Create a light sensor using an ESP8266 and an LDR for 5€, directly send the measures to a Redis database via a Wifi/TCP/IP stack and publish a push notification to notify your frontend user interface (described in a future blog post) that there is a new value to display in realtime. Redis is an in-memory key-value store with persistency to disk, high-availability and clustering. It is so easy to use that a simple micro-controller, such as an Arduino or an ESP8266, with a TCP/IP stack can connect and use it.

You can find links to the related materials at the end of this post.

Prerequisites

Hardware

I had to build about 200 such devices, for the cheapest price, within as short time as possible. The idea was to build these devices and teach how to use them with Redis in our French Redis meetups. The attendees can keep the device after the meetup to continue to play with it at home.

It had to be as cheap as possible (reached less than 5€), as easy and fast to build as possible (30 seconds less per device is important when building 200 of them), as versatile as possible to connect other sensors (thermistors, digital sensors, I2C sensors, SPI sensors, …).

First of all, you need some hardware. Despite I am an Arduino addict it would require a Wifi shield (expensive) and some soldering (longer to build). I chose to use a small breadboard, an all-in-one ESP8266, a resistor and an Light Dependent Resistor (LDR or photoresistor), that can be easily replaced with a thermistor for the same price.

Parts and tools

Lets talk about furnitures : IoT1 Full size

My prices have a discount because I ordered more than 100 of each.

Qty Item Price (€) Link
1 170 points breadboard 0.86 Banggood
1 ESP8266 ESP-12E/F breakout 3.18 Banggood
1 10K Ohms resistor (metal or carbon) 0.03 Conrad
1 GL5528 LDR 0.02 Banggood
1 Micro-USB cable 1.10 Banggood
  Total 5.19  
  • Noze plier
  • Cutting plier

Build

Here are the steps. Links are opening detailled pictures of steps.

Congratulations !!! That’s ok for the hardware !

Software

IDE, cross-compiler and upload

We will need the software to write our code, the IDE, the software to compile our code which depends both on the combination computer/OS and targetted micro-controller, the cross-compiler, and the software to upload the compiled code to the device.

There are several IDE and toolchains available to compile C/C++ code for microcontrolers. I chose to use the very simple but sufficient IDE provided by the Arduino project. It is generic, but can automatically download the cross-compiler that runs on your computer to compile for your target device and it can also download and include a lot of helper libraries from internet repositories.

You can either go the the Arduino website to download the IDE1 or click on the direct download links :

Extract the archive in a folder anywhere in your filesystem. The IDE will be executed from this folder and all your files will reside inside this folder. The folder can be moved anywhere anytime.

Libraries, code sample and IDE Customization

The IDE can be customized to be portable and have everything in a single folder (the IDE itself, the configuration, the toolchains for the devices and your code). We only need to create a portable subfolder that will host all your code, all your libraries and your IDE configuration file (like some kind of home directory).

I prepared one and created a configuration file with the external repositories. As said previously, a cross-compiler toolchain needs to be installed. As the binaries depend on your computer and OS, I can only preconfigure the repository from which you’ll have to download and install them by youself. A Redis helper library and an empty code template are also included in the customization file.

The file will also add the full source code of this blog in your sketchbook.

  • Download the portable archive

  • Extract the archive at the top level of your IDE folder (or in the Contents/java subfolder for MacOSX users).

  • Start the IDE

  • Go to the Tools/Board/Board manager menu Boardmanager menu

  • Search for ESP8266 (there should be one found from the preconfigured repository), select it in the list and install it. Boardmanager menu

  • Close the dialog, and select the Tools/Board/NodeMCU 1.0 (ESP-12E Module) in the menu Boardmanager menu

You’re done with the IDE installation and configuration.

Redis

You need a Redis database, it can either be the community or the enterprise edition. This blog post will not tell you how to implement the sentinel protocol, nor the cluster API, respectively needed for high-availability and for clustering in the community edition. These two protocols are not needed for high-availability and clustering in the enterprise edition, these features will be automatically enabled if using the enterprise edition.

Given that the enterprise edition is seen as a single Redis instance by the applications, regardless if there is sharding and/or replication behind, and in order to keep this post not too long, I’ll describe the Redis community installation, without clustering or high-availability, and use it.

To avoid admin permissions requirement, to keep Redis portable and to master the used version, I will not use the pre-packaged redis server available in most of the Linux distributions repositories, but I’ll download, compile, configure and run the Redis version available on the official Redis community homepage .

Download the source code from the following link Redis source and unzip it in a folder. Open a terminal inside this folder and type make. Once the compilation is complete, run Redis from here without installing it in your system, by typing ./src/redis-server --protected-mode no (without turning off the protection, you will only be able to connect from localhost).

You’re done with Redis. Amazing, isn’t it, only four commands (download, extract, build and execute) and you have an up-and-running portable installation.

Micro controler application

Now, it is time to start coding!

