Play It Safe! Intelligent Design Tips for Smart Home Locks and Access Controls
December 21, 2022
The rapid development of smart homes and advanced building automation is enabled by a combination of smartphone, networking, and internet-of-things (IoT) technologies. These technologies provide increased security, automation, and control that gives homeowners and office occupants enhanced convenience, comfort, and greater safety. Wherever a homeowner or office occupant is, they can easily view the status of their door locks and window and door access.
Today’s engineers who design building and home security products need to ensure that their devices are not creating a false sense of security for their customers. An essential step to improving overall reliability and safety is understanding the protection and sensing components required to comply with applicable standards to ensure safe, robust, and reliable products.
The smart locks market alone is seeing high growth and innovation, with global growth expected to have a CAGR of 25 percent and unit growth expanding to about 23 million units in 20241. At approximately 70 percent, residential market represents most of the growth. The increased awareness of a need for personal security will drive global growth of window and door sensors as well, especially in developing economies. Shipments are expected to increase at a CAGR of around nine percent, to around 465 million units in 20242.
Protecting Smart Lock Designs
Smart locks consist of a manual access keypad, a smartphone wireless protocol link for access through a software application, a sensor that monitors the door handle position, door lock/unlock actuators, and sensing to detect any attempts to circumvent the lock. Figure 1 provides an example smart door lock with recommended circuit protection and sensing to ensure reliable operation. Figure 2 shows a detailed block diagram of a smart lock with the suggested protection and sensing components placement.
The primary danger to smart lock electronics is electrostatic discharge (ESD). The wireless and user interfaces are susceptible to ESD from end-users.
The User Interface provides a keypad that a person must contact with their finger to enter the pre-programmed access code. All users are a source of ESD, particularly in very dry environments. Designers should protect the User Interface circuitry from ESD to avoid damage to sensitive electronics.
Figure 1. Smart lock with protection and sensing recommendations
Figure 2. Smart lock block diagram
Consider using a transient voltage suppressor (TVS) diode or TVS diode array for ESD protection. TVS diodes are Zener diodes that utilize silicon avalanche technology that provides a minimum protection level of ±15 kV of ESD voltage. A TVS diode array with up to six Zener diodes can protect five signal lines and provide a ground reference (Figure 3). In one space-saving component in an 0402 surface-mount package, a TVS diode array can protect up to five lines. The impact on the circuit is minimal as TVS diode arrays have a leakage current of only 1 µA. If more ESD protection is needed, individual diodes can provide ESD protection for the individual signal lines. A single TVS diode (Figure 4) withstands as much as ±30 kV. Regardless of the configuration used, place the TVS diodes as close to the circuit’s input as possible to prevent any ESD transient from penetrating the circuitry.
Figure 3. Example 5-line TVS diode array
Figure 4. A single TVS diode
The Wireless Interface links to a cellular network, wireless LAN, or WiFi network to communicate with smartphones or other networked devices. Because it is exposed to the external environment, the Wireless Interface requires ESD protection. A polymer ESD suppressor is the recommended component. The benefit of a polymer ESD suppressor is its ability to respond to and absorb any ESD transients while having very little impact on the Wireless Interface output's characteristic impedance. Polymer ESD suppressors withstand a direct contact ±8 kV ESD and an air strike ±15 kV. Typical capacitance is a low 0.06 pF, with a very fast response time to a transient of under 1 ns. Be sure the ESD suppressor is as close to the input antenna's connector as possible. Figure 5 shows two configurations for bi-directional polymer ESD suppressors.
Figure 5. Bi-directional polymer ESD suppressors configuration
Smart Lock Sensing Recommendations
Ensuring doors are completely seated in their door frames requires sensors at each location for detection. Consider a reed switch with a magnetic actuator as a low-power sensing solution for battery-operated smart locks. Reed switches do not require drive power, can switch 10 W with ratings up to 0.5 A or up to 200 V, and are hermetically sealed for long life in outdoor environments. Reed switches are well-suited for use in low-voltage controller circuits. Surface mount versions are available for automated printed circuit board assembly.
Because they are designed for mounting on door frames, cylindrical magnetic actuators are recommended. An AlNiCo magnet is the recommended material, which can be as small as 5 mm x 25 mm.
The Tamper Detection circuit also requires a sensor that alerts the user if the lock has been compromised, and the door has been opened. Again, using a reed switch and actuator is recommended. The combination reed switch-actuator consumes a minimal amount of power while maximizing battery life. To ensure a fast response to a tampered lock, consider using a reed switch-actuator pair with adjustable sensitivity.
It takes only four components to provide protection and sensing for smart locks. These components consume a minimum of printed circuit board space and ensure safe, reliable operation.
