Introduction: One Part $1 Computer Security Power Lockout

About: Just getting into the microcontroller craze. I used to do this sort of thing building circuits for 8 bit microprocessors back in the early 80s, so this is kind of like reliving a bit of my childhood. Back then…

It's a PC... The Apple sticker came with my iPod...LOL.

I have been looking at a number of ways to easily secure a PC in an environment where not everyone may be 100% trusted.
Maybe you have a sneaky roommate / Mother / Father / brother / sister that you know is using your computer when you are out, at school or at work (possibly reading your chat logs and going through your files).
Maybe you just want to give potential thieves the impression your system is dead as a doorknob.
And I don't know how many marriages have broken up or fights started with "I went into their Facebook, and saw...."
Whatever your reason, you don't want people messing with your computer.

First - if your computer allows itself to be powered on from the keyboard, go into the CMOS setup (the BIOS) and disable that feature.
The procedure for this will vary from system to system, so you may need to consult your motherboard manual in order to do this.
We need to disable anything that can make the system power up other than the power switch.
Most systems are set this way by default, and most modern keyboards don't have a power button anymore because people got tired of losing their work because the cat decided you were done anyway..

Step 1: Where and How to Hide the Hardware

If you have a keypad, or switches or a card reader or something like that, this gives someone that wants access to your system a starting point. They know WHAT they have to bypass to get into your system, even if they don't know how yet.
So the answer is - don't put your safe in the middle of the room if you know someone wants what's inside.
Make them think you don't have a safe.
A safe-cracker can't get your goodies if he is convinced you don't even have a safe.
So the question is...where and/or how do we hide the security function?
So it hit me - what do all PC cases seem to have that we can use?
That seemingly innocent reset button hidden in plain sight!
The question then becomes how to use it.
Sure, we could reverse the power and reset buttons, but someone that hits the power and sees that your system won't turn on may simply guess that he got the wrong button and try the reset.
We could put a microcontroller on them and make up some sort of sequence requirement to power it up, but that requires money, programming skill, and more effort than most people have time for. I happen to be both lazy and cheap.

But... what if you simply had to hit BOTH buttons at the same time?
This is something someone would normally never do, and we can wire that up quickly, easily, and cheaply !

Step 2: How It's Done (The Hardware)

Ok, so it's easy to wire up the switches in series - but then you lose your reset function.

So we need a way to power it up with the buttons in series, and return the buttons to their normal functions after power on.
We also want it to go back to the "locked" position after power down!

Well, fortunately for us, motherboards use active low signals for the power and reset functions. This means that the signal pins are held high, and perform their designated function when connected to ground.

To do this, we can use a DPDT (double pole double throw - the equivalent of two 2-way switches) relay to do the switching for us.
I used a 12V relay. When the system powers up, the 12V outputs of the power supply become active, and when it powers down, of course there is no longer a voltage there. This means any 12V source from the power supply will work for us.
Similarly, if you have 5V relays handy use a 5V line.
Since I am using 12V and didn't want to hack my supply at all, I tapped into a fan power line.

When power is OFF we want the power button connected to the POWER signal line of the motherboard, but we want it to have to go through the reset button to get to ground.
This means that in the off state, we want the power button's ground line severed and connected to the reset button, whose reset signal line is severed. The reset button must after all provide the path to ground.
So, we get the relay to tie the power switch's severed ground line to the reset button's severed signal line.
The moment power is on, the relay can then connect the power button's ground line back to ground, and the reset button's signal line back to the reset signal of the motherboard.

Here's the basic circuit. Please note that lines that cross each other are not interconnected.

Step 3: Hiding Hardware and Finding Common Ground

So we're going to hide the hardware so that even if someone opens the case, they won't see anything out of the ordinary.
The best place is the face panel of the PC itself where the switches are.
Anything you do past this point is at your own risk, and I will not be held responsible for any damage you may cause to anything anywhere ever as a result of anything I say or have said in this instructable.

Now that the disclaimer is out of the way, let's get started.

First, we unplug the computer's power supply, and then open up the front of the case by removing the case covers, finding the screws or clips that hold the front panel in place and removing / un-clipping them to remove the front panel.

Once the front panel is open, locate the wires going into the power and reset switches.
Separate the wire pairs for about a 2 inch stretch each using a sharp knife. Just separate them if they aren't already separate, don't cut through them.
Time to find out which wires are ground, and which are signal!

