Category: Arduino

Along with the power monitoring project I’ve been working on, I’ve also developed a simple Internet-enabled screen. It uses another ESP8266, a few wires, and a 16×4 LCD display.

The display used is one of the standard HD44780 compatible ones – so you can use the LiquidCrystal library, which is one of the standard libraries in the Arduino IDE.  Wiring diagram to a standard Arduino is available here, as well as the library reference and documentation. You can find these types of displays in various sizes, ranging from $5 to $20. You’ll also need a 10k potentiometer to control the display’s contrast.

The ESP8266 variant used is the WeMos D1 Mini – http://www.wemos.cc/Products/d1_mini.html.  It’s available online for about $4.

For the box, I used a couple of scrap pieces of 1/4 inch oak plywood, and a scrap piece of 1×8 pine board. I measured the screen, cut, and gave the USB wire a small hole. Here it is painted and all put together:

To help parse JSON, I used this fantastic library, called Arduino JSON (clever name). It’s pretty easy to use and is well documented.

To program an ESP8266 using the Arduino IDE, you can also use a custom board type available here.

My simple code is available on my Github page. It interprets the JSON, formats it to fit on the screen, and scrolls through it.  Someday I might add a button or rotary encoder to help control it.

So, after a… while… I finally got around to getting this project together.

Below, a quick picture of the completed and populated board. This required quite a bit of soldering, and I discovered a couple of problems during testing (hey, it’s supposed to be  a learning experience!).

The Teensy 3.2 and the ESP8266 are removable – they have plugs for all their pins.

So the finished board has 12 inputs. The board size is the limit of the free version of EAGLE. On the back is where you’ll see some airwires (aka mistakes).

Two airwires – one because I forgot to route a pin from the Teensy 3.2 to the ESP8266, and the other because I may have fried one of the input pins on the Teensy trying to get the device powered from AC.

So when I was designing the board in EAGLE, I knew a lot less about circuit design than I do now (especially AC power). During testing, I discovered that powering the circuit via AC isn’t really that feasible for high currents (the ESP needs >200ma). Not only does the taking power from the AC-AC transformer deform the AC waveform slightly (affecting measurement), you also must use a separate ground for the ADCs.  Oops. Here’s some blog posts describing this –  here and here.

I haven’t yet uploaded the code to my GitHub account – that’s coming in a future update on the code that supports this project.

I got something in the mail today from Seeedstudio!

 

And here they are:

Here’s some close up shots of both sides:

I’m very impressed with the quality and value. I only paid $26 for five of these boards! I’ll definitely be ordering from Seeedstudio again in the future. So now the rest of the components have to arrive, and then I’ll be able to begin soldering components to the board.

The last post describes a bit about the design process.

A while ago, I stumbled upon this great organization: OpenEnergyMonitor. They’re doing a lot of things right!

Anyway, they prompted me to attempt to create a derivative of their EmonShield TX. I loved the idea, but I wasn’t sold on the RF nature that needed a base station. Since it’s easy to provide WiFi connectivity to an Arduino with the venerable ESP8266, why not make it so? Also the number of circuits (four) wasn’t exactly calling my name. I wanted the ability to do more.

So.

For non-intrusive current monitoring, current clamps are great. They’re simple current transformers – they themselves are coils of wire, and the conductor around which the clamp is placed forms the other coil.  Thus two things are achieved – isolation and voltage step-down. However, without a burden load, transformers can be dangerous – they build up potential until they reach their insulation’s break-down voltage and fail (sometimes dangerously). So to use these, you MUST have in place a burden resistor, before clamping around a current carrying wire. OpenEnergyMonitor provides a great circuit diagram describing how to both limit the peak voltage, and bias it to remove negative potential (for measurement reasons).  The burden resistor must also be matched so the peak voltage produced by the CT doesn’t saturate or exceed the limits of the device used to measure it.

Once the CT output is biased and limited to the correct voltage by the burden resistor, it’s ready for measurement by an Analog-to-digital converter.

To measure voltage, you can simply use a 9VAC transformer to provide a voltage signal (you can also use it to power the contraption). It just takes a bit of biasing and dividing, similar to the CT sensors. Again, OpenEnergyMonitor has diagrams here.  If you wanted to measure both phases of common residential service, you’d need two, and thus two ADC inputs on the measurement device.

In the US, to measure a 120v device, you need 1 CT sensor and 1 ADC input. However, to measure a 240v device, you must measure both phases and add them together. So each 240v device requires two ADC inputs.

Now that we have low voltage analog signals, how to we read them? Arduino time! or… not.

An Arduino Uno has 6 ADC inputs. Not enough for me.

An Arduino Mega has 16 ADC inputs, but is rather large, and there are many reports of being unable to power the hungry ESP8266. Again, out.

The Teensy 3.2 board has many more features and ADC inputs in a much smaller package. I use the same board in my revamped Motorcycle Computer project. Its 3.3v regulator can also support the ESP8266’s power requirements. Bingo.

Now, for the board. I’ve been using EAGLE 7.5 to learn and play around with circuit design. Since this circuit doesn’t have any high speed signals (only 60hz AC), I don’t need to worry much path impedance or other factors in design.  Here’s what I was able to come up with:

The board layout. I experimented with some surface mount components in the bottom left, but the rest of the components are all through hole.

And now that I’ve got it designed, I sent it away for manufacturing. I used SeeedStudio for my first ever self-designed PCB creation.  The next update will likely be when I get these in the mail. I’ll make sure to note the turnaround time as well.