How to design a smart thermostat complete with intuitive human machine interface (HMI) and secure wireless connectivity

There are many different kinds of thermostats available to homeowners. The three most common varieties are non-programmable, programmable, and smart. No matter what kind you choose, a thermostat has one basic function. The thermostat controls your heating and cooling system to make your indoor air climate the temperature that you want it to be. You set the thermostat to the temperature that you want your home to be. The thermostat then tells the heating and cooling system what to do to get your home’s climate to this temperature.

In the winter you set the thermostat to heat. In the summer you set the thermostat to cool. You also decide if you want the fan to be on constantly, or if you only want the fan on only when the heating and cooling system needs the fan to operate. If your home’s air temperature is already at or very near the temperature that is currently set on the thermostat, then the thermostat directs the heating and cooling system to do nothing.

In winter when the temperature outside dips, your indoor air temperature will go down too. The thermostat will detect that the current air temperature has fallen below your desired setting, and will direct your heating system to activate and heat the indoor air. When the thermostat detects that the air has reached the desired setting, the thermostat then directs the heating system to stop. It follows the same procedure in the summer but using the cooling system instead of the heater.

Next I will introduce you Smart Thermostat, and recommend you the products of Smart Thermostat and reference designs.

What's the Smart Thermostat?

The new breed of thermostat, the smart thermostat, learns the behaviors of your home. By using Wi-Fi, a smart thermostat can be controlled by your computer, tablet, and smartphone remotely. This is why they are called “smart”. Once a smart thermostat has been in place for a while, it can adjust itself. It can show you energy consumption in real-time.?

As shown in the figure below, the intelligent thermostat consists of 18 parts, they are Clocking、Non-isolated AC/DC power supply、Input user interface、Temperature sensor、Proximity sensor、Ambient light sensor、Audio interface、Digital processing、Self-diagnostics/monitoring、Signal isolation、Memory、Non-isolated DC/DC power supply、Output user interface、Wired interface、Relay system、Wireless interface、Energy storage、Signal input/output protection.

Clocking

A clock generator with support for spread-spectrum clocking (SCC) may be used to reduce electro-magnetic interference (EMI). A real-time clock (RTC) can be used to keep track of time during system power loss. Clocking includes Real-time clocks (RTCs) & timers, the corresponding product models are TLC555 、LMC555 、NA555

Non-isolated AC/DC power supply

Thermostats are generally powered from an existing 24V AC power source. A bridge rectifier and a FET can be used to create a DC voltage from an AC power source. E-fuses can also be used in place of traditional fuses for added current protection. Non-isolated AC/DC power supply includes Buck converters (integrated switch)、Flyback controllers、N-channel MOSFETs、Shunt voltage references、P-channel MOSFETs, the corresponding product models are LMR43620 、UCC28700 、CSD17382F4 、LM4040 、CSD23280F3

Input user interface

The user interface allows the user to control and program the thermostat. Designs with a more traditional button experience may use an I/O expander device to interface the system microcontroller (MCU) to a mechanical keypad. Designs desiring to replace physical buttons with a capactive touch keypad may use a microcontroller with integrated capacitive touch technology. A capacitive or resistive touchscreen may be also used in lieu of a mechanical interface. Input user interface includes MSP430 microcontrollers、Touchscreen controllers、Haptic motor drivers、Piezo drivers、Inductive sensor AFEs、Signal conditioners, the corresponding product models are MSP430F5437A、TSC2004 、DRV2605 、DRV2667 、LDC1314 、FDC2212

Temperature sensor

Ambient temperature measurement can be accomplished in one of multiple ways in a thermostat design. A digital output device with integrated humidity and temperature sensor can be an option for designs requiring both sets of data. An analog output temperature sensor provides a low-cost solution if an analog-to-digital converter is available on the system microcontroller, or as a discrete solution. Finally, a thermistor with a corresponding operational amplifier can be a choice for designs only requiring temperature data, although accuracy will often not be as high as other solutions. Temperature sensor includes Humidity sensors、Digital temperature sensors、Precision ADCs, the corresponding product models are HDC3021、TMP1075、TLA2024

Proximity sensor

A proximity sensor allows the system to turn on the LCD backlight based on user presence and control temperature using occupancy detection information. A PIR sensor is the most common proximity sensor used in thermostat designs. Multiple operational amplifiers can be used to filter and amplify the output of the PIR sensor. Comparators or autonomous monitoring ADC's are used to maximize power savings by keeping the system in a low-power mode until a motion interrupt generated. Proximity sensor includes General-purpose op amps、Comparators、Precision op amps (Vos<1mV)、Precision ADCs, the corresponding product models are LM2904B 、LM339LV 、TLV369 、ADS7142

Ambient light sensor

An ambient light sensor allows the system to adjust the amount of backlight in thermostat designs which include a display. A light-to-digital sensor with integrated amplifier, analog-to-digital converter, and digital interface can be used for faster time-to-market and ease-of-use. A custom design based on a photo-diode and corresponding operational amplifier can be used at a lower cost, but will require a microcontroller with an analog-to-digital converter input. Ambient light sensor includes Light sensors, the corresponding product models are OPT3005、OPT3001、OPT3002

