Regardless of how or where we connect them, an understanding of how sensors work, what their capabilities are and, equally, what are their limitations.
The inexpensive ultrasonic twin-transducer modules that we use have the very simple ability to measure distance at a range of up to roughly 13 feet in ideal circumstances. Ultrasonic distance (or rate of change of distance) sensing works best with hard targets (like metal cars or concrete floors) but may not work well with soft targets, like hanging drapes. They work by emitting a ping (short burst of sound around 40 KHz, well above our range of hearing) and listening for its echo. It is a time-of-flight measurement. Variations in air pressure and humidity, for example, can alter the local speed of sound but those variations are insignificant to the relatively coarse measurements we need to make.
While their 15-degree sensitivity angle provides some protection against adjacent modules interfering with each other, we choose to add the additional protection of time sharing through sequential activation. A complete trigger, ping and echo cycle at the module's maximum range takes less than 0.04 seconds. That allows us to activate each of 5 modules in rotation 5 times per second (a total of 25 readings per second).
The popular infrared distance measuring sensor we use has a maximum range of 6 feet, but that assumes the target is a bright white card and that the sensor is not exposed to direct sunlight or incandescent lighting (either of which can flood high infrared levels onto the sensor). This sensor uses triangulation and at limited ranges offers accuracy with non-white surfaces; when the sensor is positioned such that a surface it encounters is out of sight of occupants, a small patch of white on that surface can aid in reliable operation of the sensor.
The Panasonic Grid-EYE does infrared thermopile temperature measurements of 64 locations arranged in an 8 x 8 grid and also includes an internal thermistor as a basis for internal temperature reading comparisons. Its view embraces 30 degrees from center in each of 4 directions (each of the 64 thermal pixels has an approximately 8-degree viewing angle) and depending on the specific variation of the chip, provides the results of those 64 temperature measurements either 1 or 10 times per second. The sensor is not well-qualified for outdoor applications. The readings it provides can, upon external analysis of temperatures in both individual and adjacent cells, indicate the presence of beings with an expectation of being able to discern among adults, children, infants or larger pets. Its data can also alert to heat sources, pre-ignition heat signatures or active combustion. It can also map the effectiveness of ventilation airflow in heating or cooling a room.
This time of flight of light sensor uses an infrared (850 nm) laser, slightly diffused so it isn't just a tiny dot, to create a 3-degree beam. It pulses that beam up to 70 times per second and that pulsed IR illumination gets seen by an array of 6 detectors. This lets it make distance measurements up to 130 feet with an error of less than 2 inches. Sequential range measurements can, after analysis, also indicate the speed of approach or departure.
Beacon services are available under recent Bluetooth Low Energy standards but need to be separately implemented. A beacon transmitter transmits a unique identifier signal. A beacon receiver can identify all of the identifiers within its range. No prior pairing is needed. We use beacons embedded in our cars, powered only when the engine is running. Separate beacon receivers at both the apron and garage ends of the driveway enhance the presence detection with an ability to recognize entry and exit of a vehicle and, in the garage, when an engine has started and when it is turned off.
While we do not give this device separate attention, the house does have a pet door equipped with an RFID reader programmed to recognize the RFID chips injected under the skin of our pets. This is a near-range proximity detection that, in this case, unlocks the pet door only for our pets and for no other small creatures seeking entry into the house.
IR beam break
Each 2-piece (transmitter and receiver) set uses twin infrared beams separated by 100 feet or less; it indicates (through a relay dry contact) a beam interruption only when both beams are interrupted for at least a (user-specified) 50-700 ms period. Each pair can be set to any of 4 channels, so multiple pairs may be used in proximity with one another by a combination of separate channel selection and careful positioning.
We also involve other sensors. PIR (passive infrared) sensors built into Broan ceiling fans in lavatories work in conjunction with timers to automate their operation.
Built-in humidity sensors in Broan shower exhaust fans respond to both set trigger points and to any rapid rise in humidity for automatic activation. The Honeywell TrueDry DR90 dehumidifier in the crawl space also has its own humidity sensor.
An Excelitas thermopile above the cooktop uses the ability of that type of sensor to remotely sense temperatures to operate the exhaust fan. (There is also a double relay present there so that fan can be turned on or off by other requirements, from the need to exhaust air when outdoor barometric pressure drops to the need to stop airflow during a fire).
An Intermatic dual-technology (both PIR and ultrasonic) motion detector on the porch roof by the front door tells us when someone is there. Inductive coils in the driveway signal the presence of any motorized vehicle. A thermostat in the equipment rack we house near the apron of the driveway turns on its cooling fan.
Leak sensors (primarily 2-wire devices providing dramatic changes in resistance when in contact with water) go under sinks and such major water appliances as clothes washers, dishwashers, hot water heaters and the water manifold blocks.
The overall count of sensors in the house: more than 2000.
A “four horsemen” sensor grouping to measure ambient light, temperature, relative humidity and barometric air pressure is the most commonly used sensor pod, deployed in more than 100 locations.
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