the facility rails, however they positive are helpful for this! Second, there’s quite a lot of variance between these LEDs. The picture does not fairly match actuality, however you’ll be able to inform that inexperienced 0603 on the proper is manner dimmer than the remaining, and the blue 1206 on the left is totally saturating the digital camera sensor. Time to begin tuning!

I did not need to be the one to dictate the “greatest” brightness, so I surveyed a number of folks across the workplace. I had all of them alter the brightness of every LED till they had been pleased with it, and I recorded the resistance they set for every (3.3V enter). Here is my knowledge:

Survey data

As a lot as I really like watching numbers, it is normally greatest to visualise them ultimately. So I made a histogram of every column to match:

Histograms

That is fairly the unfold! I then took the median resistance for every LED, then chosen the closest commonplace resistor worth as our ultimate LED resistors for 3.3V inputs.

We additionally use 5V on quite a lot of merchandise, so I added a column to the spreadsheet for that. I may have redone the survey at 5V, however I truly did not must! The brightness of every LED is set by the quantity of present flowing by means of it, which we simply decided! So I can as a substitute use Ohm’s Regulation to calculate the resistor that leads to the identical present at a 5V enter, and the LEDs may have the very same brightness as earlier than! I then examined it in particular person, and was happy to see every LED on the similar brightness as earlier than. Now we lastly have our excellent resistor values for every of our LEDs:

Final Resistor Values

And the way do they appear? You inform me!

Assembled LED Testers

Custom-made Resistance – Even Brightness

Take into accout, these resistor values solely apply to the precise LEDs that we use! Your LEDs may require very totally different resistors, so you will must run your individual checks if you wish to obtain excellent brightness. However no want to fret about designing a customized PCB for testing, we have you lined there!

The entire time I used to be designing this board, I had the aim of creating this a product for customers to simply take a look at their very own LEDs. I’ve carried out my greatest to make it so simple as doable and promote it as a package, then customers solder the boards themselves. The package consists of:

  • 1x naked PCB
  • 1x 10k potentiometer
  • 1x 100 Ohm resistor
  • 2x 5-pin headers

LEDs will not be included with the package, as a result of the concept is for customers to check out their very own LEDs. The first aim is to be a great tool, however it may additionally function an academic useful resource for anybody new to electronics. It would be a enjoyable challenge for studying through-hole soldering, or perhaps a place to follow floor mount soldering!

It additionally does a great job of demonstrating Ohm’s Regulation, the place reducing the resistance will increase the present by means of a circuit. And it is a nonlinear relationship! For these of you accustomed to Ohm’s Regulation, you will know the present is definitely the inverse of the resistance. For some extra visualization, I’ve plotted the resistance and the corresponding present:

Linear Taper Current

Resistance vs. Present

So, because the potentiometer knob is turned to the left, the present (and the LED brightness) shoots up very quickly! I truly regarded into utilizing logarithmic potentiometers because of this, however could not discover one which I used to be pleased with. If there’s sufficient demand, we might look into sourcing a great logarithmic potentiometer for a future revision.

There have been a handful of different issues to scrub up earlier than releasing it into the wild. I initially made the board width precisely 0.6″, which made for a reasonably comfortable match that might trigger some boards to get jammed. So I decreased the board width by 0.015″ on either side to present some wiggle room. We additionally determined to launch it below SparkX, since that is extra of an experimental product, so I needed to change the logos from the primary prototypes. Lastly, we put a enjoyable graphic on the again aspect, which is the usual LED image with manner too many arrows popping out of it.

Final PCB

And that is that! In the event you’re all for having a few of these to play with, you’ll be able to choose them up beneath. Glad hacking!


LED Brightness Adjuster Package

In inventory


SPX-21226

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