The goal of this project is to make a temperature/humidity monitor that wirelessly logs the temperature and humidity to a remote server. We will use an Arduino Uno (or clone), a DHT11, an ESP8266, and ThingSpeak.com for this project. This is a great project to start using the ESP8266 for some basic data logging so let's get started!
Step 1: Gather Your Supplies
For this project I used an Arduino Uno clone but any Arduino board or clone will work.
Step 2: Create a ThingSpeak Channel
In order to log your temperature and humidity measurements online you need to have a website or webservice that can accept GET http commands. In this Instructable I am using ThingSpeak but there are other, similar services out there so feel free to use something else to log your data. To create a ThingSpeak account click here.
If you use ThingSpeak you will need to make a channel with two fields, one for temperature and one for humidity. They have a number of options for displaying data in graphs so I encourage you to explore these options and see what you like best. I settled on two simple line graphs with a 10 point average to smooth out occasional blips in the data. ThingSpeak will give you a Key for your channel that you will need to put into the code so that your channel will accept your data.
Step 3: Wiring
First we should cover some basics of the ESP8266 and the DHT11. Then we will move on to wiring them to the Uno.
The image above shows the front of the ESP8266 with the 8 broken out pins labeled. The ESP8266 runs at 3.3 V so make sure you connect the Vcc pin to the 3.3 V pin on the Uno. This also means that you will need to use a logic level shifter or make a voltage divider for the serial connection from the arduino board to the ESP8266 or you risk ruining it. If you don't want to buy a logic level shifter then you will need to make a voltage divider which you can do using two resistors. Sparkfun has a great tutorial on this so you can see how they build theirs and calculate which value resistors you will need to use. I used a 220 Ohm resistor and a 470 Ohm resistor for mine which got me close enough to 3.3 V. Since the ESP8266 outputs 3.3 V and the Arduino boards can take that voltage there is no need to do anything to the serial line from the ESP8266 to the Arduino.
The DHT11 spec is only +/- 5% humidity and 2 degrees Celsius so it is not a super accurate sensor. It's also only good for temperatures above freezing. The DHT22 is a bit more accurate so if that's important to you opt for it instead. Both sensors work with voltages between 3.3 - 5 V. The signal pin will need to be connected to a pull-up resistor, which I used a 4.7 kohm resistor for.
The Fritzing diagram above shows the wiring. For ease of use all connections to 5 V or 3.3 V are red and all connections to ground are blue. The wiring connections are as follows:
- Uno | ESP8266
- RXD | TXD
- TXD | RXD (through voltage divider)
- 3.3 V | Vcc, CH_PD, Reset
- GND | GND
- Uno | DHT11
- 5 V | Vcc
- GND | GND
- D7 | Signal (connect to Vcc via pull-up resistor)
Step 4: Code
Before you can use my code you will need to download and install the DHT library from Adafruit. They have a description of how to install and use their library and a link to their GitHub repository. Their library comes with other features such as a heat index calculator which I don't show here but feel free to log that as well if you're interested in it!
The baud for the ESP8266 is set near the top of the .ino file. The version of the ESP8266 I bought comes in either 9600 baud or 115200 baud. Both of the chips I have ordered are set to 115200 baud but if you're trying to use this code and it doesn't seem to work try changing the baud to 9600 as your first step in debugging.
There are several places in code with values in '<' '>' brackets. Those are places where you will need to paste in your own values to make this work. This includes the name and password for your WiFi network and the Key for your ThingSpeak channel. There is code in place to use a red and green LED for debugging and status monitoring but those are not shown in the wiring diagrams. If you'd like to have a quick way to visibly see if things are running smoothly you can wire up a couple of LEDs.
A final thing that tripped me up for a while is that the serial pins CANNOT both be connected between the ESP8266 and the Uno while uploading the sketch. The sketch will fail to upload if they are connected so you must first disconnect one or both serial pins on either the Uno or the ESP8266 and then upload the sketch.
The code is attached to this step and can also be found on my GitHub page here.
Step 5: Use Your Sensor!
Congratulations, you've created your very own wireless temperature and humidity logger! I usually leave mine in my basement so that I can monitor the temperature, and more importantly, the humidity since it's unfinished and I don't want my things getting moldy from too much humidity. I have also mounted it to a piece of cardboard to make it easy to move around. I hope you found this helpful and had fun making it! In the future I plan to program the ESP8266 directly and bypass the Arduino altogether so look for that to come soon.