NodeMCU is an open source IoT platform which intergrates the ESP8266 wifi chip. It includes firmware which runs on the ESP8266Wi-Fi SoC from Espressif Systems, and hardware which is based on the ESP-12 module. It s benefits are the following:

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• Arduino like hardware IO

• Event driven API for network applications, which speeds up coding process

• Lowest-cost wifi networking. It integrates a cheap WI-FI MCU ESP8266.

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In this project it is used to enable wireless data transmission of the pulse rate and accelerometer data to ThingSpeak. 

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The details of how it has been connected to Arduino can be found in the "Arduino" section of the blog, and coding involved to connect it with it in the "ThingSpeak" section.