Last year my flat developed a bit of a mold problem, or maybe I just found out about it then. It's possible the fungus already lived a long, fulfilling life before being discovered. It wouldn't be surprising for a building from an era when thermal- and hydro-isolation were pretty far down on the priority list. In any case, it made me want to monitor the relative air humidity and dew point levels a bit more closely. I had the apartment pretty well covered with sensors already, but the room with the mold in particular lacked a hygrometer.
Not to re-invent too much hot water, I more or less replicated the nicely documented temperature and humidity web server project from Adafruit. It was doubly appealing because I still had a full bag of old ESP8266 modules that I bought for pennies back when they were the exciting new thing. The only problem was the fact that the humidity sensors supported out-of-the-box by that project were only available from their shop, which is relatively expensive for small items with oversea shipping. Since I was in a hurry, I bought a few DHT11 modules anyway (which turned out to be a mistake).
There's not much to say about the hardware. It's the minimalistic Adafruit's circuit soldered on a perforated board. For the power supply I used a small 3.3V switch-mode converter module I had left over from another project. I was nicely surprised by how easy ESP8266 support was to install into the Arduino IDE. Another pleasant discovery was that ESP8266 with the Arduino-based firmware seems to consume much less power than with the stock AT-command firmware.
The Arduino IDE got updated since Adafruit's tutorial was written, so I had to experiment a bit with the firmware upload settings. Following values seemed to work with my particular modules. Another thing I discovered was that the RST line on ESP8266 has to be left floating for the firmware upload to work reliably. On my previous ESP8266 project I tied it to VCC.
Unfortunately, the DHT11 modules are pretty bad as far as accuracy is concerned. I only discovered Robert's wonderfully in-depth comparison of hygrometer modules after the fact. I played a bit with power supply filtering, but that doesn't seem to be the source of the noise in the data. I ended up modifying Adafruit's firmware so that it reads the sensor every 5 seconds and returns the average of last 8 readings. This alleviates somewhat the problem, but I definitely recommend using some other sensor to anyone wanting to build this.
For comparison, here is the daily humidity graph recorded using DHT11 with averaging:
And here is humidity recorded at the same time (albeit in a different room) by a TEMPerHUM USB dongle with no extra averaging applied:
After several months of running, the Arduino-based ESP8266 turned out to be pretty reliable. I haven't seen any big outages in the log of sensor readings. This is a nice improvement over the stock firmware that I used in my Munin display, which still regularly gets lost to the point that it requires a power cycle.