Create a new application

You should have an empty application skeleton in your IDE when you start it. Basically, there are two mandatory functions : void setup() and void loop(). When you reset it (either hard reset with the button, from the USB connection, by powering it or soft reset from the application), it executes the Arduino bootloader. if data is available on the serial link (bridged with the USB connector), it stores the data in the flash memory and execute the program from flash. If there is no data, the bootloader directly executes the program from flash memory.

At each power cycle or reset, the program executes once the setup function and then iterate on the loop one, endless.

So, you need to initialize the serial console (for debugging) and open a Wifi connection from the setup function. Then, at each loop iteration, you will check if you need to read the sensor’s value and send it to Redis, if yes, you will check that you have an active connection to Redis and open it if needed, and you’ll send your data.

Check sensors values

First, initialize the feedback with the serial console and read the sensor’s values.

In the setup function, initialize the serial console, wait for its initialization (only useful for some Arduino such as the Yun, but without drawback on others).

void setup() {
  // Serial console initialization for debugging
  Serial.begin(115200);
//  Serial.setDebugOutput(true); // Wifi debugging
  while (!Serial);
  Serial.println("Serial initialized.");
}

In the loop function, print the sensor’s value, from the Analog-Ditial-Converter, to the serial console.

void loop() {
    Serial.print("Sensor value (0-1024) : ");
    Serial.println(analogRead(0));
}

Test your sketch with the tick icon in the toolbar, or from the menu Sketch > Verify/Compile or with the keyboard shortcut Ctrl+R.

If you have no error, upload it to the device using the arrow icon, the menu Sketch > Upload or the shortcut Ctrl+U. As soon as it is uploaded, it is executed.

Start the serial monitor with the right magnification glass icon, from the menu Tools > Serial Monitor or with the keyboard shortcut Ctrl+Shift+M. Choose the same baud rate as in the sketch (115200) and observe.

You can see the speed of the loop function. Despite you could introduce a void delay(long ms) call in the loop, it would be a bad practice, the loop function needs to be executed as fast as possible, without blocking, this is the main event loop. It is better to initialize a millisecond timestamp at each print and to only read a new value after a timeout :

//  your network SSID (name)
#define WIFI_SSID "YourWifiNetwork"
#define WIFI_PASS "YourWifiNetworkPassword"

#include <ESP8266WiFi.h>

unsigned long lastSensorRead=0;
void loop() {
  if ((millis() - lastSensorRead)>5000) {
    lastSensorRead = millis();
    Serial.print("Sensor value (0-1024) : ");
    Serial.println(analogRead(0));
  }
}

Compile, upload and observe… It is better, one value every 5 seconds.

WIFI network connection

Lets begin with the setup function. You need at least a Wifi network name and password to connect to.

Add the ESP simple Wifi header, define your Wifi credentials and add the following at the end of the setup function. I also included a LED blinking during the WIFI connection, to have an hardware status feedback, and resetting lines (commented) in case of unexplainable issue.

//  your network SSID (name)
#define WIFI_SSID "YourWifiNetwork"
#define WIFI_PASS "YourWifiNetworkPassword"

#include <ESP8266WiFi.h>

unsigned long lastSensorRead=0;

void setup() {
  // Serial console initialization for debugging
  Serial.begin(115200);
//  Serial.setDebugOutput(true); // Wifi debugging
  while (!Serial);
  Serial.println("Serial initialized.");

  // WIFI connection
//  ESP.eraseConfig();           // in case of need
//  ESP.reset();                 // in case of need
//  WiFi.softAPdisconnect(true); // in case of need
//  WiFi.disconnect(true);       // in case of need
  Serial.print("Connecting to ");
  Serial.print(WIFI_SSID);
  WiFi.begin(WIFI_SSID, WIFI_PASS);
  // Blink LED twice per second while waiting for connection
  pinMode(LED_BUILTIN, OUTPUT);
  while (WiFi.status() != WL_CONNECTED) {
    digitalWrite(LED_BUILTIN,HIGH);
    delay(250);
    digitalWrite(LED_BUILTIN,LOW);
    delay(250);
    Serial.print(".");
  }
  Serial.println("");
  Serial.print("WiFi (");
  Serial.print(WiFi.macAddress());
  Serial.print(") connected with IP ");
  Serial.println(WiFi.localIP());
  Serial.print("DNS0 : ");
  Serial.println(WiFi.dnsIP(0));
  Serial.print("DNS1 : ");
  Serial.println(WiFi.dnsIP(1));
}

Compile, upload and watch.

TCP Redis connection

Now, it is time to check that you have an opened Redis connection. For that, we use a WiFiClient object. It can open a TCP connection to an IP or an hostname and a port number. It will take care of the DNS resolution, but might keep the IP in cache (this could be an issue if the DNS are involved in the high-availability architecture). The SSL encryption support is quite limited in the ESP, but we wont use it in this post.

