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Hi guys,
I'm in the first stage of planning a school project.  This includes operation of solenoid valves through a computer.  However, I am totally lost were to start from here.  Some of the questions I have right now:
1) When is it necessary to use relays?
2) Would this (http://www.paia.com/proddetail.asp?prod=K74) board be suitable?
3) Is there an alternative board that has USB connectivity?
4) I heard Darlington transistor arrays can replace relays and minimize the circuit size. Would you recommend this approach instead of relays?
4) How do I write a simple program in VB that opens/closes up to 8 valves?
Your help is very much appreciated. Cheers, Philipp

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Hello phinder and Kevin,
I am not sure what type of power you need for these solenoids, but here is an example of controlling the mains voltage from a PC. I can't imagine you would need any more power than this for the solenoid.
You can use the parallel port of the PC to set a pin high, which turns on a solid state relay. I prefer solid state relays to the mechanical types as posted in your link.
You can duplicate such a circuit and use D0, D1, D2, D3, D4, D5, D6, and D7 to control 8 different SSRs (Solid State Relays).
Here is an example. Click on the image below to enlarge it.

MP

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I don't know why you wouldn't have the minimum 3 to 5 volts on the data lines of your parallel port unless you either measured incorrectly or you have a problem with your port. Check to see if it operates your printer. The ULN chip is not needed for parallel connection to an SSR. It needs no more current or voltage than your printer, if it were attached.

MP

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