(July 2019)

Remote power on/off and remote serial via ESP8266

What is this about?

It's about a single-board computer (SBC) that I wanted to control remotely - my Atomic PI. I wrote about this SBC a month ago - it's an unbelievably cheap (35$) Intel-based computer, with many features. Amongst them: 2GB of RAM, 16GB of EMMC storage and a 4-core Atom CPU.

The AtomicPI needs to be powered by a good, solid 5V power supply - and since I measured its current consumption (see article for details) I knew that my Raspberry PI's power supply is more than up to the task.

So I had to splice a jack - that would allow me to plug into my custom AtomicPI power adapter.

After some cutting, kapton tape-ing, soldering and heat-shrinking...

Splicing power from the Raspberry PI power supply

 
...the power supply was ready - and I could connect it to an ESP8266-based power module:
 

Stealing 80mA, but giving a remotely-controlled 5V supply.

 
The ESP8266 board (more specifically, ESP01/relay module) is a dirt-cheap one (1.8 Euros as of July 2019) that I bought from AliExpress.

What I didn't know when I bought it, is that the PCB design had two flaws:

• The 10K Ohm R2 resistor had to be removed
• The VCC (Pin 1) and CH_PD (Pin 3) had to be bridged (that's the small yellow wire you see protruding in the picture above).

Oh well - for that price, including shipping, I shouldn't complain too much. Diagnosing and fixing things is, after all, what makes this fun :-)

Now, all that was left was to program the ESP8266 with my own software.

What, you actually expected me to trust closed-source SW connecting to my Wi-Fi?
Are you serious? :-)

A bit of coding

There are tons of examples on the Web, demonstrating how to boot up a simple Web server on the ESP8266 - I did a very simple implementation that controls the GPIO0 pin (since that's the one that the PCB routes to the relay control):

...

const char* ssid = "Your SSID goes here";
const char* password = "Your WiFi pass goes here";

ESP8266WebServer server(80);

...

server.on("/", handleRoot);
  
auto redirect = []() {
    server.sendHeader("Location", String("http://") + WiFi.localIP() + String("/"), true);
    server.send(302, "text/plain", "");  
};
server.on("/poweron", [&]() {
    if (!poweredUp) {
        poweredUp = true;
        digitalWrite(controlPin, 1);
    }
    redirect();
});
server.on("/poweroff", [&]() {
    if (poweredUp) {
        poweredUp = false;
        digitalWrite(controlPin, 0);
    }
    redirect();
});

I am sure you'll agree that modern C++ is much better than the old ways. The code is a lot closer to what one writes in higher-level languages - passing lambdas that do the work, as callbacks for specific Web-server end-points.

So what does that translate to, in the end - how do I actually control the relay remotely?

bash$ curl http://IP_ADDRESS/poweron

bash$ curl http://IP_ADDRESS/poweroff

Not too shabby :-)
In case it's not clear, the IP address is not accessible from the outside; this is a static IP allocated to the ESP behind the firewall.

I also added a mainpage, offering direct links that I can just click on - from any browser (e.g. from my phone).

Executive summary: Here's a video with the whole thing in action.

UPDATE - the following weekend...

I used a small piece of perfboard, to get access to - and connect - the TX/RX/GND pins of the ESP8266 to the corresponding ones on the AtomicPI (i.e. the CN10 connector):

The ESP01 now plugs in my perfboard - which plugs in the module.

The perfboard from a different angle.

The TX/RX/GND signals from the ESP end up on the CN10 connector of the Atomic PI.

This allowed me to enjoy a remote serial port!
And be able to fully control the booting process and the BIOS:

...which in turn allowed me to perform an OpenBSD installation over this remote serial!

I just needed to type this in the boot menu:

stty com0 115200
set tty com0

Half an hour later, I concluded an installation of a UNIX OS over a remote serial port.

That was a new experience :-)

An OS for true Spartans :-)

Overall, this was a very fun and educational process - highly recommended!
And for anyone else that is interested in reproducing this, the code I hacked to run inside the ESP8266 is here.

P.S. Obligatory trolling - who needs Intel's Management Engine? :-)

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