Reddit mentions of HiLetgo ESP-WROOM-32 ESP32 ESP-32S Development Board 2.4GHz Dual-Mode WiFi + Bluetooth Dual Cores Microcontroller Processor Integrated with Antenna RF AMP Filter AP STA for Arduino IDE

Yep, that's the jist of it.

• The Current Transformers can be these as long as they fit around your mains cables in your breaker box: https://www.amazon.com/gp/product/B075541WVT
  (And you'd have to cut off the connectors since the board has screw terminals)
  
  
• The AC Transformer for measuring voltage, and powering the board and ESP32: https://www.amazon.com/gp/product/B00B886CWS
  
  
• This ESP32 (or ESP8266) will work: https://www.amazon.com/dp/B0718T232Z Yeah, it's like a Ras Pi, but smaller. Can be programmed with the Arduino IDE.
  
  
• Jumper wires with Dupont connectors to connect the two boards.
  
  
• Software from my GitHub to load on the ESP32 to link everything together: https://github.com/jdeglavina/EmonESP and https://github.com/jdeglavina/ATM90E32_Arduino
  
  
• EmonCMS has their own servers that you can send data to. It looks like they just started charging a small fee per feed for this though: https://emoncms.org/ You can host the software locally if you know how to setup a web server (or rent one from a host for cheap)
  
  Honestly, the hardest part is calibrating the transformers with the board. You need something to measure against, like a hair dryer and a kill-a-watt. Then there's some math and changing some values in the main program. After some time I will probably have some baseline calibrations for various CTs.
  
  
  
  

This is fine; it works. For a high-school project, this is really cool (my HS robotics team never did PCB design; we were scrubs). They're just saying this because there are some other options you may want to look into. This is one of my own favorites. It has a lot more memory and a faster CPU than the Arduino boards. It has built-in wi-fi and bluetooth. It's cheap. It's small. It can even be programmed in the arduino IDE. It's a pain to solder, but you can find cheap boards that already have that done quite easily.

That has proven to be less than intuitive. Which manual should I be consulting? I've got a "HiLetGo" ESP32S. Nothing came with it pointing to documentation. So I can find scads of docs for the ESP-WROOM-32 chip, but that doesn't tell me anything about the design choices made for the board itself (like which header pins connect to the GPIO pins). The way I eventually figured it out was flipping it over and realizing that they had printed (in very tiny text) the pin assignments on the back of the board. Programming the thing was also a bit confusing, because you have to push a switch labeled "IO0" down when the upload says "Connecting...___..." otherwise it times out. I've got another one that times out, even when you do that.

So, it's not like these little boards are without quirks, especially if you're used to a traditional Arduino. I agree with /u/holytoledo760. It appears the most sensible approach is to figure the thing out on a breadboard. I made the mistake of building a PCB with a bunch of connections before I really understood the thing's quirks. Now I'll have to design a new PCB most likely (unless I get really lucky).

https://www.amazon.com/gp/product/B0718T232Z/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

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it says Nodemcu on the back

I would actually advise against a development board. The time spent getting up to speed on the MCU/SOC, tooling, datasheet style, etc, is huge. Usually for most designs the IC is chosen specifically to cater to the design. For example, one design needs very low power (like an MSP430), one needs many fast ADC's, one needs one or two high resolution but slow ADC's, one needs a very beefy CPU with tons of RAM, etc. It's unlikely that the knowledge gained from that development board will translate well to other designs.

Instead, consider getting them a tool. An oscilloscope is always a very safe bet, but they tend to be a few hundred dollars. The DS1054z is cream of the crop for hobbysts right now, and is $350 roughly.

If you need cheaper, then a nice proper soldering iron like the TS100 here for $80 with a few tips is a very, very, safe bet.

Those tools will stick with him/her for years and years and will be used often. The development board on the other hand likely won't.

