Introduction
My electronic endeavors, lately, have taken me down two paths. One path was dealing with a salvaged LED display, and the other was the ESP8266. Those paths have now been joined in a challenging project.
Here is a video of the project:
All Design Documentation
All of my design files including schematics, board layout, and all programming sketches are on my GitHub repository. There is an extensive readme file with plenty if details.
The Display
Sometime ago a friend gave me a board with 18 five column by seven row LED matrices, Thanks Mike.
The board was missing the four most critical ICs, namely the microprocessor and two RAM chips. I'm guessing the forth IC was a programmable read only memory used to translate ASCII characters to row and column bit patterns - a character generator.
I probably would have removed those ICs anyway.
Fortunately, I had enough information to reverse engineer the display PCB and was able to design an interface board using an ATmega328P microprocessor. The ATmega device has enough internal SRAM and an internal EEPROM. I built my character generator in the EEPROM. You can see that interface board and its connections to the display PCB as the still shot for the video.
There are 90 columns and the most typical programming scheme would be to start at one end, load the column data for one column, turn that column on, then move down the line doing the same for each column. Each individual LED would be accessed 1/90 of the time. This display PCB divided the columns into six zones. The data for six columns are loaded and all six columns turned on simultaneously. An individual LED is accessed 1/15 of the time. If an LED is meant to be on, it will be six times brighter.
I wrote Arduino sketch for the user to write messages to a serial EEPROM on my interface board, and select one of 28 possible messages stored in the EEPROM to display. I wrote separate sketches to display in four direction:
- from right to left which I call my Times Square Scroll
- from left to right. Challenge here was that the letters in each word had to be reversed, otherwise "the" would become "eht"
- from top to bottom. My favorite. Challenge was to not split up words between lines. See second video in my youtobe video, above.
- from bottom to top. Like top to bottom with the same challenge
I wanted the display to do more, to be dynamic, to display stuff I did not write - like the weather or latest news.
ESP8266
At the same time, while working on the display, I got on the ESP8266 bandwagon. One of the limitations of the Arduino platform was realistic WiFi access. There Arduino are shields like the Xbee series of boards, but they are pretty expensive.
Along came a tiny 8 pin PCB from China:
This device is a little less than an inch long and a little over one half inch wide. I have seen it being sold for $2.40 USD. The small 32 pin IC is the ESP8266 chip itself. It contains a very capable microcontroller along with the WiFi circuitry - everything but the antenna. The antenna is those wavy lines at the top of the board. I happened to come across a YouTube video from electronupdate where the ESP8266 was photomicrographed. Here is what it looks like inside.
All the red is the logic circuitry for the microcontroller. All the rest is for the Wi-Fi. You can recognize inductors in the picture - those oval items. The eight pin device is a 8 Mbit serial Flash Memory with an SPI.
Although this module is commonly called an ESP8266, its proper name is the ESP-1. It has a couple of limitations. There is only one useful I/O pin and it's not RFI shielded. The pins do have a 0.1" pitch which is nice for mounting on vectorboards.
There are newer generations using the same ESP8266 integrated circuit. The latest seems to be the ESP-14, but the most common seems to be the ESP-12, of which the ESP-12f is the latest flavor. I'm familiar with the ESP-12e, so will not talk about the ESP-14 (same WiFi as in ESP8266 but another microcontroller). the advantages of the ESP-12 are seven fully useful I/O pins, and FCC certified RFI shielding. It does have 2mm pitch castellated pads though. While designed for surface mount placement, I have been able to solder 24AWG solid wire through the holes to connect to other circuitry. I have seen a price of $2.50 USD for the ESP-12e. I'm still going to use the term ESP8266 for all the members of the family.
What's the Big Deal With the ESP8266
Besides the price, this little device can act as a TCP/IP or UDP server or client. It can connect to your WiFi router, and, as a server, contains its own access point. It's often called the "Internet of Things" device.
I have used it as a client where the server is a Raspberry Pi. I employed two in a remote control project, one as a client, the other a server. I utilized the access point in the server device so it does not need my my WiFi router. The access point has an SSID and a password you choose.
If you do a search on the ESP8266, and search YouTube videos, most often you find projects involving the internet. And, indeed, my weather project uses the ESP8266 as a client to get the weather from weatherunderground.com. Other common projects fall into the "internet of Things" category. I have not explored this, myself.
"Internet of Things" projects use the ESP8266 as a server. It's connected to some device you wish to control - your front door electronic lock, for example. You call up the ESP8266's IP address on your browser from anywhere in the world. Let's say you wrote a little javascript in your ESP8266's program to create "Lock" and "Unlock" buttons. These appear in your browser window. Push a button and control your lock. Next project, perhaps (though not control my front door lock).
One of the best things about the ESP8266 is how you program it. There are several options including issuing AP commands and using the Lua interpretive language. I'm not going to discuss those but will talk about what I use - writing C code within the Arduino IDE. I've been using the Ardino IDE for quite some time, writing Arduino sketches. My LED display uses the same microcontroller as used on the Arduino Uno, and I use the Arduino IDE to write all the firmware to control the display.
Two wonderful things happened. The Arduino IDE was opened up to other devices, and the people at NodeMCU provided the functionality for writing ESP8266 firmware using the Arduino IDE. I now had one platform for writing code to both the ATmega328P and my ESP8266. Many of the same libraries as used for Arduino devices were adapted for use on the ESP8266. NodeMCU adapted the existing Arduino WiFi shield library to become the ESP8266WiFi library.
Putting the ESP8266 together With My Display
Here is my interface board that connects to the rear of my display
Above is the layout of the breadboard PCB that controls my display - my interface board. Until recently, there was nothing to the right of the serial EEPROM. The ESP8266 came later. My ESP8266, the ESP-12e version, exists on a development module designed by NodeMCU. It's $8.79 USD from Amazon. Bit more expensive but adds an USB to serial chip for programming, a voltage regulator (5V to 3.3V) and fits nicely on the breadboard PCB having pins on a 0.1" pitch. It's worth every penny. The TXB0104, between the serial EEPROM and the ESP8266 translateS voltage levels of two signals between the ATmega328P, operating at 5V, and the ESP8266, operating at 3.3V.
I'm not going to go into a lot of detail here. The readme file on GitHub explains a lot, and the sketches have lots of comments. I will say that the ESP8266 connects to the weatherunderground api every 10 minutes. It retrieves two .JSON files - one with the current weather conditions and the other, eight forecasts. It signals the ATmega328P via an interrupt that new data is ready. In the meantime, the ATmega328P is reading the weather data from the serial EEPROM and writing it to the display. I functionally divided the serial EEPROM's 64K memory into 2 kbyte blocks. The first 28 of these blocks are for static messages I write that I wish to display. The weather data sits in the next 2 kbyte space. Once the ATmega328P has written all the weather data to the display, it looks to see if the ESP8266 has sent the interrupt. If, not, it simply repeats the same weather information. If the interrupt occurred, the ARmega328P and the ESP8266 talk to each other using a pseudo SPI handshaking scheme that sends the new data to the ATmega328P. The data in the .JSON file is parsed for the desired information, and along with other text, is written to the serial EEPROM.
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