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Help with Relay circuit

  1. Jul 5, 2006 #1

    I'm new to this forum, and I'll greatly appreciate your help. If I can get this electrical/computer engineering problem solved, I'll be that much closer in pursuing a career as a bio-physicist.

    With the help of my intor physics professor, I'm designing a juice delivery system in which the main components are a computer parallel port, a relay, a 12-volt battery, and a pump. As you can guess, once the computer sends the TTL signal, the relay switches to normally open, which triggers the 12V battery to turn on, which then causes liquid dispensation.


    1.) The computer's parallel port sends out a TTL signal of 2.5 to 3.5 volts, though I was told that it's supposed to output 5V.

    2.) The smallest relay I can find is a 5V mini relay that Radio Shack sells. If I cannot find a smaller relay, can anyone explain to me other alternatives?

    How is it that the parallel port's maximum signal is 3.5V?

    Again, any help will be greatly appreciated.
  2. jcsd
  3. Jul 5, 2006 #2
    There are different TTL standards. As far as I know, a logical high for TTL is anywhere between 2.4V and 5V, so I wouldn't count on it for switching.

    If you can manage a 5V DC voltage source somehow (dividing the 12V may or may not work, I dunno) , a simple transistor switch would probably work okay.
  4. Jul 5, 2006 #3


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    You could always use a power transistor to kick the voltage up enough to drive the relay. On the other hand, if you're going to do that, you might as well make the whole thing solid state.
  5. Jul 6, 2006 #4
    BoTemp and Danger, thank you very much.
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