Using 5V motherboard pin

Question

I want to create an electrical circuit that powers up the computer when a sensor is activated.

This sensor is in parallel with the power switch button.

As far as I understood from reading on different parts of the internet, the power switch button is part of a pull-up resistor circuit that goes into the motherboard as seen here in schema 1.

My idea of the sensor electrical schema that I want to implement is schema 2

But the sensor also needs a +5V supply. I noticed there is a +5V pin on the motherboard where the power button switch connects, but I am not sure if I can use this for my sensor. Is it reserved for something else (speaker?), is the current limited to the motherboard (sensor is going to take up at most a few milliamps) or can I go ahead and use this +5V/GND for my project?

Thanks in advance

Note: I am new to hardware and electrical engineering, so I apologize if the question is a bit silly.

Edit: Thanks for the replies. The sensor is a regular motion sensor (doesn't take more than 200mA if I read it correctly). Also, what's the advantage of using a relay instead of using a transistor for this project, other than separating the current for the 2 circuits. Would one of these relays work?

Answer 1

This wont work. The power switch on a computer is set at the bios/bridge level to turn the computer on and off. You could disable the power off option in your OS but hold the button down long enough and it will force power off.

Your motion sensor is likely to keep the button pressed too long, or if already on, trigger it to force off. Especially if it's set to retrigger modes.

If your dead set on doing this, a transistor, mosfet or relay should work. There should be a +5VSB standby voltage somewhere (read the motherboard manual) that should be enough to power your project.

You could implement further features like sensing if the pc is already on if so don't trigger the power button.

You are better off trying to control this via wake on lan. A esp8266 on the same network as your PC could be used to trigger the WoL for the PC allowing it to power on and bonus no need to wire into the pc at all.

Answer 2

• For the 5V from the header pin, if it's always on (when the PC is off if you also use it to power on the PC, you can test this with a multimeter for example) and you're not drawing a lot of current (can you specify what's your sensor?) that could be a good solution.
• For shorting the button of the PC, if you want to be absolutely sure to make it work without any risk of damaging anything because of a wrong voltage I recommend you to use a relay instead:

^{simulate this circuit – Schematic created using CircuitLab}

You can find really cheap relay modules with integrated optocouplers on the Internet to avoid pulling to much current on your sensor