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Thread: What is a relay? Laptop auto-WOL question.

  1. #1
    Maximum Bitrate DiNASTY's Avatar
    Join Date
    Mar 2005
    Concord, California

    What is a relay? Laptop auto-WOL question.

    Ok. I'm using my old HP ze5470us laptop as my carputer, instead of going ITX, to save money. I have every thing planned out except for the dreaded auto-start problem. I know there are other solutions for auto starting a laptop besides WOL or WOR, but the laptop is still under warranty and I donít want to void that by breaking it open.

    I have enabled WOL (Wake-on-Lan / Wake-on-Link) and can successfully wake the laptop from standby by plugging in a CAT5 cable with pin1 to pin3 and pin2 to pin6 connected. (I think Ambush came up w/ this) the problem now is how to connect/disconnect those 4 wires when the car is on/off. Ambush mentioned something about using a simple relay to do this.

    Can anyone explain how a relay could be of use in this situation?

    From what I understand, if I hook up a relay to a switched power source (cigarette maybe?) it will connect/disconnect using some sort of magnetic device depending on if it is getting power or not

  2. #2
    Variable Bitrate Saab9-5's Avatar
    Join Date
    Mar 2005
    Ponta Delgada, Azores
    Relay - A device that contains a coil and a set of contacts which close when the coil is activated.

    DPST (Double Pole Single Throw) - A four-terminal switch or relay that can open or close two separate circuits. Essentially, two SPST switches or relays combined into a single component.

    Your understanding is correct. It connects one (or more) switched circuits when power is applied to it. So, a relay with two contacts, one for 1-3 and one for 2-6, will work for you. You'll need one with a coil that handles 12 volts. Radio Shack has several. This site will also help you understand relays:

  3. #3
    Variable Bitrate
    Join Date
    Jul 2004

  4. #4
    Low Bitrate
    Join Date
    Jul 2007
    Do the pins need to stay connected the whole time the car is on, or would it be better for it to be only connected for a few secconds? I just didn't know if it would slow the laptop down at all making it think there is network info to process.

    This is just a question to help me understand this fix. Because this is really enticing to me.

  5. #5
    Raw Wave
    Join Date
    Nov 2009
    It depends on the laptop switching...

    If it is a main power switch (on else off), then yes, they need to stay closed (eg, relay terminals #30 & #87 using Bosch etc nomenclature).

    If it is merely a "momentary" to signal the PC to start, the it may need to be momentary - especially if the start signal is on the release of the switch.
    That too can be done... as linked above have "Constant to Momentary" relay circuits. (Not that I like delaying the relay itself - I prefer a transistor or FET delay that then switches the relay. IE - the RC (resistor-capacitor) are small and are matched to the transistor or FET which can switch on almost any relay. The RC for the relay (coil) direct need to be bigger for the much higher current, and will vary a lot depending on the relay....

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