NodeMCU is a firmware for the ESP8266 SoC that makes it easy to program the device in Lua. NodeMCU is also the name of a development board based on the ESP8266. What’s interesting about the ESP8266 is that it is not only very cheap — you can buy the SoC itself for under 2 USD on AliExpress, shipping included — but also WiFi-enabled. A NodeMCU development board goes for about 5 USD including shipping.
After attending a workshop on the NodeMCU at 32C3 I was intrigued and ordered a few devices from AliExpress to play around with (this is the one that I bought). I used one of the devices to build a WiFi-connected temperature sensor.
My goal was to have a small device that I can connect two temperature sensors to and that will push the readings from both sensors to Librato every 10 seconds. I used the DS18B20 temperature sensor, a one-wire device. There is a one-wire library available for the NodeMCU, and lots of sample code that deals with reading from the DS18B20, so that part was easy.
NodeMCU also comes with a HTTP library and SSL support, so pushing the readings to Librato should be simple as well. The only problem was that the controller crashed every time I tried to open a SSL connection, so I had to come up with a workaround. Since I don’t consider the measured temperatures particularly sensitive data, I was okay with using plain old non-SSL HTTP as a transport. The solution was a simple proxy written in Go that I run on a small Digital Ocean droplet. NodeMCU has a crypto module that supports HMAC-SHA256, so doing secure authentication via plain-text HTTP was relatively straight-forward.
The code can be found on GitHub. Once the software and hardware worked on a breadboard, I built a simple enclosure and connected the temperature sensors via headphone jacks so that I could place them whereever I wanted. The result in Librato looks like this: