ESP8266 sending data to Google spreadsheets

In the past couple years, the ESP8266 platform has flourished dramatically and emerged as one of the most popular hardware tools among electronics hobbyists and IoT enthusiasts. Packed with a 32-bit RISC CPU running at 80 MHz, a fully integrated Wi-Fi radio with TCP/IP protocol stack, serial peripherals (I2C, SPI, and UART), an ADC channel, and general purpose I/O pins, the ESP8266 is the most integrated and affordable Wi-Fi solution available in the current IoT market space. An ESP8266 hardware, like Node MCU and ESP-01, can directly interface with sensors using its peripherals and upload the sensor measurements to a local or a remote web server via internet. Right now, there are already quite a bit of cloud IoT platforms (Thing Speak,,, Xively, … the list is getting bigger everyday) that provides APIs and tools to allow the ESP8266 users to directly upload their sensor readings online for real-time visualization and global access. If you are a regular user of Google Drive, like me, you would find a Google sheet more approachable than all those IoT cloud platforms. In this tutorial, I will describe a method of connecting the ESP8266 device directly to a Google sheet for storing the sensor data without using any third party plugin. For illustration, I am using a Node MCU board that reads the analog output from a soil moisture sensor inserted into one my flower pots and directly connects to a spreadsheet on my Google Drive for storing the data.
Basically there are two parts to this project. The first part is setting up the ESP8266 hardware to read the soil moisture sensor output. The second part involves creating a Google sheet and configuring it to accept the sensor data sent by the ESP8266 module over the internet using a Google App Script that is provided with this tutorial.

Hardware setup

This project uses a very simple and inexpensive soil moisture sensor (one you can buy on eBay or Ali express for $1) kit consisting of two exposed metal pads and an instrumentation circuit board. The exposed pads act as probes for sensing the moisture level within the soil. The more water in the soil means higher conductivity between the two pads, and vice-versa. The accompanying instrumentation board provides an analog output voltage that varies with the moisture level of the soil. The instrumentation circuit is powered with a 3.3V supply and its output is fed to the analog input channel (A0) of the Node MCU.
Note: The ESP8266 analog input can handle the maximum input voltage of 1V. However, the Node MCU implements a voltage divider circuit on board to expand the range of input voltage to 3.3V.
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