Electric Imp Temperature and Humidity Sensor

Continuing the project from my last post. I designed a PCB for my Electric Imp sensor but instead of the 10K thermistor I used on the prototype i changed it to use the Silicon Labs Si7021 Temperature and Humidity sensor. This sensor allows me to measure and trend both the temperature and relative humidity with pretty decent accuracy; it’s specced at +/-3%RH (from 0 to 80%) and +/-0.4 °C (from -10 to 80°C). I also added a boost converter from the battery to the electric imp so when the battery voltage gets below the threshold needed for the Wifi to operate (2.5V) it will get boosted to about 2.8V so that I can still use the remaining energy in the batteries.

Electric Imp Temp/Humidity Sensor

From testing I found the boost converter works well but if the battery voltage is too low the startup current of the electric imp will cause the board to brownout before it has time to enable the boost converter. I found this happens at battery voltages less that about 2.2V. Probably adding more low ESR bulk capacitance on the Electric Imp’s supply rail would help with this.

I originally designed it to use an off the shelf case but eventually just decided to 3D print a case for it. I would like to redesign this at some point to make it smaller; it’s pretty bulky right now especially in the case.

Electric Imp Sensor in it's Enclosure
Electric Imp Sensor in it’s Enclosure

I setup the electric imp to wake up once per hour read both the temperature and humidity and post the data to Thingspeak.com. I built two of these sensors and placed one inside my apartment and one out on the deck. To read the sensor data I wrote a small Android app that reads the last sensor reading plus pulls in the graphed data from the last 5 days.

From the bit of testing I have done I expect to get at least a year off two AA batteries but probably much better than that.

(Left) Android App Main Page (Center) Plotted Sensor Data (Right) Plotted Batter Voltage
(Left) Android App Main Page (Center) Plotted Sensor Data (Right) Plotted Batter Voltage

The schematic for the sensor is located Here:

Wifi Temperature Sensor Schematic

Next I want to build a Wifi power switch and expand this Android app to be able to control things in my apartment using the electric imp platform and after that maybe some wifi controlled LED lights for my workbench.