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How to Design Robust Smart Locks, Window, and Door Position Sensors with Robust Circuit Protection and Sensing

– Ryan Sheahen, Global Strategic Marketing Manager, Electronics Business Unit, Littelfuse Inc.

Sensor door systemSmart home products and advanced building automation products give families and building occupants and the owners more convenience and the comfort of greater safety. These products are enabled by the advancements in smartphone, networking, and internet-of-things (IoT) technologies. The incorporation of these technologies into locks and position sensors gives the occupants peaceofmind as they can view the status of their door locks and windows on internet-connected devices.

The market for smart home and building access controls is a young and growing market. The smart lock market is expected to grow by a compound annual growth rate (CAGR) of 25 percent. That represents a unit growth from 7 million in 2019 to 23 million in 20241. About 70 percent of this growth will come from the residential market. As with smart locks, the demand for security from both consumers and commercial/industrial establishments will result in the global growth of window and door sensors, particularly in developing economies. The CAGR for these sensors is projected to be about 9 percent with unit shipments increasing from about 300 million in 2019 to 465 million in 20242.There is a significant opportunity for innovation and financial success in this market.

To be successful in this market, it is essential that home and building security products such as smart locks and window and door sensors have high reliability and do not fail in a manner that would create a false sense of security for their owners. To achieve high reliability, designers should be aware of the external phenomena that can damage electronic circuitry. Examples include current overloads and transients induced on the power line from lightning and large loads turning on and off. Also, electrostatic discharge (ESD) from human contact is another example of a potential cause of damage.This article will give designers recommendations for components that protect circuits from damage due to overloads and transients, provide efficient control, and compact sensing. Designers will also be made aware of the applicable safety standards with which these products must be compliant. The objective is to assist designers with developing robust, reliable, and safe products in a cost-effective manner.

Protecting smart lock circuits

A smart lock consists of a manual access keypad, wireless protocol link for internet-connected devices access through a software application, sensor to monitor the position of the door handle, actuators to lock or unlock the door, and sensing to detect any effort to circumvent the lock. An example smart door lock with suggested protection and sensing components to ensure reliable operation is shown in Figure 1.A detailed block diagram of a smart lock is provided in Figure 2. The diagram provides the recommended placement for the suggested protection and sensing components.

Electrostatic discharge (ESD) is a prime danger to smart lock electronics. Both the user interface and the wireless interface are susceptible to ESD from a person accessing the door. The User Interface contains the keypad which a person contacts to enter the pre-programmed access code. People area source of ESD, particularly in a dry environment. Designers should protect the User Interface circuit block from ESD to avoid damage to sensitive electronics.

Smart lock
Figure 1. A smart lock system
Smart lock block
Figure 2. Smart lock block diagram

For ESD protection, consider a transient voltage suppressor (TVS) diode or a diode array. TVS diodes are Zener diodes built with silicon avalanche technology and can offer a minimum protection level of ±15 kV of ESD voltage. A TVS diode array can house six Zener diodes to protect five signal lines and provide a ground reference (Figure 3). The advantage of an array is that one space-saving component in an 0402 surface-mount packagecan protect up to five lines. The impact on the circuit block is minimal; a TVS diode array can have a leakage current of only 1 µA. If a higher level of ESD protection is desired, an individual diode can provide ESD protection for each signal line. A single TVS diode (Figure 4) can withstand a ±30 kV ESD strike. Whichever configuration is used, place the TVS diodes as close to the circuit input aspossible to prevent an ESD transient from penetrating into the circuitry.

TVS diode
Figure 3. Example 5-line TVS diode array
single TVS diode
Figure 4. A single TVS diode

The Wireless Interface links to the cellular network or a wireless LAN (Wi-Fi)network, to communicate with a smartphone or another networked device. Since it is exposed to the external environment, the Wireless Interface should have ESD protection. The recommended component is a polymer ESD suppressor. The value of a polymer ESD suppressor is its ability to respond to and absorb ESD transients while having a negligible impact on the characteristic impedance of the Wireless Interface output. Polymer ESD suppressors can withstand a ±8 kV direct contact ESD and a ±15 kV air strike. Typical capacitance for the component is a low 0.06 pF. Response time to a transient is extremely fast, under1 ns. Placement should be as close to the input antenna connector as possible. Figure 5 shows two configurations for polymer ESD suppressors, bi-directional components.

Figure 5. Configurations for polymer ESD suppressors, bi-directional components

Sensorrecommendations for smart locks

Detection to ensure the door is completely seated in the door framerequires a sensor. A reed switch with a magnetic actuator is a low-power sensing solution for a battery-operated smart lock. Reed switches do not require any drive power and are hermetically sealed for long life in any environment. Versions can switch 10 W with ratings up to 0.5A or up to 200V. The switches are very well-suited for use in low voltage controller circuits. In addition, surface mount versions are available for automated circuit board assembly.

Designers should consider a cylindrical magnetic actuator designed for mounting on a frame such as a door frame. AnAlNiCo magnet is the recommended material; and, the size can be as small as 5 mm x 25mm.

