Need to control devices based on environmental parameters such as temperature, relative humidity or ambient light level? It?s easy when you use the zero-to-five Volt input port on your whole house controller!
Making ?Sense? of Your Environment
Environmental sensing is a common requirement when designing a home automation control system. Several whole house controllers on the market today allow the user to input analog signal voltages in the zero to +5Vdc range. With the right sensor product, these analog inputs may be conveniently used to monitor common environmental parameters such as temperature, relative humidity or ambient light level. Such parameters might, in turn, allow a whole house controller to intelligently control domestic HVAC (heating, ventilation & air conditioning) appliances such as dampers, humidifiers, dehumidifiers, furnaces and air conditioners. Other devices associated with environmental control may include window coverings, lamps, space heaters, attic fans, and so on.
Making ?Sense? of Sensors
The word ?sensor? is often used to describe a simple sensing component with a ?raw?, uncompensated and unconditioned output signal. For example, a simple temperature-sensing component such as a thermistor is actually a ?raw? sensor element that cannot be advantageously connected directly to the analog input of a whole house controller through a zero to +5Vdc analog input. A thermistor requires at least a stable power source and current limiting electrical resistance and even these additions may not be sufficient since most thermistors are inherently nonlinear and somewhat inaccurate. In the end, linearization circuitry and calibration must typically be employed to fabricate a usable sensor product using a thermistor.
Some sensor devices are offered as small electronic components and claim to be ?stand-alone? since they include on-chip linearization circuitry and are sometimes pre-calibrated. However, the user should beware since many of these sensors usually are not designed to cleanly drive wire or cable more than a few feet in length and so are not easily applied in remote sensing applications. When driving long wires, a sensor of this type may supply a ?noisy? signal to the whole house controller, which usually results in readings that appear to ?jump around? or contain large errors.
In most situations, the system designer will do well to use a more complete sensor product. Such a product should incorporate a ?raw? sensor as well as additional circuitry, which can provide an accurate, linear output signal to be conveyed over hundreds of feet of cable.
What?s a Scale Factor?
Simply put, a scale factor is a number, usually expressed in Volts per Unit, which can be used to convert a raw voltage signal into a usable parameter. When the signal voltage being output by a particular sensor product is divided by its scale factor the result should equal the value of units for the measured parameter.
It turns out that a sensor, which outputs a signal with a scale factor of ?10mV per unit? (i.e. 0.010Vdc per unit) may be easily and directly read by a commonly available digital voltmeter. For example, if the analog output of a temperature sensor with a scale factor of ?10mV per degree Fahrenheit? is measured as 0.753Vdc by a digital voltmeter, then the temperature being measured by the sensor is actually 75.3 degrees Fahrenheit (i.e. 0.753/0.010 or 753/10).
Most whole house controllers incorporate some type of digital computer chip. This means that while analog voltages from zero to +5Vdc can be measured directly by a voltmeter, they must first be converted to a digital form (i.e. digital number) to be handled by the computer chip. Many controllers also use a device called an Analog-to-Digital Converter or ADC to convert a zero to +5Vdc analog voltage to the digital number that the computer chip requires. However, an ADC only responds to a minimum step-change in voltage before the digital number at its output will either increase or decrease.
Many controllers use an ADC, which can only distinguish a minimum step change in voltage of about 19.6mV (i.e. 0.0196Vdc). From our temperature sensor example above, we can see that almost two degrees of temperature change or about 20mV must therefore occur before the controller?s ADC will register a change at its output. Unfortunately, this amount of temperature ?granularity? may be unacceptably coarse in many cases. One practical approach is to provide a temperature sensor output signal with a scale factor of 19.6mV per degree Fahrenheit. A 19.6mV scale factor will cause the ADC output to adjust whenever a minimum change of one-degree occurs, thereby increasing the resolution of the measurement.
Interfacing a sensor with its environment while making it either look good or be inconspicuous can be a challenging task. Creative mechanical techniques must often be utilized in the effort to derive the best accuracy and response time from the sensor while at the same time achieving an acceptable outward aesthetic quality. Simple, reliable electrical interconnect and installation methods are also important.
Unfortunately, a raw sensor component dangling from the end of a cable or even loosely housed inside a perforated plastic box may be neither aesthetically pleasing to the eye nor a particularly useful or reliable device. Good packaging practices should take both aesthetics and reliability into account where practicable.
Environmental sensor products can be a welcome addition to almost any Home Automation system whether they are used to actively control HVAC devices or to simply monitor key environmental parameters. Sensor products most sought after, are accurate, reliable, easy to install and directly interface with most whole house controllers with a zero to +5Vdc analog voltage signal.
A complete line of analog environmental sensors designed to monitor parameters such as temperature, relative humidity and relative ambient light level are produced by Automated Environmental Systems, LLC and are sold both directly and through distribution. AES products include Decora-style sensor plates as well as low profile, flush-mount sensors. Analog output signals are available with scale factors of 10mV/Unit and 19.6mV/Unit. AES products are designed to directly drive cat5 cable, coaxial cable or shielded cable.
Bert Centala is the President of Automated Environmental Systems, LLC, a company he founded in 1994 to provide practical sensor solutions to Home Automation enthusiasts. For sensor data and pricing, please visit www.automated1.com.
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