This post attempts to gather all of the important Point Source Pixel information in one area. Click on any picture below to enlarge.
For pictures, video clips and more, take a look in the ‘archives’ section and peruse older posts.
The point source pixel system was designed in 2008 to be bright, easily chained and easy to use. To that end, we settled on a system comprised of driver boards, a ribbon cable wiring harness, and small circuit boards containing the RGB LED and appropriate DMX reception hardware.
Each pixel includes a standard RS-485 receiver and a small microcontroller. The receiver chip converts a balanced differential signal into a standard logic level that the processor can understand. Each pixel requires three consecutive DMX channels: one each for red, green and blue. Red is always the first channel, green the second and blue the third.
Of course, the start address of each pixel can be set to any number between [1 510]. Thus, 170 pixels can be discretely controlled by a single DMX universe.
Ribbon Cable Adapter
A driver board has headers for power and DMX in/through. It contains an RS485 receiver chip and 2 RS485 transmitter chips. Since RS485 is meant to be used in shielded, twisted pair cables (rather than ribbon cable) we decided it made most sense to drive moderately short (10′ – 20′ long) cables. If the ribbon cable is much longer, signal integrity of the high-speed DMX data is compromised.
Thus, up to 32 driver boards may be daisy chained together. Each driver board has two outputs. Each output can drive between 1 and 32 RGB pixels.
The system’s backplane is based on regular 10-conductor ribbon cable. Since these are very light cable gauges, we use 4 conductors each for power and ground. The remaining pair transmits the DMX data.
To build the backplane, we installed 10 conductor IDC female connectors at regular points along the ribbon cable. Spacing between 6″ and 8″ seems to work well.
Point Source Pixels have a 10-pin male header, which conveniently attaches to the ribbon cable backplane. Naturally, the header pinout matches that of the splitter / driver boards.
10 pin male header #67821, $0.21 @ qty 10 (included on assembled circuit board)
10 pin female IDC socket #32492, $0.25 @ qty 10
10 conductor ribbon cable #643815, $15.18 / 100′
Each pixel has a built-in voltage regulator which converts Vin (usually 8-12V DC) to +5v required by the processor and LED. Note that the RGB LED is driven from the processor’s pins. This means that the board input voltage can vary somewhat, so long as it is high enough that the voltage regulator can function properly. We’ve had spectacular success using switching power supplies designed for / removed from computers. They output +12v DC at many, many amps, and for a very reasonable price. Try www.weirdstuff.com if you don’t have anything lying around. At full power, each pixel draws just over 60 mA at 5 volts. A bit of Ohm’s law shows that
.06 A * 5V = .3W
12V / .3W = 40 mA drawn from a +12v supply.
So choose a properly sized power supply for the job.
Naturally, for most effective operation each pixel requires a unique DMX address.
Since each board is too small to contain a DIP switch or display + pushbuttons, there are two ways a pixels DMX start address can be set.
In firmware at compile time. Using a PIC programer and Microchip’s MPLAB, each pixel’s start address can be hard-coded into program memory. This works well (we’ve done it thousands of times) but gets tedious after a while. Also, a special programming harness is required to access the proper pins on the chip.
With an external programmer. We designed the firmware to listen for a specific sequence of data, beginning with a ‘non-zero’ start code. This start code / data sequence would never occur in a regular lighting system. Think of it as a secret knock on the pixel’s back door. When the proper data and checksum is received, the start address is stored in the processor’s permanent memory. Changing addresses is easy and takes only a few seconds and can be done without any special computer programs. See this link for a description and video clip of the field programming system.
That’s a decent overview of the system. Questions? What have we missed? Send an email to john AT response-box.com.