LED - 3. Let's make a large led display - Part 1 (LED Basics, Parts list)
What is LED?
A light-emitting diode (LED) is a semiconductor light source that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresponding to the energy of the photons) is determined by the energy required for electrons to cross the band gap of the semiconductor.[5] White light is obtained by using multiple semiconductors or a layer of light-emitting phosphor on the semiconductor device.[6]
Appearing as practical electronic components in 1962, the earliest LEDs emitted low-intensity infrared light.[7] Infrared LEDs are used in remote-control circuits, such as those used with a wide variety of consumer electronics. The first visible-light LEDs were of low intensity and limited to red. Modern LEDs are available across the visible, ultraviolet, and infrared wavelengths, with high light output.
Early LEDs were often used as indicator lamps, replacing small incandescent bulbs, and in seven-segment displays. Recent developments have produced high-output white light LEDs suitable for room and outdoor area lighting. LEDs have led to new displays and sensors, while their high switching rates are useful in advanced communications technology.
LEDs have many advantages over incandescent light sources, including lower energy consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. LEDs are used in applications as diverse as aviation lighting, automotive headlamps, advertising, general lighting, traffic signals, camera flashes, lighted wallpaper, plant growing light, and medical devices.[8]
Unlike a laser, the light emitted from an LED is neither spectrally coherent nor even highly monochromatic. However, its spectrum is sufficiently narrow that it appears to the human eye as a pure (saturated) color.[9][10] Nor, unlike most lasers, is its radiation spatially coherent, so that it cannot approach the very high brightnesses characteristic of lasers.
<From Wikipedia>
Type of LED
This contents are from https://www.solarlightsmanufacturer.com/cob-led-smd-led/ .LED lights are constantly improved to bring about better use, greater versatility and more efficient use of power. The different models of LED lights are highly popular compared to traditional incandescent lights, mainly because they can last for about 25 times longer with less energy use. LED lights come in a variety of models. Start comparing COB vs SMD LED Lights, or before you look at the difference between LED light COB and SMD.
SMD
The
SMD LED chips come in a variety of sizes. SMD LED can accommodate chips
with complicated designs, like the SMD 5050, which is 5mm wide. The SMD
3528, on the other hand, are 3.5mm wide. The SMD chips are small,
almost close to the design of the flat, square computer chip.
One of the distinct features of SMD LED chips is the number of contacts and diodes they have.
SMD LED chips can have more than just two contacts (which makes it
different from the classic DIP LED). There can be up to 3 diodes on a
single chip, with each diode having an individual circuit. Each circuit
would have one cathode and one anode, leading to 2, 4 or 6 contacts in a
chip.
This configuration is the reason why SMD chips are more versatile
(comparing SMD vs COB). The chip can include a red, green, and blue
diode. With these three diodes, you can already create virtually any
colour simply by adjusting the output level.
SMD chips are also known to be bright. They can produce 50 to 100 lumens per watt.
SMD,
or “Surface Mounted Device” LEDs, are the most common LEDs in the
market. The LED chip is permanently fused to a printed circuit board,
and it’s highly popular due to its versatility. You can find it in light
bulbs and string lights, and even in the notification light on your
mobile phone.
|  |
COB
One
of the most recent developments in LED, COB or “Chip on Board”
technology is a step forward more efficient energy use.Like the SMD, COP
chips also have multiple diodes on the same surface. But the difference
between LED light COB and SMD is that COB LEDs have more diodes.
|  |
COB
chips typically have 9 or more diodes. COB chips also only have 1
circuit and 2 contacts, regardless of the number of the diodes. This
simple circuit design is the reason for the panel-like appearance of COB
LED light (SMD light, on the other hand, appears like a collection of
smaller lights).
But unlike SMD, COB LED lights can’t be used to create colour changing
bulbs or lights. This is because there are only 2 contacts and 1
circuit. Multiple channels for adjustment are required to create the
colour changing effect. Because of this, COB LED lights are efficient in
single-color applications, but not in more versatile technology.
Another aspect of the COB vs SMD LED Lights difference is in the use of
energy. COB is known for better lumen-per-watt ratios and heat
efficiency. This has a lot to do with the design of COB LEDs, and the
cooling ceramic substrate of the chips.
Before, heavy duty technology like spot lights and flood lights made
with LED were non-standard, because you’d need multiple LED sources to
produce that kind of high lumen output.
But now, COB chips can produce a large amount of lumens with less
energy. You can find it in all kinds of bulbs and applications, such as
the flash of your mobile phone or point-and-shoot camera. Its range is
higher with a minimum of 80 lumens per watt.
LED Display Pixel Pitch and Resolution
This contents are from http://www.finepixelled.com/led-display-basics/led-display-pixel-pitch-and-resolution.htmlIf you are looking for LED display China, you run across the technical terms like LED display pixel pitch or LED screen pixel pitch. Understanding the pixel pitch and relation between the pixel pitch, resolution and viewing distance helps you to make best decision according to your requirements, space and budget.
What is the pixel pitch?
