Despite the continuing automation of many monotonous tasks, human interface always has been (and always will be) a requirement for some tools and machines to operate properly. There are many types of devices available to accomplish this interaction, but only a few that provide tactile feedback to the user. In this article from CUI Devices, learn more about tactile switches, including construction, design specs, wiring and more.
The inception of tactile switches can be traced back to the early 1980s when they first emerged as membrane or screen-printed switches, primarily intended for use in keyboards and keypads. However, their initial lack of tactile feedback and subpar performance faced significant resistance from users. This spurred the need for improvement, leading to the introduction of switches featuring metal domes as part of their design in the late 1980s.
The incorporation of metal domes brought about notable enhancements in tactile feedback, offering users a distinct sensation upon actuation. Moreover, this design modification resulted in a more robust method of switch activation and improved longevity, addressing the previous durability issues.
As a testament to their advancements, modern tactile switches have now become ubiquitous as input devices in numerous commercial and consumer products. Their widespread use attests to the significance and value they bring to a diverse array of electronic devices and interfaces.
Tactile Switch Basics
When a user applies pressure to a tactile switch, it completes the circuit, leading to current flow. This action results in a “tactile” sensation, often a noticeable click or haptic bump, confirming the switch's operation. Upon releasing the pressure, the current flow is interrupted, turning off the switch. In essence, a tactile switch is a momentary action device that provides perceptible feedback through touch, reassuring the user that the switch has functioned, and the signal has been transmitted. Additionally, certain tactile switch models are designed as "normally closed" switches, which means they deactivate the current when pressed and reactivate it upon release. CUI Devices offers a selection of these "normally closed" tactile switches as well.
User confusion can indeed arise from the terminology used to describe tactile switches, often being referred to as push button switches. While both types share similarities, it's essential to differentiate their characteristics.
A push button switch is engineered to permit current flow within a circuit when the actuator is depressed and travels a specified distance, and it interrupts the current flow when pushed again. These switches can be mounted either on panels or directly on printed circuit boards (PCBs). To delve further into push button switches, refer to CUI Devices’ Push Button Switches 101 blog.
On the contrary, a tactile switch allows current flow when the actuator is pressed and held, ceasing the current flow upon release, or vice versa. The actuator's travel distance is minimal. Such switches fall under the category of "momentary action" switches, and while certain push buttons are also momentary, all tactile switches fall into this classification. Notably, tactile switches are typically much smaller than push button switches and often carry lower voltage and current ratings due to their design. Furthermore, they offer audible or haptic feedback and are exclusively designed for direct mounting onto PCBs.
Basic Construction
A typical tactile switch consists of only four main parts: a molded resin base (4), contact dome (3), plunger (2), and cover (1). It’s due to this simplicity, with very few moving parts, that allows tactile switches to typically last longer than other mechanical switches.
The contact dome possesses an arched shape that fits into the base, which houses the terminals and contacts for PCB mounting. When subjected to force, the dome undergoes deflection or shape reversal, which generates the distinctive audible and haptic click characteristic. The flexing of the dome also bridges two fixed contacts within the base, effectively completing the circuit. Once force is removed, the dome regains its original shape, causing the circuit to break. Domes can be fashioned from various materials, including metals, depending on the desired level of haptic and audible feedback.
Positioned atop the contact dome is the plunger. This serves to push and flex the dome, activating the switch. Plungers can be crafted from materials such as metal, rubber, or others, and may feature a flat or raised actuator. The material utilized for both the plunger and the contact dome significantly influences the tactile click feel and sound.
Resting atop the plunger is the cover, responsible for safeguarding the internal mechanism of the switch. The choice of material for the cover, which could include metals or other materials, is contingent on the switch's intended usage and the level of protection desired. Certain covers may also incorporate a ground terminal to mitigate static discharge risks.
Key Specifications
Selecting the most suitable tactile switch for your product entails more than just scrutinizing specifications on a datasheet. Tactile switches, with their distinct "feel" and "sound," can significantly impact the user's perception of the overall product's quality. Although challenging to quantify, this perception plays a crucial role in user satisfaction.
The force required to activate the switch and the haptic feedback perceived by the user must align with the specific application, and this may vary significantly across consumer and industrial products. Hence, it is prudent to test the actual switch in prototype use, ensuring its characteristics match the intended application. For example, an automotive switch might necessitate a higher actuation force to counteract input errors due to vehicle vibrations, while a switch used in a printer or consumer gaming product could be less sensitive. Nonetheless, regardless of the application, the switch must maintain reliable functionality over the product's lifetime.
Various design specifications are relevant to tactile switches, including:
- Voltage Rating: Indicates the maximum voltage the switch can endure when open or closed. Tactile switches typically have low voltage ratings.
- Current Rating: Specifies the maximum current (in amps) that a switch can safely carry without damage.
- Activation Force: Also known as operating force, this represents the amount of force or pressure (measured in gram force or gf) required to actuate the switch.
- Deflection: Refers to the overall travel distance of a depressed switch, often called actuator travel.
- Contact Force: Indicates the force or pressure (in grams) necessary for the switch to establish a connection between its terminals and allow power flow.
- Actuator Height: The height of the actuator above the switch's body, influencing the user's tactile experience. See Figure 3.
- Lifecycle Range: The anticipated duration of a switch under normal operating conditions, essential for assessing durability.
- Temperature Range: Specifies the temperature range in which the switch operates optimally and complies with specifications.
- Mounting Style: Describes how the switch is mounted on the PCB, which can be either through-hole or surface mount.
- IP Rating: An international standard that classifies the degree of protection of a switch (or other products) against the ingress of dust and liquids, providing valuable information for specific environmental conditions.
- Illumination: Tactile switches come available in a variety of illuminated LED color options, providing users with additional visual identification and feedback in their application.
Tactile Switch Wiring
Tactile switches commonly consist of 4 pins, which are internally connected in 2 sets. The inclusion of 4 pins serves to enhance the stability of the device when mounted on a circuit board. From a wiring perspective, you technically only need to utilize 2 of the leads, but it is considered best practice to make use of all available pins while understanding that the pairs are internally interconnected. Additionally, tactile switches can be found with just 2 pins as well. Moreover, there are 5-pin tactile switches available, specifically designed to enable joystick-like control within a compact package.
Summary
Tactile switches have earned their popularity due to their compact size, low height, and long lifespan, making them well-suited for a wide range of consumer and industrial products. Moreover, with advancements in technology, tactile switches are now finding their way into newer applications, such as wearables, that capitalize on their small footprint, lightweight nature, and durability. Tactile switches ultimately prove to be a smart choice when a product demands low-power, momentary action power control, or data input, and where tactile and auditory feedback is a desirable feature. CUI Devices offers a comprehensive line of tactile switches that come in compact packages, various actuator heights, and multiple configuration options, providing versatile solutions for diverse design requirements.