Step 6 : Building a prototype (cont)


Here is the PCB layout for the schematic in fig 1. I like this design and plan to make a bunch of these. I especially like it because it is a "cookie cutter" type of design. In other words, you can make SSR boards with as many channels as you need. Just cut off the design where you want to stop and transfer it to the clad board. This layout is for the prototype design and some minor parts placement changes took place on the final version. See my how to entitled "How to build a 12 channel SSR" and you'll see the final layout.

Step 7 : Controling the whole mess


Let me introduce you to my little friend! This is the ULN2803/2804 octal high voltage Darlington transistor array. The 2803 is for TTL level input signals, the 2804 is for CMOS. This wonderful little chip has 8 channels that can be individually controlled. Each channel will sink up to 50 VDC at up to 500ma (The 500ma is for all channels combined so you have to do a little math if you are switching high current loads). Application of this chip couldn't be easier. The output of any type of controller, whether it be Hill's 300 port controller or the parallel port itself, is connected to pins 1 thru 8. When a signal is applied to the input pin, the corresponding output pin (pins 18 thru 11) is connected to ground. Using this chip as a buffer between your controller and your hardware will allow you to use different types and voltages of SSR boards as well as directly control relays with the same controller.

Step 8 : That's about all

That's really all there is to it. From here the choices of application are endless. Some folks put all their controllers and SSRs in one place and then run extension cords outside to the various displays. I myself prefer to have the controller indoors, and the SSR boxes outside spread throughout the display. That way all the power is handled outside, and the only thing that goes out the window is 1 controller cable.