Smart Home Humidity Measurement

This feature covers the measurement, capture and analysis of humidity data in our smart home. We take a very similar approach with temperature sensors but, there are many more of them in our smart home. A single humidity sensor is installed in our kitchen, all bathrooms, our conservatory, garage and outside our home.

Our Home Control System (HCS) is a single logical entity built using a distributed architecture. It is also a hybrid technology technology solution using technology abstraction to enable it to work independently of the implementation technologies. We currently use many different sensors and three main technologies to capture temperature in our smart home. The use of technology abstraction makes it very easy to add new technologies and remove old ones.

HIH-4021 humidity sensor
The HIH-4021 (datasheet) is an analogue humidity sensor, with an output voltage proportional to the percentage humidity.

Typically we connect these to a slave HCS processor (a Raspberry Pi) using an I2C 8-channel analogue interface board.

Everspring ST814 Z-Wave Temperature and Humidity Sensor
We have one Everspring ST814 temperature and humidity sensor connected using our Z-Wave mesh network. It is much less configurable in terms of reporting frequency. One advantage of this approach is the ability to use Z-Wave association to directly link the sensor and actuator (whilst still collecting the associated events). Z-Wave sensors are typically around £35 each.

Design Factors

We only capture and process humanity to a resolution of 1%. Few sensors are accurate to this level but it is a convenient resolution to work with.

Because of the applications we are using humidity data for, we require a very fast response time (i.e. low latency). Correct location of a humanity sensor is key to rapid sensing of humidity changes and then taking action.

We model all of the humanity sensors in our smart home using a Sensor Java class. This class checks every sensor update against upper and lower limits, which are defined for each sensor in the XML configuration for our smart home. This enables advanced features to be implemented (once) and applied to every single humidity sensor, effectively making them all 'smart' sensors. Our Sensor class monitors actual humidity and the rate of humidity change.

Example Implementations

Main Bathroom

Vent Axia ACM100 in-line fan
In our main bathroom, we have been running an experiment with Z-Wave technology for several years now. Part of this experiment involves a Z-Wave humidity sensors controlling the bathroom extractor fan, to reduce humidity levels automatically.

Other Bathrooms

Based on what we learnt from our main bathroom experiment, we have implemented similar features in the other bathrooms in our home but, sign different technology.


Climate Control

The primary application for these temperature sensors is intelligent climate control, which is also linked to the temperature sensors in our smart home.

Weather Station

The HIH-4021-001 sensor is used in our weather station project.

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