Already found them . Was waiting for the go ahead on this. Going to pop out and buy one now. Makes this entire project a hell of a lot simpler for me.
Thanks, I'll update when it's built.
Connect the ULN2803 outputs to one side of each relay coil (or directly to your load, if it's small and you don't want to use a relay).
Connect the other side of the relay coil (or load) to 12V.
Connect the common free wheeling diode output to 12V.
Connect the GND pin to ground.
That's it, besides ULN2803 you only need the relays.
If you are not familiar with the term "darlington transistor" you can read about it here: http://en.wikipedia.org/wiki/Darlington_transistor
Here's one that can handle 1A DC continuous load current (Connection C): Omron G3VM-61B1 http://uk.farnell.com/omron-electron...pno/dp/9918965
And here's one that can handle 3.6A DC continuous load current (Connection C): NEC PS710A-1A-A http://uk.farnell.com/nec/ps710a-1a-...p-6/dp/1682874
And another UK source: http://uk.rs-online.com
Same Omron G3VM-61B1 as above http://uk.rs-online.com/web/search/s...duct&R=4801166
Thanks for the replies.
I have just designed this. Looks good to me but could do with approval?
Hard to tell anything from that drawing. The only thing in there is a ULN2803 darlington array and a diode. YOu can't tell what all the connections are going to in your drawing.
What's the diode in the drawing for? You can just connect pin 10 directly to 12V.
It would be easier if you made a schematic.
If you don't have a program for drawing schematics and PCBs, you can use CadSoft Eagle which is available in very popular freeware version: http://www.cadsoft.de
But the circuit is pretty straightforward.
If you want to connect relays to the ULN2803, the schematich should look like this, just add 7 more realys to the other outputs if you want.
And instead of AC mains you could of course connect the load to 12V and GND.
Also beware that your circuit will behave extremely unpredictably upon booting. The ports send out random junk on initialization. You may want a simple SIPO register ANDed with your outputs to make a very very simple "key" almost.
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Found this parallel port kit with ULN2803.
This is pretty much how a relay interface should look like unless you want to add the functionality 2k1 Toaster mentioned.
Look for kit 74 http://www.kitsrus.com/kits2.html
Instead you can also use a serial input latch to connect to your parallel port. This way you can get more than 8 outptus.
Read here: http://www.phanderson.com/printer/serial/serial1.html
Or this application note: http://www.micrel.com/_PDF/App-Notes/an-2.pdf