I know this sort of project has been done before but nevertheless it’s still an interesting project to try and put your own spin on it. The idea is to have an ESP8266 which can press/hold the power button and might as well have it also read the power LED state, you could also wire up the reset button if you wanted to, it’s a good concept for servers (or PCs) which don’t have a remote management and you need something basic.

One option might be using the 5V standby power which most power supplies provide when the PC is off. Some PCs might have 5V standby available in the BIOS, the keyboard and mouse might stay on, maybe you could find some pins on the mainboard but perhaps some mainboards might not have it? Let’s just avoid it all, I’m thinking I could have the ESP8266 powered off a 3.3V LiFePo4 battery in a small little case which when the PC is on, it will recharge itself via USB, this way we don’t need the PC to be on or have the ESP8266 powered via a mains adapter.

For the circuit, we can sense the power LED, it will have a voltage drop out say 2-3V with the LED but could have 5V if the LED isn’t connected for whatever reason, we’ll just use a NPN to switch 3.3V on or off to our input pin. The power switch we can use an opto-isolator to turn it on or off which can be inline with the power button. We can also integrate the USB LiFePO4 charger into this project to charge the battery.

That’s pretty much all there is to the hardware, now for the software which is where it’s all at. We want to make it as simple as possible to setup without needing any port forwarding on the client’s end, so the design is to have the ESP8266 reach out to a server to fetch a command over HTTPs. We don’t want the ESP8266 to be awake all the time otherwise the battery won’t last long so we’ll utilise the deep sleep function and say wake up once every 5-10 minutes.”

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