I am lazy, and didn't want to trace them to the motherboard because there's 8 drives and various ports in it, so it's not easy to get to the system board itself.
So, I put the multi-meter into resistance / conductivity mode, and clipped the negative lead to the computer case for ground.
I clipped the positive lead to a metal box cutter / utility knife, and GENTLY cut the wire insulation just enough for the blade to contact the wire itself underneath. Your meter MUST have conductivity with the blade of the knife !!!
When your ohm meter reads 1 or 0, or your continuity checker indicates continuity, you have found the ground half of that switch's pair of wires.
I started with the power switch wires. Once I find the one that is ground, I cut it, because on the power switch side, we want the switch wired to the power signal on the motherboard, and the ground to go to either the reset switch, or ground depending on if the system is on or not respectively.

Step 4: Finding Common Ground - Continued

Now we do the same thing for the reset line, except this time when we find the ground wire, we cut the OTHER wire of the reset switch pair, because we want the reset switch to be the path to ground for the power switch when the system is off.
So, we cut the reset signal wire, as it will be either connected to the power switch in series, or the reset signal on the motherboard depending on if the system is off or on respectively . Here you can see the already cut power switch ground wire (white).
BONUS - since the ground of the power switch will be connected to the relay for switching anyway, we can also use the same line for ground for the relay coil. One less wire to run into the system for relay power.
The other side of the coil is wired into the front fan power in my system because (A) I didn't want to hack my power supply at all, and (B) it was right there, and I am lazy. You can of course use any 12V power line in the computer.

In this photo you can actually see the meter lead connected to my knife. The green alligator clip on the casing is connected to the other meter lead. Here you can see the resistance is 1 ohm (ground). My meter never reads 0...just the way it is.

Step 5: Connect the Hardware.

Now it's time to connect the relay.
There are 2 ends of each wire that you have cut. One side connects to the button, the other goes to the motherboard.
A DPDT (double pole, double throw) relay has 2 switches in it. Each switch has a pole. This is the line that connects to one of the 2 contacts of that switch in the relay. The contacts are normally open (off when there is no power) and normally closed (on when there is no power)
The "Button side" of the cut wires go to the common or POLES of the relay (one to each of the poles), and their corresponding other cut ends go to the normally open (N/O or NO) connection of the respective pole for that wire. This means the cut wires will be connected through when there is power to the relay. The normally closed contacts (N/C or NC) are tied together. This connects the power and reset switches together in series when the power is off.

***Don't wire your relay the exact way mine is pictured, because some relays have the pole contact at the end (either end, opposite ends etc.), others have them in the middle. Look at the diagram printed on your relay or in the datasheet for the relay you purchased, pilfered or pulled. What you see in the photo is right for my relay, but may not be right for yours.
If you are unsure, post a photo of your relay, and I'll try to help you out. Photograph the diagram printed on the relay, unless it doesn't have one, in which case you can just say what the part number is.

Step 6: Close It Up, We're Done!

Hot glue or otherwise secure the relay where it won't interfere with the closing of the front panel.
Usually the reset and power buttons stick out enough that you can see that you can put a small relay just about anywhere in the front panel cavity.
Close up the panel and plug your computer back in. Test it by pressing the power switch (nothing should happen) then press just the reset switch (nothing should happen again) and then both buttons together or hold one button and press the other (computer should power up, relay clicks, and all switches are now in normal operating mode).

Once powered up, your reset and power buttons should work as normal to reset or power down the system.
If you find they don't (for instance, pressing and holding the power switch doesn't shut it down, or the system is in permanent reset and doesn't boot), then you may have gotten the pole and contacts backwards somewhere - check your connections and check or rewire your relay.

Most of these miniature relays cost about a buck (unless you buy only 1, then it might be more - I usually buy 5 or 10 at a time) and sometimes you can just salvage a relay from somewhere. Just make sure it's either 5 or 12 volts, and wire it accordingly.
5V won't turn on a 12V relay, and 12V will fry a 5V relay!

Now when someone tries to turn on your system , it'll just sit there looking stupid.
If someone opens the case, they won't see anything unusual.
You don't need a key you can lose or a password you can forget to power it up whenever you want to.

I hope you have enjoyed this, and find it useful!

I'd love to see the look on your sneaky roommate /  mother / father / brother / sister's face when they see you use your computer every day, but can never power it up themselves when they try to use it without permission!

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