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Thread: usb cutoff switch

  1. #1
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    usb cutoff switch

    I got a small utilitarian speaker for my system for basic feedback. It's connected through usb for power and audio jack for sound. Problem is when computer has NO power (as in 12V feed is cut)...not just off, I get a hum. I'm assuming this is because the ground disconnected by unplugging the computer (I'm doing this in my home right now). I want to know if once I connect the dcdc usb power regulator to the computer if it will do the same when I turn off the car, or is the ground constant and it's just the 12V that's turned off/interupted by the regulator(I plan on connecting the + and - of the battery instead of grounding to the chassis somewhere)?
    Or do I have to get some type of usb cut off switch to cut connection to the computer when it's off?

  2. #2
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    OK, this is what I'm thinking, need some help because I haven't actually done this before.
    If put a relay on the usb + wire I should be able to cut the power with the added benefit of preventing the thump when the car shuts off. However, if I understand relays properly, it needs constant power to hold a circuit closed. That's fine if I wanted to use the 12V for the relay, but what I wanted to do was use the ignition on the relay. That way when I shut the car down, the speaker shuts down before the dc-dc shuts down the computer. That way I get no thump and no hum. So,

    1/ Is the ignition wire constant power that can hold the relay closed?
    2/ Do I have to use the relay on both + and -, or can it just be +?
    3/ I understand there are many types of relays, could someone point out the right one to use in a situation like this?

    Thanks.

  3. #3
    MySQL Error soundman98's Avatar
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    1. typically you would use the accessory wire for that-- the wire that turns the radio on/off. this wire should have power when the key is turned to the 'acc' and 'on' postions, but not have power at the 'off' or 'start' positions.

    2. switching the + only is fine.

    3. any standard relay will work. because your only switching one wire, even the most basic relay will meet your needs.-- normally it is referred to as a 'SPST' relay, or Single Pole Single Throw relay--the 'single pole' means that when it gets activated it switches a device on only(double pole relays have a second set of contacts to allow something to stay on until the relay is activated), and the 'single throw' means that it can only activate one wire.

  4. #4
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    I'm a little confused about the pins on the relays. I only need 3, as far as I know, but all the ones I see have 4 and 5.I did find one relay with 3 wires, but seeing that it is the only one I found like that, I don't know if it's special for something and wont serve my needs. Which do I use?

    (5 pin) http://www.amazon.com/HELLA-00779430...6652054&sr=1-9
    (3 wire) http://www.amazon.com/Directed-Elect...d=PRH5W1ENIWF6

    Which should I get? If it's the 5 pin, how do I connect that?

  5. #5
    MySQL Error soundman98's Avatar
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    pins 85 and 86 are the 'trigger' one gets connected to the positive acc wire i recommended before, and the other one gets connected to the metal car chassis--which is directly connected to the negative terminal of the battery.

    so when power flows through that acc wire, it completes the circuit, making the relay turn on.


    pin 30 connects to your speaker, and pin 87 connects to the motherboard--these are the 'normally open' terminals, meaning they are not connected unless the relay is turned on.
    Last edited by soundman98; 01-15-2012 at 03:58 PM.

  6. #6
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    Thanks for your help. I got everything and put it together, it didn't work...as expected. I realised after the fact that the pop would still happen because the speaker is also self powered. So I put the relay on the headphone cable and it seems to function as it should. But I now have another minor issue. Is there a quieter relay, or do all of them have this loud click (loud relative of course)?

  7. #7
    MySQL Error soundman98's Avatar
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    i am sure you can locate a quieter version, but i really don't know where you could start looking-- the 'loudness' of relay's isn't usually something that they list in the product specs...

    maybe try to locate models with lower amperage-- the theory is that when the relay can handle less amperage, the contacts that cause all that clicking should also be smaller, hence less mass to make noise..

  8. #8
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    You could buy a (more expensive) solid state relay. This would eliminate the clicking noise.
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  9. #9
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    I found this:

    http://www.amazon.com/Opto-22-Contro...hu-rd_add_1_dp

    But it doesn't say anything about "SPDT". Will this suit my needs? Or do I need to get a higher amp version? Or something different?

  10. #10
    MySQL Error soundman98's Avatar
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    it might suit your needs. though this worries me:

    "Drop out voltage: 1.5 volts at 3 amps"

    so that means that any source connected to it will loose 1.5v-- if this was connected to the main power wires in the car, when your car is normally running at the typical 14v, by the time the power gets to the device, it will be 12.5v. i don't believe that would be adequate for a usb device.

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