If you must go for a development board, I would actually recommend a teensy for $30 instead of something like an Arduino. The teensy 3.6 is an awesome platform that is very arduino like but uses an actually modern beefy MCU on it. The library support for it is fantastic, and it has a decent bit of I/O for future designs. It also uses a Kinetis chip which is in my opinion very well documented, so once he has to go beyond the Arduino library he can easily do it himself.

Then on the side, get him an ESP32 which is very cheap. Sparkfun has their own ESP32 module for $20 but is very thoroughly documented. You can get them cheaper form Amazon for like $10 like this. The cool thing about the ESP32 is it's insanely cheap and very well documented in terms of API. If he wants to use an ESP32 in a design, you can get modules for $4 each. This thing is a very fast chip with Wifi built in and very easy to work with.

TLDR; Get him an ESP32 module for like $20 and a Teensy 3.6 for $30 and he should be set. Ideally get him a piece of equipment like a TS100 for $80 or a DS1054z for $350 if you are willing to spend that much money.

We have the same problem at my office; 2 stalls shared with 30 people.

Since your goal is to get the "occupied" status to a web server, you'll need something that has network capability.

For ~$10, I recommend an ESP32-based solution. It'll allow you to send read your presence detection and send it via WiFi to a service like IFTTT.

If you don't want to use mechanical pressure sensors, a ToF distance sensor, like the STM VL53L0X can be bought already mounted to a breakout board, for around $10, and can be connected to the ESP32 over SPI. From there, poll the sensor (set "occupied" status whenever the distance reported is smaller than the distance between the door and the wall).

Adafruit is always going to be overpriced. Their ESP32 board is about the same price as the Sparkfun board. Avoid adafruit whenever possible, you can usually get the exact same stuff on Amazon or Ebay for a far better price.

You can't really go wrong with a $10 ESP32 dev board from Amazon or wherever. Once you learn enough about your first dev board, and decided you need something else, you'll know better what to purchase for your specific needs. You can get quite a lot done with just the simple $10 dev board. I wouldn't go with a custom-made dev board with an exposed ESP32 chip on it (like the Sparkfun board) - you should definitely use a dev board with the ESP32-WROOM module on it, like most of them out there. You would most likely be using the ESP32-WROOM (or WROVER) modules in any project you'll do, and learning how the ESP32-WROOM module works and its pinouts is invaluable knowledge you won't get with the Sparkfun board.

There's a whole world of stuff, it really depends on what kind of thing he finds exciting.

Here are two ideas of things I think are really cool and would make a fun birthday presents for an Arduino fan.

Little OLED display

https://www.amazon.com/Heltec-Automation-HTDS-WI96-0-96inch-comminication/dp/B0742BJPNF/ref=sr_1_29?ie=UTF8&qid=1543406669&sr=8-29&keywords=oled+display

Basically an Arduino with built in WiFi ( and a lot more features )

https://www.amazon.com/HiLetgo-ESP-WROOM-32-Development-Microcontroller-Integrated/dp/B0718T232Z/ref=sr_1_3?s=electronics&ie=UTF8&qid=1543406789&sr=1-3&keywords=esp32

Do you have a link? Lowest I found is this one for $11, which I may order one of just to confirm it will fit, but that’s still $4-$5 more than Ali.

Thank you and sorry for the late response.

Is this an example of what you mean?

https://www.amazon.com/HiLetgo-ESP-WROOM-32-Development-Microcontroller-Integrated/dp/B0718T232Z/ref=sr_1_6?keywords=esp32&qid=1574788418&sr=8-6

I read reviews that there are many issues getting it to work with the Arduino IDE (although I'd want to use Visual Studio or Visual Studio Code). Also, forgive me if this is a stupid question - I'm more of a server-side developer - but I would be able to program it with C++, right?

Thanks for your time.

ESP32 I'm thinking?

https://www.amazon.com/HiLetgo-ESP-WROOM-32-Development-Microcontroller-Integrated/dp/B0718T232Z