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LPT port relay

Discussion in 'Electronic Basics' started by Test, Sep 22, 2008.

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  1. Test

    Test Guest

    I am trying to use LPT port to control a small 5 V relay (Meisei ps-5). The relay
    appears to have 1.75 ohm resistance.

    I connected the data pin to the + pin of the relay and ground to the gnd pin. No
    click. Voltage drops to about 2V. Then I tried with DC power supply at 4.5V
    (1.5A). A nice click is audible from the relay.

    (Bear with me: I am a newbie) I conclude that LPT is not suppliying enough
    current (live and learn!). After googling for some time I am presented with a new
    thing that should: a transistor - as in "transistor radio".

    This helpful site has a cirquit that fits the bill:
    here is it:

    | __|__
    Relay /^\ Diode 1N4002
    Coil /---\
    | |
    | /
    4.7K B |/ C
    parallel port >-\/\/\/\/---| NPN Transistor: BC547A or 2N2222A
    data pi |\ E
    | V
    parallel port >--------------+
    ground pin |

    Above need an extra power supply (Vcc). Is any way to use power (perhaps from
    multiple data pins) from the LPT port itself? One pin gives out 0-5V and a few
    milliamps. The Meisei relay I have seems to need more.
  2. You can't drive the relay directly because, IRC the LPT port can only source
    about 20mA and sink about 5 or 10.

    This is why they use the above circuit with the external power.

    I don't recall how much power the LPT can source total but probably not
    enough for your relay.

    Sure you can tie all the output pins together and hope for the best. If on
    goes low then you got a direct short. You can use diodes to prevent sinking
    any current but that will lower your voltage.

    LPT output ---- diode ----
    LPT output ---- diode ----
    LPT output ---- diode ---- All tied together here
    LPT output ---- diode ----

    The current will basically add up.

    So, suppose each output will give a max of 20mA before protection kicks in
    and you have 10 pins tied together, then thats a maximum of 200mA that
    you'll get. That's assuming the whole chip can output that much.. if not I
    imagine you have a real chance of burining up the port.

    It's pretty easy to get a wall-wart power suppose and use that with a
    transistor as the circuit shows... it's also a lot safer for your parallel
  3. Test

    Test Guest

    I've seen parallel port relay boxes that don't use an external power. How do they
    do it? Use low power relays?
  4. probably...

    Theres tones of info on using the lpt port for various applications. I'm
    sure if you search google you'll find the exact info your looking for.
  5. neon


    Oct 21, 2006
    If you think that a realy can follow an PLT data stream is just plain wishfull thinking
  6. neon


    Oct 21, 2006
    a relay following an LPT data stream.?.
  7. Jasen Betts

    Jasen Betts Guest

    1.75 is too low. 175 or 1.75K possibly?
    excellent conclusion!
    that's the way to di it!
    multiple data pins will probably not be enough.

    you could try pulling a 5V supply from the keyboard (or mouse) socket
    (pinout here: )

  8. Jasen Betts

    Jasen Betts Guest

    3 possibilities:

    latching relays (with extra circuitry)

    latchiong relays don't needed energy to stay turned on - a cuircuit
    with a capacitor could be used to store up energy to stwitch the relay.

    stronger parallel ports (some can put out 12mA on each data line)

    this relay would work if driven by 2 data lines:

    I did not find any that were able to be powered from a single data line

    solid-state relays.

    pretty much all solid-state relays only need a few miliamps.
    and so can be powered from the even the weakest parallel port.

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