Building a device to power on/off computers

I want to build a device that will interface with a computer via USB or RS232 that will power a different computer on or off.  I'm envisioning the device having  a lead that will connect directly to the power leads on the target computer's motherboard, and a USB/serial cable heading to the "controller" computer's USB/serial port.

I know there are a million retail solultions out there -- I won't bore you with the details, but none of them work for me.  I need to build one.  

My question is simple:  can anyone point me to readily available electronic components that would allow me to build this?  Again, I don't want a packaged retail solution.  I want the actual electronic components.  

Thanks in advance.

Reg Natarajan
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Gary CaseRetiredCommented:
Without more details (distance involved in particular) it's difficult to say what might work for sure.   But assuming the distance involved does not exceed the length constraints for an RS-232 cable run, all you need is a 12v momentary contact micro-solenoid switch, and you can simply control it with any of the control signals from an RS-232 serial port.  Check with any auto parts store -- there are quite a few 12v solenoid switches used in automotive electronics.  You need one with a very low current draw.

Here's a simple relay already mounted on a PCB that you could use in a kit box (or mount inside of the "target" PC):

If you would prefer to get a pre-built interface board for this project, you can get them here:

(a) for a paralle port:

(b) for a serial port:

(c) for a USB port:
Gary CaseRetiredCommented:
... just tested the links and the last one doesn't go directly to the USB switch => but it's listed on that page.  (It does a lot more than you need; and is the most expensive of the above options)
regnatarajanAuthor Commented:
Thanks, Gary.  That's exactly what I needed.
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