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Well, living in Sweden with a dry climate during the winter and having an high end guitar is a challenge. So I wanted a way to keep an eye on my guitar to keep it healthy during the period with dry atmosphere. I needed a device with the following features:

- measure air humidity
- so small it can fit into the case, or even better inside the guitar
- keep track of all measurements some how
- low power consumption, to be used with batteries

My solution was to build a really small device with a power efficient micro controller that could connect wirelessly to internet and send measurements to a cloud service where I could see the result in a nice graph.

Step 1: Material

The case, an empty KinderEgg
Micro controller - ESP8266-12 (US $2.5)
Humidity sensor - DHT22 (US $2.5)
USB to 3.3V DC power step down (US $1)
Resistor ~10kOhm
Female socket connector
A piece of prototype circuit board

You will also need an USB to TTL Serial Adapter (not in picture) so you can connect the ESP to your computer to be able to flash firmware and upload code. I used an 'FT232RL' (US $2).

Step 2: Prepare Micro Controller

I used an ESP8266-12 WiFi SOC from Espressif, flashed with the NodeMCU firmware, allowing you to write Lua-scripts. The NodeMCU Documentation site has all information you need, but here in short what you need to do:

- Build the firmware
There is an excellent Cloud Build Service! (Remember to include the NodeMCU module 'DHT' when creating the build. I was also using the module 'enduser_setup' to simplify wifi configuration.)
- Flash the firmware, I'm using the 'esptool' to flash the micro controller
- Upload code "init.lua", I'm using ESPlorer to upload my code to the controller.

The ESP will measure every 15 minute and then go into deep sleep mode saving battery.

Step 3: Soldering

Pic 1: prototyping on breadboard

The USB to 3.3V DC will be used so it can be powered by an ordinary USB power bank.

Pic 2: soldering the ESP8266, USB to 3.3V, humidity sensor DHT22 together on the circuit board.

Pic 3: (back)

Pic 4: (close up) as seen there are some female socket connectors to break out some important pins from the ESP to be able to reset it, connect serial adapter to upload code (I put a small switch in there to set the ESP in flash-mode) and so on.

Pic 5: Schema.

Step 4: Assembly

Cut a hole in the bottom of the "case" for the USB connector and a hole in the top for the DHT22 sensor. Fit everything into the case, done!

Step 5: Installation

Pic 1: The "HumiGit" will be powered by a standard USB power bank. (Even a small 2200mAh battery will last for more than a month).

Pic 2: With an USB extension cable the HumiGit can be placed inside the guitar!

Pic 3: Measured values can be viewed at thingspeak.com as a graph.

Mission completed!

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