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ESP8266 WIFI serial to emoncms link

For applications that only require basic posting of data from one emonTx to a remote server such as an emonTx with this WiFi module provides a lower cost route than an emonBase or emonPi base-station installation.

#The core of EmonESP is also used for emonPixel and OpenEVSE ESP WiFi 2.0.

EmonEsp WiFi AP Setup Portal



  • ESP-12E module with 4M Flash

User Guide

Hardware Setup

First Setup

On first boot, ESP should broadcast a WiFI AP emonESP_XXX. Connect to this AP and the captive portal should forward you to the log-in page. If this does not happen navigate to

Note: You may need to disable mobile data if connecting via a Android 6 device

1. WiFi Connection

  • Select your WiFi network from list of available networks
  • Enter WiFi PSK key then click Connect

Wifi setup

  • emonESP should now connect to local wifi network and return local IP address.
  • Browse to local IP address by clicking the hyperlink (assuming your computer is on the same WiFi network) On future boots EmonESP will automatically connect to this network.

Note: on some networks it's possible to browse to the EmonESP using hostname http://emonesp or on windows http://emonesp.local

If re-connection fails (e.g. network cannot be found) the EmonESP will automatically revert back to WiFi AP mode after a short while to allow a new network to be re-configued if required. Re-connection to existing network will be attempted every 5min.

Holding the boot button at startup (for about 10's) will force AP mode. This is useful when trying to connect the unit to a new WiFi network.

Wifi setup

2. Emoncms

emoncms setup

EmonESP can post data to or any other Emoncms server (e.g. emonPi) using Emoncms API.

In the Emoncms Server field, enter just the hostname or address without any path (e.g., in the Emoncms Path field enter the path including the leading slash (e.g. /emoncms) or leave it empty if not required.

Data can be posted using HTTP or HTTPS. For HTTPS the Emoncms server must support HTTPS ( does, emonPi does not). Due to the limited resources on the ESP the SSL SHA-1 fingerprint for the Emoncms server certificate must be manually entered and regularly updated.

Note: the fingerprint will change every 90 days when the SSL certificate is renewed.

To obtain the certificate fingerprint, you can use several methods, some examples:

  • Chrome under Windows: click the secure icon next to the address bar and click on the certificate row to get the details, in the Details tab copy the hexadecimal digits from the box Thumbprint substituting spaces with colons and paying attention not to include any leading invisible character;
  • Firefox under Linux: click the secure icon next to the address bar, Show connection details, More information, in the security tab click View Certificate and copy the SHA1 Fingerprint
  • openssl under Linux: issue the following command substituting your host in place of

echo | openssl s_client -connect -servername |& openssl x509 -fingerprint -sha1 -noout


mqtt setup

EmonESP can post data to an MQTT server. Each data key:pair value will be published to a sub-topic of base topic.E.g data CT1:346 will results in 346 being published to <base-topic>/CT1

  • Enter MQTT server host and base-topic
  • (Optional) Enter server authentication details if required
  • Click connect
  • After a few seconds Connected: No should change to Connected: Yes if connection is successful. Re-connection will be attempted every 10s.

Note: emon/xxxx should be used as the base-topic if posting to emonPi MQTT server if you want the data to appear in emonPi Emoncms. See emonPi MQTT docs.

5. Admin (Authentication)

HTTP Authentication (highly recomended) can be enabled by saving admin config by default username and password

HTTP authentication is required for all HTTP requests including input API

admin setup

7. System

Displays free system memory and firmware version


OTA Firmware Update


HTTP API Examples

View units status:


Example return in JSON:


4. Data Input

Data can be inputed to EmonESP via serial UART or HTTP API.

input setup

UART Input

Data in serial:pairs string format can be inputed to EmonESP via serial UART (115200 baud) e.g:



Data in string:pairs can be sent to EmonESP via HTTP API. This is useful to emulate the serial string data function while using the UART for code upload and debug. API example:


Save Emoncms server details


SSL SHA-1 fingerprint is optional, HTTPS connection will be enabled if present

Save Emoncms MQTT server details


MQTT user and pass are optional, leave blank for connection to un-authenticated MQTT servers


EmonESP uses ESP8266 Arduino core

Firmware can be compiled and uploaded either using PlatfomIO (see blog post) or Arduino IDE. Firmware can also be flashed using the pre-compiled binaries using

Option 0: Flash using pre-compiled binaries

Using EmonUpload

Use our emonupload tool to download latest pre-compiled firmware release and upload to EmonESP:

Using Esptool

Find and Install esptool, python required. Navigate to the Releases section of the github page and get the firmware.bin and spiffs.bin files. Use the command below to flash the ESP. --baud 460800 write_flash 0x0 ./firmware.bin 0x300000 ./spiffs.bin

The file paths in the line above are relative, the command is run from the directory where the files are contained, you might need to substitute them for absolute file paths. The 460800 baud can be too high for some programmers, 115200 is a failsafe option.

Option 1: Compile Using PlatformIO

For more detailed ESP8266 Arduino core specific PlatfomIO notes see:

1a. Install PlatformIO command line

The easiest way if running Linux is to install use the install script, this installed pio via python pip and installs pip if not present. See PlatformIO installation docs. Or PlatformIO IDE can be used :

$ sudo python -c "$(curl -fsSL"

1b. And / Or use PlatformIO IDE

Standalone built on GitHub Atom IDE, or use PlatformIO Atom IDE plug-in if you already have Atom installed. The IDE is nice, easy and self-explanitory.

Download PlatfomIO IDE

2. Clone this repo

$ git clone

3. Compile

$ cd EmonESP
$ pio run

The first time platformIO is ran the espressif arduino tool chain and all the required libs will be installed if required.

3. Upload

  • Put ESP into bootloader mode
    • On Heatpump monitor use jumper to pull GPIO0 low then reset then connect power (simulates reset) or pull RST pin low.
    • On other ESP boards (Adafruit HUZZAH) press and hold GPIO0 button then press Reset, LED should light dimly to indicate bootloader mode
a.) Upload main program:

$ pio run -t upload

b.) Upload data folder to the file system (html, CSS etc.) (SPIFFS):
  • Put ESP back into bootloder mode, see above

$ pio run -t uploadfs

See PlatfomrIO docs regarding SPIFFS uploading

Or upload all in one go

This wil upload both the fimware and fs in a single command

Put ESP into bootloader mode write_flash 0x000000 .pioenvs/emonesp/firmware.bin 0x300000 .pioenvs/emonesp/spiffs.bin

c.) OTA upload over local network

$ pio run -t upload --upload-port <LOCAL-ESP-IP-ADDRESS>

Upload SPIFFS filesystem over OTA (and don't flash):

pio run -e emonesp_spiffs -t upload --upload-port <LOCAL-ESP-IP-ADDRESS>

OTA uses port 8266. See PlatformIO ESP OTA docs:

4. Debugging ESP subsystems

The ESP subsystems have a lot of logging that can be enabled via setting various build options.

Using Platform IO the easiest way to configure these is via the PLATFORMIO_BUILD_FLAGS environment variable.

First select the serial port to output debug;


Then add one or more of the debug options;


For example from the Windows Power shell you may do something like;

pio run -t clean
pio run
pio run -t upload --upload-port

Option 2: Compie Using Arduino IDE

An overview of this process is:

  • Install ESP boards using board manager.
  • Install Arduino IDE plug-in for uploading spiffs files.
  • Download the emonESP source files.
  • Upload src.ino and Upload spiffs.

1. Install ESP8266 boards for Arduino IDE with Boards Manager

Install steps from:

  • Install Arduino IDE 1.6.8 or later from the Arduino website.
  • Start Arduino and open ‘Preferences’ window from the menu bar.
  • At the bottom of this window find the field “Additional Board Manager URLs” and enter ‘`. You can add multiple URLs, separating them with commas.
  • Open Tools > Board > Board Manager, scroll down and click on esp8266 platform, select the latest version then install. Now the Arduino IDE can communicate with ESP8266 based modules.

2. Install ESP spiffs filesystem file uploader plug-in.

Install steps from:

  • Download the plug-in (.zip) from (
  • Open Arduino IDE ‘Preferences’ from the menu bar. Note ‘Sketchbook location:’ and navigate to there using your file explorer, then:
  • Create folder ‘tools’ if it doesn’t exist, and within ’tools’ create folder ‘ESP8266FS’.
  • The folder structure should be “…/Arduino/tools/ESP8266FS/
  • Unpack the plug-in from the .zip here, in total it should look like “…/Arduino/tools/ESP8266FS/tool/esp8266fs.jar”.
  • Restart Arduino IDE

3. Clone this repo

$ git clone

or click the green ‘Clone or download’ button at the top of this page.

4. Compile and Upload

  • Open src/src.ino in the Arduino IDE from this cloned or downloaded repo.
  • Select 'Tools > Board > Generic ESP8266 Module'.
  • Select 'Tools > Flash Size: > 4M (1M SPIFFS)'
  • Put ESP into bootloader mode/
    • On Heatpump monitor use jumper to pull GPIO0 low then reset then connect power (simulates reset)
    • On other ESP boards (Adafruit HUZZAH) press and hold GPIO0 button then press Reset, LED should light dimly to indicate bootloader mode.
  • Upload main sketch: Compile and Upload as normal using Arduino IDE [CTRL + u]
  • Reset into bootloader mode again (optional if next step doesn't work consistently).
  • Upload 'data' folder: Upload data folder (home.html web page etc) using tools > ESP8266 Sketch Data Upload tool.
  • If compiling fails because PubSubClient.h library (or any other libraries) cannot be found. Open the Library Manager again (Sketch > Include Library > Library Manager) and search for 'PubSubClient', install.

Troubleshooting Upload

Erase Flash

If you are experiencing ESP hanging in a reboot loop after upload it may be that the ESP flash has remnants of previous code (which may have the used the ESP memory in a different way). The ESP flash can be fully erased using esptool. With the unit in bootloder mode run:

$ erase_flash

sudo maybe be required

Output: v1.2-dev
Running Cesanta flasher stub...
Erasing flash (this may take a while)...
Erase took 8.0 seconds

Fully erase ESP-12E

To fully erase all memory locations on an ESP-12 (4Mb) we need to upload a blank file to each memory location write_flash 0x000000 blank_1MB.bin 0x100000 blank_1MB.bin 0x200000 blank_1MB.bin 0x300000 blank_1MB.bin

Development Forum Threads


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