# Remotely Press a Power Button

This blag post describes how to use a [Raspberry Pi](http://amzn.to/2wjICvo) to remotely "press" and potentially "hold" the power button on a PC. This is my first non-trivial (still pretty-trivial) hardware-related project. So don't expect anything too fancy. ## Mainboard Your PC probably has a power switch and a reset switch. In "modern" ATX power supplies those switches don't directly control the power but merely tell your power supply to toggle power or reset the connection. This is the reason your operating system is able to shutdown properly when you press the power switch on your PC. Please go ahead and try it now... I hope it worked and am happy that you found your way back to this post. We are now interested in the Pins normally connected to the PC's power switch. They are normally grouped together with the Pins responsible for resetting or that power the case's power LED. To tell an ATX power supply to switch a PC on you have to connect two Pins (and then disconnect them again). This happens when you press (and release) your PC's power button. To find those two Pins, consult the motherboard's manual and find "System panel connector" or similar. In good manuals you will find a schematic displaying the position of the pin group on the motherboard together with a description for every pin. Sometimes pin descriptions are also printed directly onto the mainboard.
![manual system panel connector](https://blag.nullteilerfrei.de/wp-content/uploads/2017/08/system-panel-connector.png)
We are interested in the Pin labeled PWR and an adjacent Pin labeled GND which stands for *power* and *ground*. Connecting those two is the exact same as pressing (and holding) the power switch. ## Optocouplers To protect the mainboard from me [screwing up too much](https://blag.nullteilerfrei.de/wp-content/uploads/2017/08/no-idea.png) I decided to isolate the motherboard from all the things I'm doing now. For this a friend recommended so called *optocouplers*. Despite from having a pretty cool name they are like a switch but not usable by humans but by other circuits. For this they have 4 legs:
![optocoupler](https://blag.nullteilerfrei.de/wp-content/uploads/2017/08/optocoupler-1.jpg)
They are numbered from 1 to 4 counter clock-wise starting at the upper left leg (when you orient the thing correctly). If you apply potential to leg 1 and 2, legs 3 and 4 get connected. This is archived through a LED and a transistor: If you apply potential to leg 1 and 2 the LED lights up making the transistor conduct. Hence the name *opto*coupler.
![optocoupler schematics](https://blag.nullteilerfrei.de/wp-content/uploads/2018/10/Optoisolator-schematics.png)
Don't go crazy with the amount of voltage though! ## RaspberryPi The Pi has a bunch of Pins.
![raspberry pi pins](https://blag.nullteilerfrei.de/wp-content/uploads/2017/08/raspberry-pi-pins.jpg)
I installed raspbian, GPIO and its Python bindings. GPIO is something you can use to set a potential between a pin and GND just by calling a command. For this you first have to *mark* a pin as an *output pin* and then set it to *high* (meaning "connect") or *low* (meaning "disconnect"). Then one can do (as root on the pi):
import RPi.GPIO as GPIO
from time import sleep

GPIO.setmode(GPIO.BCM)  # set board layout
GPIO.setup([22, 23, 24], GPIO.OUT, initial=GPIO.LOW)  # mark pins 22 to 24 as output

def press(pin, seconds):
GPIO.output(pin, GPIO.HIGH)
sleep(seconds)
GPIO.output(pin, GPIO.LOW)

Calling press(24, 0.2) then sets pin with number 24 "high" for 200 milliseconds. I set up pin 22 to 24 as output pins because I want to control 3 computers. ## Wiring If you just connect pin 24 and GND on the Pi with leg 1 and 2 of the optocoupler and then set pin 24 to high most certainly the [magic smoke goes away](https://en.wikipedia.org/wiki/Magic_smoke). So add a resistor to the mix. The rest is straight forward: just connect pin 3 to GND on the motherboard and 4 to PWR:
![final wiring](https://blag.nullteilerfrei.de/wp-content/uploads/2018/10/wiring.png)
Happy switching. Edit (2018-10-03): James Foote pointed out a mistake reg. the connected legs of the optocoupler. Thanks and fixed!

## 6 Replies to “Remotely Press a Power Button”

1. James Foote says:
your opto-coupler diagram is incorrect, the transistor is reversed in yours, emitter is 3, collector is 4, they way you show it should not work, but because your diagram is incorrect that is why it does! Just flip the legs around in your diagram to fix it!
2. ukhan says:
Does this post/diagram reflect the corrections James Foote brought up.
3. Andreas says:
Hello Lars, first of all thank you very much for this great article. I have a question: Could you please give me a hint which certain type of optocoupler (specs) I do need? Tried to figured it out by searching for the label which is shown on the picture but had no luck. - Or I was too dumb.... Greetings, Andrew
4. @Andrew: just get any (google for "buy optocoupler" for example). I _think_ they are similarly generic as, say, resistors. Make sure to get one with 4 legs though ;).
5. Arne Michael Neumann says:
Another tip: if your power supply adheres to the atx standard re coloring of wires, the power on wire is pur green and there should be only one green wire. Any black wire will be ground. To power the raspberry, you could use a purple wire, i.e. 5V-standby, and enclose the raspberry within your tower without any awkward power cables going into the enclosure somewhere.