The pixel pitch is center to center distance of adjacent pixels measured in millimeters. In a plan term, pixel pitch shows the gap between the pixels of LED display. The smaller pixel pitch means there is smaller empty space between the pixels and there are more number of pixel, results in higher resolution and higher image quality. Figure below illustrate the pixel pitch for a high resolution SMD LED module and standard resolution DIP LED module.
This is the distance between the LED pixels. P3 has a pixel-to-pixel distance of 3mm, P4 of 4mm, and P5 of 5mm.
What is effect of pixel pitch size on image quality?
Pixel pitch determine the resolution, minimum viewing distance and best viewing distance of LED screen. The smaller the pixel pitch, the more pixel and results in more details and higher image quality. So if you need to show high resolution images or video on your display, you need LED display with small pixel pitch. The following figure shows the pixel pitch effect on image quality.Pixel pitch affect the viewing distance too. The minimum viewing distance is the distance that you can watch image and video with acceptable level of pixilation. The smaller pixel pitch, the smaller gap between the pixel and less pixilation of the image. Therefore, the viewer can stand closer to your display.
How to choose the best Pixel Pitch size?
In my other post(https://iot-for-maker.blogspot.com/2020/01/led-1-how-to-design-led-signage-and-led.html) , you can see this equation.
Required Pixel Pitch in millimeter (mm ) = “Viewing distance in meter”(m )
So the viewing distance is about 10m, P10 LED is sutable, the viewing distance of 3 ~ 5M, then P3, P4, P5 is sutable.
To choose the optimum pixel pitch two factors should be considered, the viewing distance and the required image resolution. Smaller pixel pitches are better all the time and deliver you better image quality but, it costs more. You may decrease the expenses of LED display buy using larger pixel pitch and still have almost same image quality if the viewing distance is longer than best viewing distance. The best viewing distance of a pixel pitch is the distance that your eyes won’t be able to the gaps between pixel anymore if you go further away.
Take an LED screen with P10 modules (pixel pitch of 10mm). The minimum viewing distance for P10 is 10m and the best viewing distance is 20m. So, if people watch it from 10m to 50m away, they can see an acceptable image quality with a fading level of pixilation from 10m to 20m and a very good image quality after 20m. So you may want to increase the image quality using P8 or even P6 module for close range. But If the viewing distance was from 20m to 50m, there was no point to use smaller pixel pitch with higher resolution. Because, the viewing distance was already more than best viewing distance of P10.
As a rough estimation, P10 (10mm pixel pitch) is suitable for any outdoor application with viewing distance of 10m and more. P8 or P6 (6mm and 8mm pixel pitch) can also be used if target audience are in closer distance. For indoor led display, P6 may suit any advertising needs with viewing distance of 6m and more. P4 (4mm pixel pitch) can also be used if your audience are closer such as LED video wall in retail environment.
Parts List
I like arduino, but I like linux based Raspberry Pi better. So I'll control the LED display with Raspberry Pi 3.Raspberry Pi
I recommand Raspberry Pi 3B, 3B+, Zero, Zero W.I don't recommand Raspberry Pi 4.
RGB LED Matrix with HUB75 connections
P3 32 X 64LED Matrix 2EA (P4, P5 also OK). Once you've completed , it's very easy to change your P3 display to something else. Therefore, prepare a product that is easy to get.
If you want a cheaper product, you can buy a product from China at AliExpress(https://www.aliexpress.com/item/32739844613.html?spm=a2g0s.9042311.0.0.66f94c4d7Ls382).
This product's led pixel size is 21 X 21 mm. SMD2121 (or 2121 SMD) is used.
<SMD2121 Light source>
Hub75 interface
This RGB LED matrix back pannel has two hub75 interfaces.
Here you see a Hub75 connector to be seen at the bottom of the RGB panel board including the arrow indicating the input direction. Other boards are very similar, but instead of zero-indexed color bits
R0, G0, B0, R1, G1, B1, they start the index with one and name these
R1, G1, B1, R2, G2, B2; the functionality is identical.
<hub75 connection image from https://github.com/hzeller/rpi-rgb-led-matrix/blob/master/wiring.md>
We will use the hub75 interface to communicate the Raspberry Pi and the LED matrix. It will also be used to transfer data by connecting multiple led matries.
Power Supply
5V 10A SMPS Power supply(Please see the datasheet of your LED matrix to check the power consumption)
It doesn't matter what type you have in the picture below. It only requires to supply enough current, 5V voltage.
Adafruit RGB Matrix HAT (or Bonnet)
Adafruit offers excellent products that make it easy to control LED matrices in Raspberry Pi. You can create a dazzling display with your Raspberry Pi with the Adafruit RGB Matrix HAT or Bonnet. These boards plug into your Pi and makes it super easy to control RGB matrices such as those we stock in the shop and create a colorful scrolling display or mini LED wall with ease.
< HAT Type Bonnet Type >
Bonnet type is designed for Pi Zero, however, it does not matter with the 3B model.
You can buy this product at https://learn.adafruit.com/adafruit-rgb-matrix-plus-real-time-clock-hat-for-raspberry-pi
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