Tale of two thermostats: Nest teardown Richard Quinnell - December 12, 2014 Home automation is the application where many consumers are getting their first introduction to the Internet of Things (IoT), so it seemed appropriate to explore how vendors are approaching device design in this market. I selected two devices to explore. One was the Nest thermostat, which generated considerable buzz in the market when it came out a few years ago. The other was a more recent introduction: the Honeywell smart WiFi thermostat. Both are available at various chain home improvement stores. One of the first things I noticed when working with the devices was a significant difference in packaging. The Nest is packaged in a fitted box, with all the component pieces (including installation tools) layered inside. The effect is elegant, like a jewel on display. The Honeywell device is in a cardboard box with a flap opening to a window showing the device. But when opened, there is disappointment. Plastic spacers hold the device displayed in the window, and most of the rest of the box is filled with air. The effect is that of a cheap toy in a showy container.
To see how the two compare when digging further inside, I began at the outer layer. The Honeywell provides a full-color touchscreen for controlling its operation. One attribute of this feature is that you can select the background color to complement the color scheme of your house. The Nest thermostat has a look resembling that of conventional electromechanical thermostats, with a simple color display that indicates current and set point temperature, color coded to show if it is heating or cooling. It senses the user's presence, lighting up when approached but darkening when not needed. Unlike the touchscreen of the Honeywell, the Nest has users rotate the outer ring to identify the action they wish to take, then press the unit to confirm the selection. Audible clicks accompany
these actions to provide haptic feedback. The back of the Honeywell device reveals there are no screws or other fasteners. The plastic housing unit is held together using snap-locks. The rear view of the Nest shows that its assembly is more substantial, with screws helping hold the unit together within the outer ring.
The Honeywell wall plate is a simple two-layer PCB with connectors for the wires coming from the HVAC unit. The Nest wall plate has active components, including an STMicro STM32L151 microcontroller and an 8-channel analog Mux. The bubble level in the center is there to aide in proper installation of the back plate to the wall. Comparing PCBs
The Honeywell unit's single PCB includes DC-DC converters, voltage regulators, rectifiers, an 8- channel, 12-bit ADC, 128 MB of SDRAM, 32M of Flash, and a Broadcom WiFi module (shield removed). An Atmel 926 ARM processor is the main processing unit, enclosed by the metal shield just visible underneath the flat-ribbon cable to the display. A single shield encompasses all of the Nest device's main electronics. Two add-on circuits near the bottom provide sensing functions. The PCB and the display secure together with screws, capturing the metal ring between them while leaving it free to rotate.
The main board of the Nest, with metal shield removed, shows a TI Sitara CPU, power management and USB chip, Flash, 32Mx16 SDRAM, and a Murata WiFi module. There is also a ZigBee network coprocessor, implying that the device may offer expansion options in the future.
Users rotate the outer ring of the Nest to select options. This emitter/sensor combination from an optical mouse is what detects that rotation. The inside of the outer ring shows the grooved pattern that the optical mouse element uses to sense rotation.
The Nest senses the presence of its operator using this IR proximity detector, automatically becoming active when approached. It is important to remember, however, that the IoT device is only half of the story. The other half is the software that interacts with the device, receiving data and issuing commands. Both the Nest and the Honeywell thermostats have apps for smartphones with demo modes that allow you to play with virtual devices if you don't actually have one. For the Honeywell device the demo mode is available as a link on the login page. For the Nest you will need to log in with username: demo@nest.com and password: demo. It's worth exploring the software to see which provides the kinds of functionality you would want from a high-end home automation device. For my money, I think the Honeywell device is simply a touchscreen thermostat with WiFi bolted on. There is no new functionality here. The Nest has a learning mode that can determine your usage patterns and program itself over time, eliminating the need to page through a lot of setup screens. It also makes use of local weather forecasts to anticipate when your home will need a more aggressive response in order to achieve desired settings. My feeling is that the Honeywell device is an old design with network connectivity added as a remote interface. There is no utilization of the network's ability to log data, correlate with other information sources, or the like. The Nest seems to be a true IoT design, with network connectivity an integral part of its operation and features. Let me know what you think of these devices and what questions you may have on what I discovered while tearing them apart.