Audio interface

An audio interface allows for voice recognition using advanced beam-forming and echo cancellation techniques. An audio analog-to-digital converter (ADC) can be used to receive data from two or more microphones. A digital signal processor (DSP) can implement the necessary audio algorithms to filter the audio and pass the resulting audio data to the main processor for cloud-processing. The DSP can also process and output audio data to a speaker using an audio amplifier. Audio interface includes Audio ADCs, the corresponding product model is TLV320ADC3101

Digital processing

A thermostat uses a microcontroller to perform functions such as temperature monitoring, relay control and keypad monitoring. The design can use a dedicated microcontroller or a single system-on-chip solution which combines the microcontroller and radio on a single device. Alternatively, a microprocessor may be used to support more advanced functions. Digital processing includes Sub-1 GHz wireless MCUs、Wi-Fi products、Low-power 2.4-GHz products、Sub-1 GHz transceivers、Arm Cortex-M0+ MCUs、MSP430 microcontrollers、Arm-based processors、Digital signal processors (DSPs)、Arm Cortex-M4 MCUs、I2C general-purpose I/Os (GPIOs)、Precision ADCs、Real-time clocks (RTCs) & timers, the corresponding product models are CC1354P10、WL1835MOD、CC2674P10

Self-diagnostics/monitoring

A voltage supervisor can keep the system in reset until the system voltage reaches valid levels. Current and power monitoring of the main power supply can be used to maintain accuracy between the thermostat internal temperature sensors and the ambient temperature. Low-power analog temperature sensors can be used in conjunction with autonomous monitoring ADC's to provide heat generation data accross the PCB for internal temperature compensation. Self-diagnostics/monitoring includes Humidity sensors、Digital temperature sensors、Analog temperature sensors、Digital power monitors、Supervisor & reset ICs、Precision ADCs、Real-time clocks (RTCs) & timers, the corresponding product models are HDC3021、TMP1075、TMP235、INA231 、TLV803E 、TLA2021、TLC555

Signal isolation

Signal isolation is needed to interface the system to service signals and to monitor the AC power source. Optocouplers, n-channel MOSFETs, and p-channel MOSFETs are generally used for signal isolation. Signal isolation includes Digital isolators, the corresponding product models are ISO7760、ISO7720

Memory

The thermostat may include non-volatile memory such as flash memory to store system settings and volatile memory to support microprocessor program and data accesses. Voltage reference devices and DDR termination regulators can be used in systems utilizing DDR synchronous DRAM. Level translation devices and ESD protection devices may be an option when using flash memory. Memory includes Auto-direction voltage translators、Application-specific voltage translators, the corresponding product models are TXS0108E、TXS0206

Output user interface

Traditional thermostats use an LED display and status LEDs to to communicate status and other information to the user. A capacitive or resistive touchscreen may be used in lieu of a more traditional LED display. These designs may use a backlight LED driver and an MCU with integrated touchscreen and LCD controller. Output user interface includes Backlight LED drivers、RGB LED drivers, the corresponding product models are TPS61169 、LP5024 、TLC5971

Wired interface

A thermostat may include a wired interface for communicating with a building automation system using standards such as RS-485 and the BACnet protocol. An RS-485 transceiver may be used in this design to interface the system microcontroller to the network bus. Wired interface includes RS-485 & RS-422 transceivers、Ethernet PHYs、USB hubs & controllers, the corresponding product models are THVD2410、DP83867CR、TUSB320

Relay system

The relay system allows the thermostat to operate the HVAC system. A mechanical relay system provides the lowest cost solution, but generates loud clicking noises and has physical contacts that wear out. A solid-state relay (SSR) system performs the same function without the use of mechanical parts. A digital SSR system consisting of control logic and MOSFET switches provides a low-footprint and low BOM-count solution. Relay system includes AND gates、Noninverting buffers & drivers、Inverting buffers & drivers、OR gates、D-type flip-flops、JK flip-flops、Ideal diode/ORing controllers、N-channel MOSFETs、P-channel MOSFETs、Low-side switches, the corresponding product models are SN74HCS08、SN74AUP1G17、SN74AUP3G14、SN74LVC1G32、SN74AUP1G74、SN74HC112、LM66100、CSD17483F4、CSD23280F3、TPL7407LA

Wireless interface

The wireless interface allows the user to access and control the thermostat from a remote location. A system-on-chip solution which integrates a microcontroller and radio allows designs to run the application layer and wireless protocol stack on one chip. A microcontroller with dedicated RF radio topology allows designs to separate the wireless protocol stack from the application layer. A range extender can be used if additional range is required or will be used in an area that can expect a weak radio signal. Wireless interface includes Sub-1 GHz wireless MCUs、Wi-Fi products、Low-power 2.4-GHz products、Sub-1 GHz transceivers、Other wireless products, the corresponding product models are CC1354P10、WL1805MOD、CC2340R5、CC2651R3、CC1120、CC2592、MSPM0G1107、TM4C123GH6PZ

Signal input/output protection

ESD protection devices can be used to protect wired interfaces. Signal input/output protection includes ESD protection diodes, the corresponding product models are TPD1E0B04、TPD4S010、TPD1E01B04

Hope our reference designs and integrated circuits can help you address complex challenges and deliver what is next in thermostat design.