The Tamper Detection circuit block also requires a sensor to alert the user if the lock has been compromised and the door has been opened. Again, a reed switch and an actuator are recommended.The reed switch-actuator combination consumes a minimal amount of power to maximize battery life. Designers can consider a reed switch-actuator pair with adjustable sensitivity to ensure fast response to a tampered lock.

It takes only four components to provide both protection and sensing for a smart lock. These components consume a minimum of circuit board space and ensure a safe and reliable product.

Protecting wireless door and window sensors

Wireless door and window sensors provide information on the state of windows and doors. The user can obtain information on whether windows and doors are open or closed from any location.In Figure 6, ahardware configuration is illustrated for both a wireless door andwireless window sensors. Recommended protection and sensing componentsfor each of the hardware elements are also provided.

Wireless window

The block diagram of the two main elements of the system is shown in Figure 7. The sensor circuitry detects the window or door position and reports the information to a controller that is also the interface for the user and the transmitter of information to any location. The sensing circuitry is on the door and window and must allow for movement; thus, the circuitry must be battery-operated. The User Interface Controller with the keypad is in a fixed location so it can be AC linepowered. AC line power is a typical application for commercial installations.

door sensor
Figure 7. Block diagram of a window and door sensor system

As with smart locks, consider a reed switch-magnetic actuator for proximity detection. With no activation power required, the reed switch extends the battery life of the sensor system. The Wireless Interface circuit blocks in the sensor assembly and the user interface controllercan use polymer ESD suppressors to ensure protection from ESD while maintaining the integrity of the RF transmission.The User Interface circuit block, like the smart lock 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. Potential damage to the electronics can come from lightning strikes, overcurrent conditions, other voltage transients, and ESD transients. Electronics engineers can protect their designs from these conditions with fusing and voltage transient protection devices.

There are numerous options for fuses including the fuse’s operating characteristics and the case style to meet a wide range of design objectives.Designers should consider time-delay or slo-blo fuses to avoid nuisance shutdowns.In addition, designers should 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. This parameter trades off with fuse size. If a small fuse is needed, make sure that the fuse can withstand the available short circuit current supplied by the AC line. A final consideration is the fuses’ cold resistance. If power consumption is a prime concern, then designers should look for a fuse with low cold resistance.

To safely absorb the energy from a voltage transient on the AC line from lightning or motor turn-on and turn-off spikes, designers should consider employing a metal oxide varistor (MOV). MOVs can absorb a current surge as high as 10,000 A from an 8/20 µs transient pulse. A 20mmMOV can also absorb as much as 530 J of energy.

An alternative component to an MOV is a TVS diode. Models developed for protecting circuits from lightningand other transients can withstand as much as 1500 W of power from a 10/1000 µs pulse. To minimize power consumption, a TVS diode draws less than 1 µA under normal operating conditions. Furthermore, a TVS diode can respond quickly to a transient in less than 1 ps. Surface mount versions are available to minimize assembly labor.Designers can select either a bi-directional diode or a uni-directional diode as shown in Figure 8.

Figure 8. Configurations for a bi-directional and a uni-directional TVS diode

As with a smart lock, it does not take many components to protect window and door sensing circuits. Designers have a number of options to select the most suitable versions for their products.

Complying with the applicable industry standards for electronic security products

Designers should be knowledgeable of the standards that apply to the products they are developing so that they can incorporate safety requirements in the product definition phase of their project. Failure to accommodate the standards can lead to potentially expensive design modification work and delays in product introduction. In addition to general product safety standards such as the IEC 61000 series which define requirements for withstanding ESD, electrically fast transients, and lightning, specific standards exist for electronic locking and related products. Table 1 lists these standards. They cover the North American market and China. The documents are essential reference materials for designers of smart locks and window and door sensors.

Table 1. Standards for electronic locking and related products (applicable in North America and China)

electronic Value of a robust design

Incorporating the appropriate protection and sensing components will contribute to achieving safe and robust products. Fortunately, designers only need a small number of components to fully protect their products and comply with safety standards. With low-energy sensors, designers can maximize battery life to minimize the frequency of battery replacement.One final recommendation for achieving an optimal design is to take advantage of the expertise of the component manufacturers and seek their advice. The manufacturers’experts can both recommend appropriate components and provide guidance on complying with appropriate safety standards. Their help will save design time and save on safety certification costs.

Attention to circuit protection will lead to robust designs. This will result in a reputation for quality and reliability. This reputationcreatesa strong competitive advantage for manufacturers of smart locks and window and door sensing products.

For more information on circuit protection, sensing devices, and component selection criteria, see the Circuit Protection Selection Guide and the Sensing Products Selection Guide courtesy of Littelfuse.


1Smart Lock Market Size. Grandview Research. February 2020.

2 Window Sensors Market Outlook. Outlook Market Research. May 2019.

About the author

Ryan Sheahen is the Global Strategic Marketing Manager for the Electronics Business Unit. Ryan joined Littelfuse in 2011 as an inside sales specialist. He was previously the Global Product Manager for the magnetic sensing product portfolio.  His current responsibilities include the development of marketing collateral, management of marketing activities for new product launches, and performing marketing studies and feasibility analysis for new product ideas. Ryan earned his BS in Mechanical Engineering Technology from Purdue University.


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