Protecting Wireless Door and Window Sensor Designs
Sensors provide information on the state of wireless windows and doors throughout a home or building. Users can easily check whether windows and doors are open or closed from any location. An example hardware configuration for both a wireless door sensor and a wireless window sensor is shown in Figure 6.
Figure 6. Example wireless window and door sensing system
There are two main elements of the system (Figure 7). The sensor circuitry detects the window or door position and reports the information to a controller, which also provides the user interface and the transmission of information to any location. The sensing circuitry on the door and window must allow for movement; thus, the circuitry must be battery-operated. The User Interface Controller (UIC) and keypad are in a fixed location so that they can be AC line powered. Using the AC line power is a typical commercial application.
Figure 7. Wireless window and door sensor system block diagram
Electronics designers should consider a reed switch with a magnetic actuator for proximity detection. The reed switch extends the sensor system's battery life because no activation power is required. The Wireless Interface circuits in the sensor and the user interface controller can use polymer ESD suppressors to ensure ESD protection while maintaining RF transmission integrity. Like the smart lock, the User Interface circuit with its keypad should have ESD protection from human contact. A TVS diode array can protect the sensitive signal lines from ESD transients.
Where AC power and an AC-DC power supply energize the User Interface Controller, designers need to protect the controller from potential threats from the AC line. Possible damage to the electronics can come from overcurrent conditions, lightning strikes and other voltage transients, and ESD transients. By using the correct fusing and voltage transient protection devices, designers can protect their designs from these conditions.
Designers should consider numerous fuse specifications. For example, evaluate the fuse's operating characteristics and case styles to meet a wide range of design objectives. Consider slo-blo or time-delay fuses to avoid nuisance shutdowns. Also, select the fuse current rating to accommodate short-term overloads, such as in-rush currents, where applicable. Other considerations include the interrupting rating, which defines the maximum overload current the fuse can interrupt. However, remember that this parameter trades off with the fuse size. If the design requires a small fuse, make sure that the fuse can withstand the short circuit current supplied by the AC line. Finally, consider the fuses' cold resistance. For example, designers should look for a fuse with low cold resistance if power consumption is a concern.
Consider employing a metal oxide varistor (MOV) to safely absorb energy from a voltage transient on the AC line, including lightning or motor turn-on and turn-off spikes. These components absorb a current surge as high as 10,000 A from an 8/20 µs transient pulse, and a 20 mm MOV can also absorb greater than 500 J of energy.
As an alternative component to the MOV, consider using a TVS diode. These components are developed for protecting sensitive circuits from lightning and other transients, and can withstand up to 1500 W of power from a 10/1000 µs pulse. TVS diodes draw less than 1 µA, which minimizes power consumption under normal operating conditions. Also, a TVS diode responds quickly to transients in less than 1 ps. Surface mount (SMD) versions are available to both reduce the PCB space required and minimize assembly labor. Designers can select either a bi-directional or uni-directional diode (Figure 8).
Figure 8. Bi-directional and uni-directional TVS diode configurations
It does not require many components to protect window and door sensing circuits. Designers have numerous options to select the most suitable versions for their specific applications.
Understand Compliance with Applicable Industry Standards
Building automation and smart home electronics designers should know the standards that apply to the products they are developing. It is important to incorporate these requirements during the development phase of any design project. Failure to comply with the standards can lead to expensive re-design work, delays in product introduction, and potential cost overruns. In addition to general product safety standards, like the IEC 61000 series, which defines requirements for withstanding ESD, electrically fast transients, and lightning, specific standards exist for electronic locking and related products (Table 1). The documents cover North America and China and are essential reference materials for engineering teams designing smart locks and window and door sensors.
Table 1. Applicable standards for electronic locking and related products
Robust Reliability Results in End-User Satisfaction
It is a tremendous competitive advantage for manufacturers of smart locks and window and door-sensing products to have a reputation for quality, reliability, and convenience. By using the appropriate protection and sensing components, designers will contribute to achieving safe and robust products for end-users. Fortunately, only a small quantity of components is needed to fully protect their product and comply with safety standards. With low-energy sensors, designers can maximize battery life to minimize the frequency of battery replacement. Electronics designers have many alternative components that they can use. A final recommendation for achieving optimal home automation designs is to take advantage of the component manufacturers’ expertise and seek their advice.
For additional information on circuit protection and sensing components and selection criteria, see the Circuit Protection Selection Guide and the Sensing Products Selection Guide courtesy of Littelfuse.
1 Smart Lock Market Size. Grandview Research. February 2020.
2 Window Sensors Market Outlook. Outlook Market Research. May 2019.