Real Time Clocks for Electronic Thermostats
Real Time Clocks
(RTCs) are IC devices that keep track of time. A typical RTC can provide the time of day as well as the date
via a serial bus such as I2C or SPI. Although similar in many ways, RTCs should not be confused with clock
generators, which are used to synchronize various parts of a circuit.
Temperature Sensors for Electronic Thermostats
A temperature sensor is a device that measures cold or heat as a temperature or temperature gradient. Many
applications require some implementation of temperature sensing and measurement. For motors, the operating
temperature inside the case is monitored by the processor and set to alarm or shut down at temperatures
higher than the normal operating temperature of the motor. If a motor runs at too high of a temperature for
too long, it can reduce the life of the motor. Operating temperature is an indication of the general
operating health of the motor. Higher temperatures inside the motor case can mean too high of a load is
placed on the motor, since as load increases, motor current consumption increases to meet the load
requirements.
Touch Panels for Electronic Thermostats
A touch overlay, or panel, is essentially the sensor portion of a touch display, but in discrete form.
Together with a controller, touch overlays can enable touch sensing for designs using LCD displays that lack
this function.
There are many different touch technologies in use today, each with their own strengths and weaknesses. For
example, resistive touch technology is relatively simple, widely available, and often capable of providing
higher resolution at a lower cost when compared with capacitive touchscreens. However, capacitive
touchscreens can offer superior optical qualities and can be made highly durable and environmentally
resistant.
Touch Screen Controllers for Electronic Thermostats
Touch controllers
interact with a touch sensor to measure tiny changes in capacitance or resistance, which are then translated
into digital signals. Most controllers are designed to interface with a host processor to indicate
finger/stylus positions (in the case of touchscreen controllers), taps, and other useful input. There are
many reasons for using a dedicated touchscreen controller instead of the common A/D converters found in a
microcontroller. For example, implementing capacitive sensing with a generalized MCU can drastically
increase the amount of "on-state" time and may require more A/D inputs than are available. Touch controllers
and specialized touch-sensing MCUs can provide superior accuracy, noise handling, algorithm efficacy, and
environmental compensation techniques
Batteries for Electronic Thermostats
Batteries
are a portable, wireless means of storing energy via the use of self-contained chemical cells. They can be
for one-time use and discarded, or recharged and reused. In essence, a battery is an energy storage device,
but can only store and release electricity as direct current. Direct current is a flat line at a given
amplitude (until it declines as it is exhausted), versus alternating current, which is a sinusoidal wave.
Fuel Gauges for Electronic Thermostats
A battery
fuel gauge
, or state-of-charge (SOC) indication, has evolved from a simple warning to a more complex system level use
of the information, such as soft shutdown to prevent data loss. Considerations that affect accurate
accounting for remaining battery charge include the effects of aging, self discharge and temperature
variations. The value for full capacity is obtained and algorithms are used to calculate the remaining
capacity. The drawback of this approach is that self-discharge is difficult to model since it is a function
of aging and temperature.
Power Management ICs for Electronic Thermostats
A
Power Management Integrated Circuit
(
PMIC
) is a special-purpose IC that provides one or more power management related functions. These can include
voltage regulation, DC/DC conversion, battery management capability and more. Many PMICs offer an I
2
C and/or SPI bus interface, and some might provide additional features such as an integrated touch screen
interface.
Processors for Electronic Thermostats
The term "
processor
" refers to an electronic device that performs computational functions and carries out the instructions of a
stored program. Other terms for processor are microprocessor, central processing unit, and digital signal
processor. Essentially, the processor refers to "the brains of a computer."
Displays for Electronic Thermostats
LCD means "liquid crystal display." It is an electronically driven flat panel screen that orients liquid
crystals within the panel in a direction that blocks or transmits light coming from behind the panel. LCDs
are a low cost, energy efficient visual display that can be controlled in segments or as individual pixels,
in shades of black and gray or in full color. LCDs have most commonly replaced bulky cathode ray tubes in
televisions and computers and are available in all sizes. Liquid crystals were first discovered in 1888, but
were first put into common use in the early 1970s as electronic digital-display watches.
LED Drivers for Electronic Thermostats
LED drivers
are a constant-current source commonly used to power LEDs for screen backlighting. LEDs are current-driven
devices whose brightness is proportional to the magnitude of forward current flow. Typical desired features
for an LED driver are direct control of current, high efficiency, PWM dimming, overvoltage protection, load
disconnect, small size, and ease of use.
Relay Drivers for Electronic Thermostats
Designers of power electronic circuits must often drive power switches that feed DC, AC, or power signals to
a variety of workloads. Logic-level electronic circuits provide the driving signals. In general, however,
the power sources and their loads have reference levels different from that of the control circuitry
(ground). MOSFET selection begins by choosing devices that can handle the required current, then giving
careful consideration to thermal dissipation in high current applications.
Backlights for Electronic Thermostats
A backlight is used in or with a liquid crystal display (LCD). Because LCDs do not produce any light
themselves, backlight illumination is often necessary to make the screen acceptably readable. Many of
today’s backlights use either light emitting diodes (LEDs) or cold cathode fluorescent lamps (CCFL) as a
light source. As its name suggests, a backlight typically illuminates the display from the back side of
(i.e. beneath) the display.
Relays for Electronic Thermostats
Simply put, a relay is an electrical switch that is controlled electrically. Relays are often necessary to
control high power/current/voltage circuits with a low-power signal while maintaining complete electrical
isolation between them.
Many relays are electromechanical in nature; the control signal energizes an electromagnetic coil creating a
magnetic field. This field exerts a force on a movable armature of ferromagnetic material (such as iron)
which makes or breaks the electrical connection.
Solid State Relays (SSRs) are another common type, and unlike their electromechanical equivalent, they have
no moving parts. Instead, one or more transistors, such as a MOSFET, are used to provide a similar function.
Electrical isolation must be added in this case, often by using an on-chip optocoupler.
Switches for Electronic Thermostats
Electro-Mechanical Switches
are used on electronic thermostats and room controllers for thermostat programming, temperature adjustment
and fan control, as well as manual reset functions. The most common switch types are tact, slide, and DIP.
Important design considerations include haptics, small form factors, and long cycle life.
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