Selecting Smart Pixels
Selecting smart pixels depends upon your end use. To help with your choice of smart pixel we will cover some of the basics, so that you can make an informed choice. There are two main places that hobbyist buy smart pixels. The first is a shop in China run by a man called Ray Wu. Buying from Ray is nice, since you can custom order the spacing of the pixels. The second place is DIY LED Express, which runs a group buy every year right after Christmas.
Which Pixel Chip?
It was my intent to place on this page a table comparing all of the different pixel controller types. However, most of the data sheets are incomplete, so there will just be some discussion on the desired characteristics.
The number one specification to review for a pixel controller would be the accuracy of the current for each of the outputs. The LED’s brightness is proportional to the current, so you want the current to be consistent for all of the LED outputs. Sadly most of the parts do not specify this tolerance. There are a few that actually mention the value and it is either the nominal value or the worst case (1% to 1.5% nominal to 6% worst case).
The data sheets for most of the RGB pixels do not specify the wavelength of color for each of the LED’s. I would want a LED’s where the brightness and color match between device to devices. Of the few manufacturers that I have perused, Cree is the only company that allows you to buy their LED’s with the colors matched.
Most of the LED’s run at 800Khz data rate, which is fast enough to output 150 pixels in 5mS. The only concern with protocol speed would be if you are running hundreds of them in series. Also, the refresh rate varies between all of the pixels varies from about 400 to 1600 hertz. This really does not matter for most people, unless you are spinning one line of pixels and trying to draw a 2D display. If this was the case you would want the fastest rate available. I used the WS2812B for the Christmas ornament, and had to lite two at a time to reduce the flicker, since they run at 400Hz.
Since there is little information on each of the pixel controllers, I would tend to choose one of the more popular types. This will ensure that from year to year you will be able to continue to buy your pixel of choice.
8/12 Bit LED Lights
Most of the LED controllers that you can buy are eight bits. However, there are controllers that provide more data bits for the brightness, such as the TLS3001 or CYT3001, which have 12 bits. In fact there are controllers with 16-bits of brightness control. Which type you buy depends upon many factors, but lets look at the supporting hardware and software.
First of all, looking at the software; Vixen, Vixen plus, HLS, and Nutcracker, they all support only 8-bit data per channel for lighting. If you look at the transport protocols; DMX, Renard, ACN1.31, and Pixelnet, they only support 8 bits of data per channel. And finally, most of the controllers also only support eight bits of color per channel. The software support for sequencing is probably the most important.
The SanDevices controller has mapping to convert the 8-bit data to 12-bit data. This is a good thing since you can map the brightness to the perceived brightness. Basically, you get better dimming at the lower brightness levels. For eight bit pixels if you do a slow dim you will see the steps in brightness at the dimmer end of the scale.
One more thing to consider is the accuracy of the pixels you have bought from China. These are low cost, low end parts. So you get what you paid for, the white color will not match from LED to LED, and many of the controllers do not specify their brightness accuracy. The data sheet for the TLS3001 indicates that the worst case accuracy between chips is ±6 percent, which is only about 4 bits of color accuracy. To be honest most of the pixels can produce fairly accurate pictures, but some correction to the pixels is required to get the best results.
12V or 5V Pixels
There are two basic voltages for LED strings, 5Vdc and 12Vdc, which one is better to use. Well that depends.
Let me explain a little bit about how LED’s work. The brightness of a LED is dependent upon how much current flows through the LED. So the voltage used to drive the LED does not really matter, it is the current. Nominally, most LED’s have a forward voltage of 2 to 3 Vdc. This means that you need to have a drive voltage greater than a the 2-3 volts, so both 5V or 12V will both work. But if you know that power is voltage times current it is clear the 5Vdc power is more power efficient.
Another interesting aspect of LED’s is that to the human eye, full current in the LED to half current brightness is barely discernible. But as you get near the bottom of the dimming curve our eyes are very sensitive and can see the small changes in brightness (for 8 bit lights or less). Most 12Vdc strings are driven at about the same current as the 5V strings, so it will take more than twice the power, however this is worth the ability to use smaller gauge wire and minimize power injection.
There are now some 12V strings that place a 5V power supply integrated circuit with each of the controller chips. These strings have very good uniformity of brightness, since each of the pixels are driven at a regulated 5Vdc. The use of each voltage depends upon the application.
5Vdc strings: Recommended use is for small items, signs and power over Ethernet.
12Vdc strings: Recommended use is for the majority of your display.
Types of Smart Pixels
There are many types of pixels that you can buy. A good place to start for your display would be with the bullet type pixels. These are the least expensive and sealed in hard epoxy, so they will survive just about any environment. The bullet pixels come in either a round sealed package as shown or in a square package. These pixels can also be bought with a bezel to cover the LED as shown below.
Pixels can also be bought on a pixel strip. These can either be in a waterproof jacket or open to the environment. The strips are available in two common pitches 30 and 60 LEDs per meter. These work well for trees, signs and icicles.
The final major type of smart pixel is a LED panel. These can be bought either as indoor or outdoor, with different pitches (3mm per pixel through 20mm per pixel). These require a special controller to drive them, since they are not really smart pixels. These pixels are either ON or OFF and the controller creates the intensity differences for each of the color red, green, blue LED's. These panels support a standard interface call HUB75. The panels are bought with either a 8:1 or 16:1 scan rate. This basically means that only 1 of 8 or 1 of 16 lines are on at any one time. So the 8:1 panels are brighter than the 16:1. Per pixel, this is the cheapest way to make a sign.
Last Edited May 31, 2015