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xihuitl
12-20-2008, 11:01 PM
I am trying to devise a 595 board for next year. I have read the DIY HOW-TO on computerchristmas.com, but have also found David Fansler's site at http://www.dv-fansler.com/Computer%20Lights/computer_lights.htm and am trying to combine the information from both designs, as they both have their merits.

The first difference that I am trying to overcome are the ULN2803's. David's design does not have them, and he seems to run the SSR's right off the 595's. Neither his controller schematic nor his SSR schematic shows them. The DIY has the ULN2803's inline between the 595's and the RJ45's. Since they are both calling for the 595's, are the ULN2803's needed? What is their purpose.

I tried looking at the data sheets for the 2803's but I am no technician/engineer.

Secondly, on the DIY board, they call for RJ45's for output, but only 4 wires are used on each (not including the +5V lead). This seems a waste to me. I would like to use DB9's with appropriate cabling. That should effectively cut the output connectors by near 1/2. If +5V is sent from the controller, what would be the effective max length of the cable? It would seem to me (no expert here) it would be pretty short.

I would like to incorporate (among other things) leaping light arches, which require 8 channels per color. By the DIY board, that would mean running two cables per color to each arch. Thinking of white and blue on each, which means 4 cables per arch. But if I am able to effectively send 8 channels per wire to SSR located near arch, then only 2 total wires need be run. This is only one example, but it would seem beneficial to put hermetically sealed SSR boxes near each display element instead of miles and miles of cords.

P. Short
12-20-2008, 11:18 PM
The purpose of the ULN2803 is to buffer the outputs of the HC595, so that they can drive more current and operate over a wider range of voltages. The raw HC595 output can only drive around 5 mA per channel at a limited voltage swing(0 to +5V). The ULN2803 is capable of driving up to around 1/2A per output, with a potential voltage swing of up 50. In other words, adding the ULN2803 provides much greater flexibility in using the boards.

The problem with trying to run eight outputs over a CAT5 cable is that you need nine wires (eight switch outputs plus power), while there are only eight wires in a CAT5 cable.

pete
12-21-2008, 01:27 PM
you may want to look at the Grinch design... It offers almost the same features as the DIYC 595. but at a lesser cost. and fewer parts.

Pete