Developing Robust Motion Detectors Does Not Have to Be a Moving Target
This article discusses some concepts for ensuring motion detector designs will be robust to external disturbances by presenting an infrared detector assembly that can significantly reduce the parts count and improve product performance and reliability.
Smart homes and buildings are benefitting from intelligent technologies that enable them to be more convenient, safer, and energy-efficient. Figure 1 shows the wide range of products available for both homes and buildings. These products can be automatically monitored and controlled by consumers in their homes and building management personnel.
One product that combines convenience, safety, and energy conservation is an infrared motion detector. An infrared motion detector senses occupancy in an area and can then activate lighting, the HVAC system, or an alarm. If the motion detector, is providing a security function, then it needs to operate reliably 24 hours a day, 7 days a week.
Figure 1. Intelligent devices that enable security, convenience, safety, and efficient energy management in homes and buildings
Passive infrared technology is the leading technology for motion sensing due to a combination of reliable performance and low cost. The market is experiencing a healthy compound annual growth rate of over 13% and is expected to exceed $3.5 B in 20251. Factors driving growth include:
- Residential demand for security using surveillance
- Reduced installation costs with wireless connectivity and IoT networks
- Government initiatives for energy conservation that are being adopted in the public and commercial sectors
- A growing market presents opportunities for new, innovative designs
Protection, Control, and Sensing Components
Since passive infrared motion detectors are used either for interior monitoring or outdoor monitoring, these products must be robust to environmental disturbances. If the detectors are powered by the AC line, they must be capable of withstanding current overloads and voltage transients that can propagate on the AC power line. In addition to circuit protection, efficient control and reliable sensing performance are essential for having a quality product.
Figure 2 shows an example infrared motion detector and indicates recommended protection, control, and sensing components that enhance product reliability and performance.
Figure 2. An example passive infrared motion detector showing recommended protection, control, and sensing components
In Figure 3, we show a block diagram of a passive infrared motion detector, and we show in which circuits where the recommended protection, sensing, and control components should be placed. We will discuss each block in which components are recommended.
Figure 3. Block diagram of a passive infrared motion detector showing the circuits where the recommended components are located
AC/DC Power Stage
The AC/DC power stage provides the DC power for the other circuit blocks. This circuit interfaces with the AC power line and is subject to overcurrent surges and overvoltage transients. Overvoltage transients and current surges can result from lightning strikes, inductive spikes from motor turn-on and turn-off, and transients from power line voltage variations.
Against these potential disturbances, we recommend a metal oxide varistor (MOV) as the first line of defense for the AC/DC power stage board. Locate the MOV as close to the entrance of the AC voltage on the circuit as possible to minimize the propagation path for AC line transients on the circuit board. Select an MOV with these characteristics:
- Safe absorption of up to 10 kA peak surge current or 150 J of pulse energy to protect downstream circuitry from a lightning strike
- Low clamping voltage which will not damage downstream circuitry
- An operating temperature range to ensure the component will function over the detector’s specified environmental range (versions of MOVs with phenolic coating can operate safely up to 125 °C)
- UL- or IEC-recognized to reduce qualification time by a nationally recognized standards lab
At the output of the AC/DC Power Stage, we suggest you use a transient suppressor (TVS) diode for further protection of all the load circuits on the power supply. The TVS diode will minimize transient stress on the power components in the various load circuits. The TVS diode offers these benefits for circuit protection:
- Absorbance of as much as 600 W of peak pulse power or 100 A of peak surge current
- ESD protection that can be as high as 30 kV either from through-the-air strikes or direct contact
- Ultra-fast response, under 1 ps, to a transient
- Versions that have clamping voltages as low as 10 V
- UL or IEC component recognition
As shown in Figure 4, TVS diodes can be bi-directional, two series diodes in a package, or uni-directional, a single diode. In addition to their protection features, TVS diodes consume a small amount of power. In normal, disturbance-free operation, the component draws under 1 µA. Finally, surface mount versions of TVS diodes are available for conserving pc board real estate.
