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Thread: Creative use of relays - will this work?

  1. #1
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    Lightbulb Creative use of relays - will this work?

    I thought up something that to me seems like a very cunning plan, but that may be because I don't know my volts form my amps... I have spent the evening looking around this forum, and I haven't found anything quite like this idea. Here goes:

    If I take the trigger power for the relay I use on my inverter from the 12 V on the PC, the inverter will turn off when the PC shuts down. And if I use another relay to send a pulse to the ATX power switch when my power doors lock, the PC will go into hibernation and cut the power. Then both the computer and the inverter will be off. And I never lock the doors if I'm not going out of the car (I usually lock it with the alarm remote, but when I leave the dog in the car I have to lock it manually, that's why I want to connect the shutdown relay to the door locks). And to turn on the PC (using automatic power on from the BIOS) I can run an extra relay for the inverter trigger with a jumper button on the dash (since I don't always run the PC when I take a short drive) for the inverter.

    I don't have to worry about what might happen if the computer does not get the shut down message every single time (actually I'd be satisfied with 90 %) since the inverter has a low power shut down and I will power this whole system on a battery separated from the main battery - my Suburban does not have it now, but I'm preparing to put in an extra battery and a multi battery isolator since I have room for another battery under the hood.

    Are there any flaws in this? I hope the regular 12 V on the computer is strong enough to power a stock car relay, like the one you use on extra lights and stuff. That really doesn't take much current, does it? And the computer's powersupply won't be in connection with the car's power supply since I only use it for the relay duty, not to send the actual power to the inverter.

    Have I found a very simple, cheap and efficient solution to my specific problem (turning the computer off when I lock the doors, but turning it on with a button) or am I a very simple, cheap and efficient idiot?

    Thanks a lot in advance!
    Tor - The MediaSUV

  2. #2
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    Well you'd need to hold down the button on the dash for several seconds for the PSU to fully power up, and you'd need to use diodes to seperate the car mains from the +12V rail on the PSU (on both + and - since the PSU ground is seperate from the car ground)

    But yes, I'm no electronics expert but that sounds like it would work for your situation.

    Basically you use the power door lock relay to trigger a ATX power switch pulse in the hopes of turning the computer/PSU off.

    You use the +12V from your PSU to keep the relay closed for the power to the inverter, and you have a pushbutton in the front that will momentarily give that relay for the inverter +12V from the car to close it until the PSU can power on, at which point the PSU maintains the relay closed.

    I think if you use 4 diodes to isolate the car and PSU power from the inverter relay you should be OK. The only problem that I can see is the computer might not always power itself off when the power door locks press the power button, there are many cases in which it might put up a user prompt or something preventing shutdown. In this case you'll drain your second battery, but as long as it is seperate from your main battery then I guess it's not that big a deal.

    You could also load a program on your computer to force a shutdown once the power button is pressed, so no matter what prompts come up it will always shutdown. This should limit the cases when it might drain your battery.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  3. #3
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    Eh...using the PSU for the inverter relay means that no current from the PSU is in contact with the car's power system, it's basically like turning the power on to an electromagnet, as far as I know. Which means that I don't need diodes (and I stay away from anything that's more difficult to pronnounce than "relay"...). And except for during a short test period with Blutetooth control of the media PC (I now use WLAN) I have never seen any user prompts, so that does not worry me. But thanks, it seems I'm not totally off my rocks with these thoughts. Gonna be fun to test it out in real life!
    Tor - The MediaSUV

  4. #4
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    How do you expect to have the PSU isolated from the car mains if the relay can be powered by both the PSU and your pushbutton switch which connects the relay to car power? While the pushbutton is depressed you have essentially connected car ground to PSU output ground and car +12V to PSU +12V output which = BAD. Otherwise you'd have to have your pushbutton disconnect the PSU leads and connect the car leads when you push it, and when you release it return the connection to the PSU leads. I think that you're gonna need to use a few diodes. But not to worry, they are simpler to use than a relay so it's really not that big a deal

    I repeat: If you are going to have both car and PSU power to power the relay, then they will be crossed as long as your pushbutton is depressed which will likely cause damage to your computer unless you use relays.
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  5. #5
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    Sorry, Telek, I was thinking something and writing something else. I meant to say that I use two relays that controls two parallell power cables to the inverter, so the PSU runs another relay, only it's the same current that's let through the switch in the relay.
    Tor - The MediaSUV

  6. #6
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    Oh OK, yeah I guess that would work, but it would have been much easier to do just using diodes (and cheaper).

    But whatever works!
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  7. #7
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    Perhaps easier, as long as I had you holding my hand through it, but cheaper? Let me give you a hint: A relay costs around two bucks at the local service station. Any other electronic material out here in the sticks will cost me about 12 bucks...in shipping and handling alone! So I think I'll just do a dual relay.
    Tor - The MediaSUV

  8. #8
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    You have no electronics stores at all in your area? These diodes are like $0.10 each

    But I guess if the relays are only $2 then it's no problem to just use two
    IN DEVELOPMENT -- '96 Mustang, lilliput with PII/450 laptop, custom DC-DC power supply, 60GB; Garmin GPS; 802.11g; compact keyboard, small graphical LCDs, OBDII.

  9. #9
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    The closest one that I know of is a couple of hours drive. But maybe a radio repair shop has these things? What should I ask for in that case?

    And thanks for your help!

    Edit: I called my brother who works for a computer company, and it turned out that he had tons of those things laying around, so he'll send me some tomorrow! It's going to be interesting to try to widen my horizont a bit...
    Tor - The MediaSUV

  10. #10
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    My brother didn't get his *** off the ground, so I got the relays and did it that way. It works better than I dared to hope! I only have to press the button on the dash for half a second, and the PC is running! And I have done around 20-30 startups/manual shutdowns so far without a single error. Next step will be hooking it up to the power doors.
    Tor - The MediaSUV

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