Printer's port unable to control relay

Discussion in 'Computing and Networks' started by Somtom, Dec 13, 2013.

  1. Somtom

    Thread Starter New Member

    Dec 13, 2013
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    Hi all,
    i would like to use my computer's printer port to control an external relay, to switch on and off common electrical's appliances.
    I can see about 5 Volts coming from the port, but even if the relay says that just 3 Volts would be enough, it doesn't works.
    I found some suggestions online about using a "Darlington's pair" and also using 2 wires from the printer's port instead of just one (plus the ground wire), but none of this worked out.....
    Can someone in the knows please point me out to the right direction ?
    Thank you so much, Tom.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    http://www.epanorama.net/circuits/parallel_output.html Take a look at this site and how they use transistors to drive a relay. Also for some ports even a LED may require to much current of the port. A circuit that may help you a loy is the ULN2003A. It contains 8 darlington transistors drivers. Feel free to ask if you need help using the ULN2003A to drive LEDs, relays or something else
     
  3. Somtom

    Thread Starter New Member

    Dec 13, 2013
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    Thanks T06afre, i am going to try that soon, as i need to buy it first from a place far away from my current location.
    It's a shame there aren't any plug & play sort of kits available for a reliable price, all i could find is some way hyper priced boxes stuff from X10 or a company with a similar name....i am sure there must be a way to do what i need for a couple of dollars instead of hundreds, i'll keep trying as i haven't fried up anything yet :)
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    You also need an external dc supply to power the relay. I use an old 5vdc wall-wart.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I use a 2n7000 for individual output or the ULN2805 for 8 out. (the ULN2003 is 7 transistors).
    If you want off the shelf PP to I/O search for CNC BOB (Break out Board) controller.
    Max.
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    A not needed and not to old cell-phone charger may also do the job. At least if it is not a fake one
     
    Last edited: Dec 13, 2013
  7. Somtom

    Thread Starter New Member

    Dec 13, 2013
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    Thanks all for all the precious suggestion, i am going to start testing with the IC first, then, i will also try to locate someone that sell those BOB locally, as they look just as what i need, this one in particular http://www.kelinginc.net/c10.pdf
    I hope it got enough "power" to trigger my 3V IKE HRS4H-S relays, thanks!
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The relay, if 5v takes 100ma coil current.
    I don't think that BOB is rated for 100ma on the outputs?
    If only switching one or two outputs, it may be worth looking at the 2N7000, it was designed primarily for this use, TTL up to 60vd and 200ma.
    Max.
     
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