Induction sensor to turn on a device?

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  #1  
Old 08-30-11, 08:01 PM
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Induction sensor to turn on a device?

Hi all, I'm not electrical savvy. I have a USB hube that is powered via A/C outlet and the computer's 5v rail.

I have a smart powerstrip that cuts power to the usb hub when the computer turns off. Unfortunately, the computer still receives 5v's and will take over powering the USB hub. (There is no way to turn this off via BIOS on my mobo or software, I checked)

I'm trying to rig something that will detect current on the a/c outlet cable, and will operate a switch on the USB 5v power. So when the a/c power source is off, it will open (break) the 5v circuit going to the computer, and vice versa.

What parts would do this? I believe there is a horseshoe shaped thing that inductively detects current. I'm looking for the simplest way, since I'm no expert.

Trying to avoid physically switching or unplugging it, since it's very hard to reach. I'd like to be able to hit the "sleep" button on the comp and walk away.

Thanks
Brian
 
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Old 08-30-11, 09:38 PM
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They're not cheap, probably start at about fifty bucks but this is what you want.

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  #3  
Old 08-31-11, 03:42 AM
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The easiest way to stop the computer from powering the hub is to cut the wire that is connected to Pin 1 of the cable from the computer to the hub. Obviously you won't be able to power devices off that cable once you've cut the wire.

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Old 08-31-11, 05:35 AM
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I have to ask why this is important?
 
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Old 08-31-11, 06:40 PM
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You could also get a wallwart and a "Normally open" small relay. When the wallwart loses power the relay opens.
 
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Old 08-31-11, 07:22 PM
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I'm thinking the OP has 'vampire power' concerns.. If so, it's not worth spending any money on, because anything you spend on eliminating that miniscule amount of waste is going to be many times what you would save on your electric bill.. That's why I asked why he wants to do it.
 
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Old 08-31-11, 09:00 PM
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If the concern IS "vampire power" then I totally agree with Matt. I had my cell phone charger on a Kill-a Watt" meter for four months and it still didn't amount to even one cent's worth of electricity at my rate of 8+ cents per kilowatt.
 
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Old 09-01-11, 04:33 PM
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Not really a vampire issue, though it is somewhat a concern since the hub is hooked up to 7 different items (mp3 player, phone, wireless headphones, etc.) charging them all the time. Plus an eye-strain relief light behind the monitor

The main reason is every thing and its cousin has lights on it, including the USB hub. I have it attached to the wall, behind the desk. All the lights put off a lot of glow at night and it's attached to my bedroom. Plugs are all hard to reach.

I would like to hit "sleep" on the comp and walk away while it saves everything and powers down, instead of standing here waiting, then crawling under desk to hit the power strip switch.

The idea of snipping wire 1 on the USB to cut the power + sounds pretty slick. My only concern is if it will affect the USB hub/comp at all, other than charging things. If not, then it's perfect and cheap.

Thanks for the info all. I read about the vampire pulling being minimal except perhaps on the cable boxes, but I wanted it for all the dang lights everything has on it that never turn off.
 
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Old 09-02-11, 07:33 AM
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Ok yeah, in that case snipping the + wire will definitely do what you want. USB devices don't care where they get their power from, as long as they get it. Only the data and ground need to be continuous between the computer and device. + power can come from the powered hub.
 
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Old 09-02-11, 02:47 PM
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Hmmm ... Since the data pair uses differential signalling I'm not sure if the ground is needed to pass data. As far as I can tell the only use of ground on the data pair is to tell the host whether there's a connected device at the other end. As long as there is a device connected it shouldn't matter. One of these days I'll get ambitious and check it.

Still, Alpha Crow, I wouldn't snip the ground wire.
 
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Old 09-02-11, 04:44 PM
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It's not required for data proper, but it keeps an equipotential drain throughout the chain to the computer for shielding/noise rejection purposes when a cheap non-shielded cable is used or the shield connection is defective. Shield and ground connect together inside the devices.
 
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Old 09-03-11, 02:22 PM
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Bummer. I went in and snipped the red wire (like in the movies!), which shouldn't be ground. Some reason, the cable doesn't work at all now, no data or power. Not sure why, the other wires are not disturbed, and the connection points aren't pulled out or even strained.
 
  #13  
Old 09-04-11, 02:19 AM
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Sometimes in the movies it's the blue wire.

How did you know the red wire was power? There is really no standard for color codes inside those (and many other) cables.

In any case, you should be able to put it back together. Strip as small a piece of the insulation as you can from each end and twist the wires together.
 
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Old 09-04-11, 03:43 PM
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Probably right, but it had all the colors that matched USB pinout and wiring @ pinouts.ru
 
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Old 09-05-11, 04:49 AM
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Plenty of the cables I've seen don't match those color codes, but if your cable matched you probably cut the correct wire. While you have it apart you could check to make sure it's the correct wire by testing continuity to Pin 1 of the connector with a multimeter. If the meter's lead is too big to touch the pins, use a sewing needle or straight pin as a meter lead extension.
 
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