The touch switch is a recognized electrical switch, which is the same as the mechanical switch of the same kind. The circuit is connected or disconnected by manual operation. This switch was originally used as a membrane or screen printed switch for keyboards and keyboards in the early 1980s. Due to its unsatisfactory performance and lack of tactile feedback, it encountered resistance in its promotion. However, in the late 1980s, the version that included metal vault in the design gained wider popularity, thanks to its improved tactile feedback effect, powerful actuating force and longer service life. It is these changes that have led to the extremely common touch switch that is widely used in today‘s consumer and commercial applications.
As mentioned above, the key feature of the touch switch is that when the actuator is under pressure, it will provide a perceptible "click" or tactile impact feedback to indicate that the switch has successfully completed its operation. As a transient device, as long as the user no longer applies pressure, the switch will release, thereby cutting off the current. Although it is usually used as a normally open device, the touch switch also has a normally closed version. When the actuator is pressed, the current is cut off, and the current will be connected immediately after release.
Figure 1: Common touch switches (picture source: CUI Devices)
Touch vs. button
Touch switch and button switch are two kinds of switches that are often confused or whose names can be interchanged. Although similar in nature and function, the button switch operates by pressing the actuator within a certain stroke to start the current and cutting off the current by pressing the actuator again. In contrast, the touch switch can turn on and off the current by simply pressing and holding the actuator within the minimum actuator travel.
Although some buttons can be used as instantaneous switches, all touch switches are instantaneous. Touch switch usually has smaller package, lower rated voltage and current than button switch. Compared with the button switch, the sound or tactile feedback of the touch switch is also a key difference. Although the button is provided with a panel or PCB installation style, the touch switch is only used for direct installation on the PCB. For more information on button switches, please read CUI Devices‘ button switches 101 blog post.
Basic structure and operation
One of the key factors that the touch switch is superior to the mechanical switch in terms of simplicity and robustness is its limited number of internal devices, which enable it to achieve the expected functions. A typical touch switch design typically includes the following four components:
1. Top cover: the internal mechanism of the component protection switch, which can be composed of metal or other materials, depending on the expected function. The top cover can also be used as a grounding terminal to protect the switch from electrostatic discharge.
2. Plunger: It is located under the top cover and above the arch contact. The plunger is a user operated component to bend the arch contact and activate the switch. The plunger adopts flat structure or convex structure.
3. Arch contact: the part is arc-shaped and matches with the base. When it contacts with the plunger, it will deflect or reverse. This bending process will produce a clicking sound and a clicking touch. At the same time, it is connected with two fixed contacts on the base to connect the circuit. Once the pressing force disappears, the arched contact will return to its original shape and disconnect the circuit. The materials of the arch contact and plunger (metal, rubber, etc.) will help determine the touch and sound of the switch.
4. Molded resin base: this is the last part of the touch switch, which contains the terminals and contacts connecting the switch to the PCB.
Figure 2: Structure of typical touch switch (picture source: CUI Devices)
Main advantages and specifications
The correct selection of the touch switch should not be limited to evaluating the specifications on the product data sheet. In addition to some common features and advantages, such as instant action, lower rated power and current, robustness due to less movement, and lower cost, the perceptible touch and sound of touch switches have a great impact on their selection. This may be a property that is difficult to quantify because the pressure required to activate the switch and its tactile feedback will vary depending on the intended use and function. For example, a touch switch used in automotive applications may require a greater actuating force than a switch used in a home or office environment to avoid accidental input due to vibration. In general, the best way to determine the correct characteristics of a switch is through prototype development and testing.
In conclusion, there are several key specifications that should be kept in mind during the selection process:
• Rated voltage: the maximum voltage that the switch can withstand when making or breaking.
• Rated current: the maximum current that the switch can carry without damage.
• Excitation (or operating force): the force or pressure (expressed in grams or gf) required to move the actuator on the switch.
• Deflection (or actuator travel): total travel required to depress the switch.
• Contact force: the force or pressure (in grams) required for the connection terminal of the switch.
• Actuator height: the height of the actuator above the switch body. (See Figure 3)
• Life cycle: the expected service life of the switch under normal operating conditions.
• Temperature range: refers to the operating temperature range, within which the performance of the switch meets the specification requirements.
• Installation method: the installation method of the switch on the PCB is divided into through hole installation or surface mounting.
• IP rating: an international standard for classifying the dust-proof and liquid intrusion prevention performance of switches (or other products). See IP67 touch switch of CUI Devices.
Figure 3: Common actuator heights for touch switches (image source: CUI Devices)
Wiring of touch switch
Most touch switches contain four pins for stabilizing during installation on the PCB. The four pins are internally connected into two groups. Although technically only two wires are required for routing, it is best practice to use all possible pins. In addition, some touch switches have only two pins available, and also have a 5-pin version, which can implement control functions similar to joystick in a very small package.
Figure 4: Common 4-pin touch switch configuration (picture source: CUI Devices)
Summary
With its small size, low height and long service life, touch switches can meet many needs of various consumer and industrial products, including the latest applications in wearable technology. When low power consumption and instantaneous performance are required, touch switch will still be an essential switch solution that can provide tactile and auditory feedback. To help users select touch switches, CUI Devices offers a series of compact touch switches with a wide selection of actuator heights and configurations.
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