Figure 4. Bi-directional and Uni-directional TVS diodes for protection from ESD and other electrical transients
Motion Sensor and MCU
The primary elements of the passive infrared detector are the infrared radiation sensor and the microcontroller unit. Be aware that complete packages are available that include a sensor, a lens, and a microcontroller unit (See Figure 5). A complete package offers:
- Reduction of the design’s bill of material since the sensor connects directly to the microcontroller chip.
- A microcontroller which contains all the circuitry needed for a complete design for the detection and processing electronics. The microcontroller contains the CPU, RAM, timers, comparators, A/D converter, communication interface, and the firmware for the sensor.
- A two-element or a four-element sensor that, combined with a lens, can offer numerous options for different fields of vision.
- Designs that allow use of lower cost and smaller ceramic capacitors compared with typical usage of electrolytic capacitors which have higher leakage and a shorter product life
- Advanced motion detector algorithms that provide sensitivity, range, and field of detection management
- Flexibility in the microcontroller to allow implementation of user-defined application features
For power savings, look for an assembly that has a low power mode when no motion is detected. Figure 6 shows an example infrared motion detector package. The combination of an integrated package with fewer components and no electrolytic capacitors improves overall product reliability, saves pc board space, and reduces cost.
Figure 5. Block diagram of a combination lens, sensor, and microcontroller assembly
Figure 6. An example lens-sensor-microcontroller infrared motion detector assembly. This assembly is from Zilog. The lens and sensor are on top of the board. The microcontroller is underneath the sensor.
The alarm circuit activates when the infrared sensor detects an appropriate amount of motion. The circuit will typically drive a flashing LED light or a combination of an LED light and a speaker. The alarm circuit will need a control component to energize the external device. Consider either a reed relay or a solid-state relay, both of which will provide galvanic isolation of the high-power drive from the low-power logic circuitry. A solid-state relay offers longer life for the output drive contacts, while a reed relay offers lower power consumption.
Reed relays are available in compact single-in-line packages. You can also get reed relays with built-in back EMF suppressor diodes to protect the coil drive circuit and with magnetic shield options to prevent the coil EMI from entering the control circuitry. In addition, reed relay contacts have a longer life than conventional electromechanical relays, and they are relatively immune to wide ranges of environmental temperatures.
Solid state relays optically couple control between an input LED and an output photodetector transistor. Solid-state relays can have as much as 1500 Vrms isolation between input and output. Many are designed to eliminate EMI/RF generation with logic that initiates switching at zero voltage crossings. Versions of solid-state relays can have low off-state output leakage current of under 1 µA to minimize power consumption. They are available in space-saving, surface-mount packages.
Your choice of drive component will depend on ensuring the contacts or the output have sufficient drive capacity for the types of outputs that will be used. Size, power consumption, and cost will be other factors that you will want to factor in your choice of drive component.
Safety Standards for Passive Infrared Motion Detectors
Table 1 lists critical standards that your design needs to conform to so that it can be certified and obtain market acceptance. Adhering to these standards as part of the development project will reduce certification costs and reduce certification time. Complying with the IEC standards will permit sales of your product in all worldwide regions.
Table 1. Standards that Govern Safety and Minimum Operational Requirements for Passive Infrared Motion Detectors
Robust, Reliable Designs Can Have a Low Component Count and Low Development Costs
For an infrared detector design, a robust design requires only a few protection components. You can take advantage of a lens/sensor/microcontroller package to reduce parts count and maximize product reliability. Make sure you include standards compliance as an important element of the development project to save certification time and costs. We suggest you take advantage of a manufacturer’s applications expertise to help save time with selection of protection and control components and save significant design time. The manufacturer can also provide guidance on which standards apply to the design and provide guidance on how to ensure compliance. These recommendations will help you achieve a robust, reliable design that will result in fewer field failures and a more cost-effective and profitable product.
1.) Occupancy Sensor Market. Markets and Markets. July 2020.
To learn more, download the Building Automation Application Guide and Circuit Protection Selection Guide, courtesy of Littelfuse, Inc.
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