Wiring up the main power switch

I know what you’re thinking.

I’ve got a bunch of components in this cabinet, I don’t want to open up the coin door and stick my head in there and turn 5 things on every time I use this machine. I just want one button!

Well, we can do that through the power of electronics.  The mastermind behind this controlled electrical distribution is known as the relay.  A relay is an electrically controlled switch.  So a voltage applied to one side of the relay will turn the switch on.  In our case, we want to be able to turn on power to a number of devices.

  1. Computer
  2. Monitor
  3. Speakers or amplifier
  4. lights

In order to do this, we need a controlled source of 12V DC power.  Fortunately, we already have that, right in the computer.  The IDE power cables (red, black, black, yellow) have a 12V DC power line in them.  The black cables are ground, red is +5V DC and the yellow is +12V DC.  SATA power cables are +12VDC (yellow), +5VDC (red), and +3.3VDC (orange).

Well, how do we turn the computer on?  I still don’t want to stick my head in there.

The key here is that we just need to be able to turn the computer on from the outside.  Every motherboard in a computer has a group of pins that connect  the motherboard to the computer case.  This pins are controlling the speaker, hard drive lights, reset switch, and yes, the main power switch.

So if we wire a switch up from our main control panel to the correct pins in the header on the motherboard, we should be able to turn the computer on, giving us control over our 12V DC power source.

So wait, I need an outlet inside that always has power and then another that I control with the relay?

You got it.  So the computer always has power unless you turn off the main switch in the rear of the cabinet.  The power to the controlled outlet runs through the relay and will only be powered on when the computer is powered on.  By turning on the power to the computer, the IDE drive power cable now has 12V DC power, turns on the electromagnet in the relay therefore closing the switch, turning the power on to our switch outlet.

In our controlled outlet I’ll just attach a power strip just to make things easy and allow for future expansion.  In this power strip we will plug-in the monitor, any lights, our speakers and amplifier.

So there we go, one button, power goes on, and everything turns on.  Within windows, we can also control what the power switch does when it’s pressed.  We may just want to have the computer hibernate instead of a complete shutdown.  I’ll cover this in a future post.

Also, keep in mind that we also have a +5V DC line available on that same IDE power cable.  That will come in handy as well.

The relay that I chose was a Tyco Electronics KRPA-11DG-12 relay.  $13.01 at Newark Electronics. I like to use relays that have a nice and easy base to work with. This one has contacts that are easily accessible so things will be kept nice and neat.

It’s rated for 10 Amps @ 240VAC, so plenty of capacity for what we’re doing here.

Relay Wiring Diagram
Relay Diagram

Overall Wiring diagram
Full Diagram

Next post: Building the power switch cable.

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