Until very recently, assuring our well-being at home was a completely hands-on experience. It took flipping a switch, physically locking a door, or moving a lever, button, or dial to make certain our food was fresh and our air or water was clean. Today, with a quick look or swipe of a smartphone screen, it is easy to remotely monitor many of these activities in the home and also certify that security remains in place.
Sensors make this possible. Within our homes, sensor applications range from measuring CO2
levels in the air, making sure our food is stored at the right temperature, to intrusion sensors that protect our houses, whether we are in them or not. With the increased availability of inexpensive low-power sensors, radios, and embedded processors, the so-called “smart home” is typically equipped with a large amount of networked sensors which collaboratively process the acquired data on the state of the home as well as the behavior of its residents.
This article will explore some of the new sensor products and technologies available for use by engineers working on smart home applications, to help them meet one of their key responsibilities, to select the right sensor for the job. All parts, data sheets, and other information referenced here are available on the Hotenda website.
Better than a watchdog
I recently experienced the frustration and trauma of having my home burglarized. Gone for only 45 minutes, my electronic devices were completely MIA upon my return. Now, of course, I have implemented a home security system that employs PIR (passive infrared) motion sensors such as the EKMC1601111 PIR sensor from Panasonic (Figure 1). The sensors are small, inexpensive, low power, and rugged. And, when used in security situations, they literally do not wear out, providing a high-degree of reliability that the application demands.
The sensor works by measuring infrared light from any objects that generate heat in its field of view. Crystalline material on the face of the sensor detects the infrared radiation. Interestingly, it is not the detection of the radiation, but a change in condition when an entity/object enters the field that changes the voltages generated, which are then boosted by an on-board amplifier. When motion is detected the sensor outputs a high signal on its output pin, which can either be read by an MCU or used to drive a transistor to switch a higher current load. The field does not have to be broken by an object with a different temperature to register change, as some highly-sensitive sensors will activate from the movement alone.
Advantages of sensors such as the Panasonic part include simplified circuitry with fully-integrated circuit design, 1 μA low current consumption, excellent resistance to electromagnetic noise, high signal/noise ratio to minimize false operation, and environment-friendliness with “lead-free” construction.
Figure 1: The EKMC1601111 Panasonic PIR Motion Sensor.
Other features of the EKMC160111 include:
- Sensing circuits enclosed in a TO5 can. The high-density embedded circuit design eliminates external sensing circuits, helping designers reduce development and design schedules.
- Low current consumption. Reduction of current consumption allows battery life to be extended for battery-driven wireless products.
- Excellent noise resistance (radiation noise, power supply noise). The entire circuitry is enclosed in a metal package for high electromagnetic-shielding capability.
- Miniaturized lenses with small elements. A short focal length is all that is required.
Not all dangerous conditions in the home are visible. Monitoring the CO2
level is relatively inexpensive and more accurate compared to monitoring any of the other dangerous gases that are in the home such as carbon monoxide (CO). As the air quality gets worse, CO2
levels increase. Outside air has an average of 350 – 450 ppm of CO2
. Inside, the levels rise to around 550 – 700 ppm. Similar to smoke detectors, most states require CO2
monitors to be installed before a house sale can be completed.
Two examples of a sensor platform for CO2
applications are the GE Telaire T6613 and T6615 CO2
modules (Figure 2). Small and compact, the modules are designed to integrate easily into existing controls and equipment. The T6613 is factory calibrated to measure CO2
concentration levels up to 2000 and 5000 ppm. Based on Telaire’s ABC Logic self-calibration software, which begins 24 hours after first use, adjusting the sensor measurement results in sensor-to-sensor consistency. The self-calibration software is designed to be used in applications where concentrations will drop to outside ambient conditions (400 ppm) at least three times in a 14 day period, typically during unoccupied periods.
Figure 2: The GE Telaire T6613 and T6615 CO2 modules measure deadly CO2 emissions.
The GE Telaire T6615 is a dual-channel sensor. The two channels can be described as a CO2
channel that measures gas concentration and a reference channel that measures sensor signal intensity. The T6615 performs periodic self-calibration using the reference channel. The self-calibrations are approximately every 24 hours but can also be initiated by sending a command. During the self-calibration, the sensor PPM reading is frozen; it will not react to changing CO2
conditions. The calibration time is adjustable, but nominally is two minutes. The dual-channel module is designed for applications where the ABC Logic calibration software cannot be used. The module is intended for making precision measurements in applications including kerosene heaters, air-to-air heat exchangers, air purifiers, and demand-controlled ventilation.
Along the same vein are sensors that identify and react to smoke, gas fumes, and more. The Parallax MQ-5 gas sensor features a high sensitivity to liquid propane and natural gas, and a small sensitivity to alcohol and smoke. The sensor is used in gas-leakage detection in both residential and industrial settings. Their simple drive circuit, fast response, and stable long life make them ideal for these and other applications.
Figure 3 shows the typical sensitivity characteristics of the MQ-5 for several gases at a temperature of 20°C, humidity of 65%, O2
concentration of 21%, and load resistance (RL) of 20 KΩ.
Figure 3: Typical sensitivity characteristics of the MQ-5.
Resistance value of the MQ-5 is different for various kinds and concentrations of gases. When using this component, sensitivity adjustment is necessary and the supplier recommends that engineers calibrate the detector for 1000 ppm H2, or LPG concentration in air and use an RL value of about 20 KΩ (10 KΩ to 47 KΩ).
Withstanding freeze/thaw cycles
A sensor that we often take for granted is one used in refrigeration applications that keep our food safe for consumption. An example is the Spectrum Sensors and Control (part of API Technologies Corp.) A1004SS22P0 ruggedized temperature sensor (Figure 4). The sensor is designed to offer a robust solution for monitoring temperature in often-difficult environments, such as high-moisture applications, where the sensor is subjected to continuous freeze/thaw cycles.
With standard temperature sensors, ice places a significant mechanical force on the wire or cable, allowing water to breach the end seal of the probe. Once water is inside the housing, freeze/thaw cycles can provide a direct moisture path to the thermistor element and resulting failure.
This sensor’s swaged-end design offers a moisture seal between the cable and the housing. It has the added benefit of providing a superior mechanical bond for strain relieving the sensor and internal connections. This isolates the thermistor element not only from the outside moisture, but also from the high stresses that can be associated with freezing and thawing.
Figure 4: SS&C’s ruggedized sensor is well-suited for freeze/thaw environments.
The temperature sensor can incorporate a wide variety of thermistor curves and resistances. Other options include cable lengths and terminals. Features include:
- Testing to 10,000 freeze/thaw thermal cycles
- Wide operating temperature range: -40°C to 105°C
- Design allows for quick response and excellent thermal tracking
- Excellent for freeze/thaw applications in HVAC, food and beverage, and refrigeration systems
In summary, whether sensors are activated and/or accessed via a smartphone or simply embedded in measuring and monitoring devices, they are rapidly evolving and delivering expanded features and functionality to provide for our safety and well-being at home. For more information on the parts discussed in this article, use the links provided to access product pages on the Hotenda website.