Not will sun shades be required to have a look at LEDs!
Howdy of us! I am Dryw, a brand new engineer right here at SparkFun! I joined in March of this yr and have already launched a handful of merchandise, similar to these Bosch strain sensors and the IoT Redboard – ESP32. I’ve additionally been engaged on some extra thrilling merchandise that shall be popping out quickly, keep tuned for these!
As I have been working right here, I’ve observed loads of our merchandise use completely different resistor values for LEDs. Constant LED brightness hasn’t been a essential design requirement for us, which has resulted in numerous brightnesses throughout our catalog. Essentially the most extreme instance I noticed was on a prototype, the place the inexperienced and yellow LEDs had been barely seen, and the pink and blue LEDs felt like staring into the solar! So I made it my mission to seek out the right resistor for every of our LEDs, so as to give a constant brightness throughout all our merchandise, and to by no means want sun shades ever once more!
This can be a pretty easy downside to unravel, so that you is likely to be pondering “simply take a look at the LED’s datasheet and compute the perfect resistor with Ohm’s Regulation.” You are completely proper, and that will be a completely legitimate strategy. Nonetheless there are some nuanced human components that make this extra tough than it first seems:
- Completely different colours will not be all perceived equally. For instance, human eyes are most delicate to inexperienced gentle, so they’ll seem to have a distinct brightness than pink or blue LEDs regardless of having the identical absolute brightness.
- The scale of the LED impacts the perceived brightness. For a similar absolute brightness, bigger LEDs seem dimmer as a result of gentle being produced over extra floor space.
- Completely different cone angles change the quantity of sunshine that really hits your eye. Some LEDs produce gentle 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 a way brilliant a millicandela is!
- Completely different individuals need completely different brightnesses. In my testing (see beneath), there was a variety of brightnesses that folks most well-liked.
- Completely different ambient lighting circumstances require completely different LED brightnesses.
On high of that, there are some further technical concerns:
- Every shade requires a distinct voltage. Shorter wavelength photons are extra energetic, so blue LEDs sometimes require the next voltage than pink LEDs to activate. That is also referred to as the “ahead voltage drop” of the LED.
- Completely different colours require completely different quantities of present to provide the identical brightness. In my testing, inexperienced LEDs often required far more present than pink or blue to realize the identical brightness.
- The identical LED produced by completely different producers have completely different brightnesses for a similar currents. Essentially 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 components could possibly be thought of 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 started working determining one of the best 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. The truth is, we truly use 3 completely different sizes of LEDs, together with 0402, 0603, and 1206. So fairly than making completely different PCBs for every, I figured I might embody 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 might check any LED!
In collection with these LED footprints, I included a potentiometer as an adjustable resistor. I knew we would not have to 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. This is what the schematic appears like:
To be clear, just one LED is added to every board. There are simply 5 completely 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 underneath 2″ lengthy. That obtained me pondering… pulls out ruler and a breadboard… Aha! Solderless breadboards have energy rails which are slightly below 2″ other than one another! That will 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 good!
Nonetheless, this result in a query that is surprisingly arduous to reply: What is the spacing between these energy rails? You’d in all probability assume it is on a 0.1″ grid, nevertheless 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 often throughout the vary of 1.85″ to 1.9″ between the outer rails, so I measured my very own breadboard:
I measured just a few 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 completely different, however the PCB makes use of header pins to fit into the ability rails, which might 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 dimension of the ability rails, and extra headers make the boards extra secure. That result in this last format:
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 superb function for pointing to these marks! Nonetheless mechanical tolerances and parallax make the exact resistance arduous to learn, so I additionally added some check factors (“R+” and “R-“) for measuring the precise resistance with a multimeter.
Observe – you will see a number of variations of the board on this publish. Some are of the preliminary prototypes, some are of the ultimate product. Extra particulars are on the finish of the publish!
As soon as the PCBs arrived, I grabbed our mostly used LEDs, inlcuding a 1206 and 0603 of every pink, yellow, inexperienced, and blue, plus a pink 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:
Identical Resistance (1k) – Uneven Brightness
First off, I really like how this appears! I’ve not seen a breadboard software that completely makes use of the ability rails, however they positive are helpful for this! Second, there’s loads of variance between these LEDs. The picture would not fairly match actuality, however you’ll be able to inform that inexperienced 0603 on the proper is approach dimmer than the remaining, and the blue 1206 on the left is totally saturating the digital camera sensor. Time to start out tuning!
I did not wish to be the one to dictate the “finest” brightness, so I surveyed a number of individuals across the workplace. I had all of them alter the brightness of every LED till they had been proud of it, and I recorded the resistance they set for every (3.3V enter). This is my information:
As a lot as I really like observing numbers, it is often finest to visualise them not directly. So I made a histogram of every column to check:
That is fairly the unfold! I then took the median resistance for every LED, then chosen the closest normal resistor worth as our last LED resistors for 3.3V inputs.
We additionally use 5V on loads of merchandise, so I added a column to the spreadsheet for that. I might have redone the survey at 5V, however I truly did not have to! The brightness of every LED is set by the quantity of present flowing via it, which we simply decided! So I can as an alternative use Ohm’s Regulation to calculate the resistor that ends in the identical present at a 5V enter, and the LEDs may have the very same brightness as earlier than! I then examined it in individual, and was happy to see every LED on the similar brightness as earlier than. Now we lastly have our good resistor values for every of our LEDs:
And the way do they give the impression of being? You inform me!
Personalized Resistance – Even Brightness
Consider, these resistor values solely apply to the precise LEDs that we use! Your LEDs might require very completely different resistors, so you will have to run your personal assessments if you wish to obtain good brightness. However no want to fret about designing a customized PCB for testing, we have you coated there!
The entire time I used to be designing this board, I had the aim of constructing this a product for customers to simply check their very own LEDs. I’ve carried out my finest to make it so simple as attainable and promote it as a equipment, then customers solder the boards themselves. The equipment contains:
- 1x naked PCB
- 1x 10k potentiometer
- 1x 100 Ohm resistor
- 2x 5-pin headers
LEDs will not be included with the equipment, as a result of the thought is for customers to check out their very own LEDs. The first aim is to be a great tool, nevertheless it might additionally function an academic useful resource for anybody new to electronics. It might be a enjoyable venture for studying through-hole soldering, or perhaps a place to apply floor mount soldering!
It additionally does an excellent job of demonstrating Ohm’s Regulation, the place reducing the resistance will increase the present via 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:
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 for that reason, however could not discover one which I used to be proud of. If there’s sufficient demand, we might look into sourcing an excellent 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 fairly comfortable match that would trigger some boards to get jammed. So I diminished the board width by 0.015″ on all sides to offer some wiggle room. We additionally determined to launch it underneath 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 facet, which is the usual LED image with approach too many arrows popping out of it.
And that is that! In case you’re concerned about having a few of these to play with, you’ll be able to decide them up beneath. Pleased hacking!