The idea is to check that we have an opened redis connection at each sensor read, if not we open it.

Define your Redis connection, add a WiFiClient object and change your loop function :

//  your network SSID (name)
#define WIFI_SSID "Freebox-AEA6A1"
#define WIFI_PASS "adherebit-commend96-sonatio#!-calidior26"

// Redis details
#define REDISHOST "redisdb.server.com"
#define REDISPORT 6379

#include <ESP8266WiFi.h>

unsigned long lastSensorRead=0;
WiFiClient redis;
unsigned long lastValue=0;
void loop() {
  if ((millis() - lastSensorRead)>5000) {
    lastSensorRead = millis();
    lastValue = analogRead(0);
    Serial.print("Sensor value (0-1024) : ");
    Serial.println(lastValue);
    if (!redis.connected()) {
      Serial.print("Redis not connected, connecting...");
      if (!redis.connect(REDISHOST,REDISPORT)) {
        Serial.print  ("Redis connection failed...");
        Serial.println("Waiting for next read");
        return; 
      } else
        Serial.println("OK");
    }
}

Send Redis commands

Now, it is time to actually send commands to Redis to insert the value at the begining of a Redis list using LPUSH. The list key is built from the concatenation of the string “v:” and the the device’s MAC address.

Add this code immediately after the connection test.

    // 10 is the base
    ltoa(lastValue,szValue,10);
    redis.print(
      String("*3\r\n")
      +"$5\r\n"+"LPUSH\r\n"
      +"$19\r\n"+"v:"+WiFi.macAddress().c_str()+"\r\n"
      +"$"+strlen(szValue)+"\r\n"+szValue+"\r\n"
    );

I used a C++ String to make the code shorter.

Redis server send back a reply to every Redis command, even when there is an error. These replies are stored in buffers on the device. When all the buffer are full, the device can not send new commands anymore, until one of the buffer expires and become available (every second). Thus, I could wait synchroneously for the reply, but it would be a busy loop, even if it is usually fine because Redis server replies very fast, it is a bad practice. I chose to test for a reply at each loop iteration and to consume it. Just before the end of the loop.

  // If there is an answer from the Redis server available
  // Consume Redis server replies from the buffer and discard them
  while (redis.available() != 0)
    Serial.print((char)redis.read());

Compile, upload and observe

Result in Arduino console Full size

Push notification with a PUBLISH

As an exercice, you can send another command using the REdis Serialization Protocol protocol to Redis, so that it will notify all listening (SUBSCRIBEd) application that you updated your value list. The idea is to send PUBLISH chan:<MACAddr> <lastValue>. With such a message, the other application won’t have to get the value from the list, as the value is already part of the message.

You will be able to observe the results if you connect to your Redis server with the src/redis-cli -h <IP> -p <PORT> tool and if you type SUBSCRIBE chan:<MACAddr>

Troubleshooting

If your ESP throw an ugly stack trace on your serial console, you can debug using a lot of Serial.print, but you can also install The ESP Exception decoder plugin in the IDE.

If you want to see what happens on your Redis server, you can monitor it using the src/redis-cli -h <IP> -p <PORT> tool and typing MONITOR. Result in Redis monitor Full size

Improvements

You can connect the LDR to a digital pin instead of 3.3V. So, you can choose to give power only when you read the value, to save energy.

Another solution is to use the internal pull-up resistor to remove the external pull-down one and connect the LDR between AD0 and GND. The values will be inverted, but you save 0.02€. Then, you can disable the pull-up resistor between readings to save energy.

You can also use sleep commands to put your device in sleep mode between reading/sending values, to save energy. This is more complex, but can lower down the current to few µA most of the time !!! Useful for battery powered devices that should last a long time without maintenance.

If you want high-availability, either you need to implement parts of the sentinel protocol, or parts of the cluster protocol or to use a proxi such as the one provided in the enterprise edition.

Add SSL between the device and Redis server, then, you can also use the AUTH redis command to add some security to your database.

You can implement a library to encode commands in the REdis Serialization Protocol and another one to manage the redis connection, with pipelining.

Materials and Links

Link Description
Full source Full source code without the publish
06/09/2017: Redis IOT meetup in Tel Aviv, Israël at RedisLabs Tel Aviv
10/10/2017: Redis IOT meetup in Paris, France at SOAT
17/10/2017: Redis IOT meetup in Toulouse, France at Étincelle coworking Toulouse
07/11/2017: Redis IOT meetup in Lille, France at Zenika Lille
14/11/2017: Redis IOT meetup in Bordeaux, France at Le Wagon Bordeaux
21/11/2017: Redis IOT meetup in Lyon, France at La cordée coworking
Redis France Redis meetup group in France

Footnotes

  1. Windows users : choose the ZIP file, not the installer nor the app from the store. We want to create a portable install without admin permissions, with everything in one singe folder that can be moved. 

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