With the Push of a Button, Tiny Board-Level Tactile Switches Control Major Vehicle Systems

Today’s vehicles rely on microcontrollers to manage everything from engine fuel-air mixtures to collision avoidance and scheduled maintenance. Board-level tactile switches are a mainstay of control interfaces between the vehicle’s operating environment and these processors. They allow low-power signals to flow to microcontrollers, which then activate high-power circuits.

The low-power nature of these switches keeps higher load currents out of the user-interface switching circuit, while mechanical robustness and sealing protect the switches from conditions inside and outside vehicles. Automotive systems that control ignition, door latches and locks, lamps and lights, fuel pumps, blowers, and other functions use board-level tactile switches.

This article will describe the function of board-level tactile switches, implementation considerations, and additional options available to design engineers when selecting these switches.

Board-level switch basics

Mounted directly to a printed circuit board (PCB), board-level tactile switches provide momentary circuit control when mechanically pressed. For instance, a human user might activate cruise control from an instrument panel, or a position input may signal a controller that a door has reached a closed position, activating a latch solenoid.

A board-level tactile switch is designed with a single-pole, single-throw, normally open (SPST-NO) architecture, meaning that it has only two states: open and closed. Because the switch is designed to be normally open, current only flows through the switch when force is applied to the actuator. Once pressure is removed, the switch springs back to an open position where no current flows.

The momentary flow of a very low level of current signals a microprocessor to start, end, or change a vehicle process or state. For instance, board-level tactile switches from CIT Relay and Switch operate up to 50 mA, which is very low compared to the amperage used by the processes they ultimately control. These switches have operating voltages of 12 V_DC, 24 V_DC, 48 V_DC, and 50 V_DC, so they can handle applications that run from 12 V battery power or from 24 V or 48 V power buses.

Electrical integration

Board-level tactile switches must be integrated into PCB circuits so that depressing the switch signals an ECU (electronic control unit) or microprocessor. The connection must be strong enough to withstand between 100,000 and 10 million cycles while being space-efficient and easy to manufacture.

Many board-level tactile switches use surface-mount technology (SMT), in which the switch terminations align with solder pads on a PC board (PCB), enabling automated manufacturing and testing. For example, CIT’s CS1104 series SMT switches (Figure 1) use gull-wing terminations.

Figure 1: CIT’s CS1104 series SMT tactile switches have 3.80 mm or 7.00 mm actuator lengths, which require 0.25 mm of travel and a 160 or 260 gf operating force. (Image source: CIT Relay and Switch)

Another SMT termination choice for switches that are placed where PCB space is at a premium is J-lead terminations that tuck under the device.

Other switches use through-hole technology, where the switch terminals are inserted into the PCB and secured with solder. These connections may be more robust but also require greater operator involvement in the manufacturing and inspection processes. CIT’s STJ and STJV spherical switches (Figure 2), for example, use through-hole termination.

Figure 2: STJ spherical-cap tactile switches (a) and their STJV vertical counterparts (b) are designed for through-hole termination. (Image source: CIT Relay and Switch)

Through-hole termination is important to the STJ and STJV design because the switches’ spherical caps allow actuation from multiple directions with 1.6 mm of travel and 130 gf, 200 gf, or 300 gf of actuation force.

Shapes and sizes

In addition to ensuring a reliable PCB connection, engineers designing vehicle control interfaces consider physical and mechanical factors when choosing a board-level tactile switch for a particular application, including the actuator’s operating force, its height, the footprint it takes up, and its form factor.

For applications where space is at a premium, engineers choose compact board-level tactile switches like those in CIT’s CS1204, CS1205, and CS1210 series (Figure 3).

Figure 3: Compact board-level tactile switches in the CS1204 series (a), CS1205 series (b), and CS1210 series (c) are 2.50 mm tall or less and take up less than 34 mm². (Image source: CIT Relay and Switch)

CS1204 flush or standard switches have an actuator height of 0.80 or 1.50 mm off the PCB, and are 5.20 mm² or 6.40 mm by 5.20 mm. CS1210 switches have a small 3.40 mm² footprint and are 1.50 mm tall. Both require 160 gf or 260 gf to operate. Slightly larger CS1205 rectangular switches are 2.50 mm tall by 6.10 mm by 3.70 mm with operating forces of 180 gf or 260 gf.

Automotive layouts may necessitate placing PCBs at different distances from the control interface, such as an instrument panel. Switches in the CS1102 and CS1102V series (Figure 4) have a wide range of actuator lengths available to meet the needs of a variety of use cases.

Figure 4: CIT’s CS1102 (a) and its side-action sibling CS1102V (b) have actuators up to 11.85 mm in length. They attach to PCBs via SMT or through-hole technology. (Image source: CIT Relay and Switch)

The cap of these switches is a plunger that transfers the 100 gf to 260 gf needed to operate them. CS1102 switches are 4.30 mm to 11.85 mm tall, and 7.10 mm by 6.30 mm or 6.00 mm². The right-angle mount CS1102V switches have actuators 3.15 mm to 11.85 mm long, and footprints that are 7.10 mm by 6.30 mm or 7.50 mm by 7.10 mm.

Strength and sealing

For applications that require more rugged construction, CIT’s ME series tactile switches (Figure 5) are made of PA6T polyamide with a silver-plated stainless-steel dome contact. ME switches provide a 350 gf operating-force option for applications requiring greater actuation force. Slightly larger than other CIT board-level switches, they have a 10 mm² outline and are 6.00 mm to 15.00 mm tall.

Figure 5: CIT’s ME series of board-level tactile switches are sealed to IP67 with silicone rubber and mount to PCBs with SMT or through-hole technology. (Image source: CIT Relay and Switch)

In addition to their rugged construction, ME series switches have an internal silicone-rubber seal. Their sealed, one-piece design lets them meet the requirements of IP67, with complete exclusion of dust and exclusion of freshwater at a 1 m depth for up to 30 minutes.

CIT’s STR, STS, STJ, and STJV series are also sealed to IP67 with silicone rubber. STR and STS switches (Figure 6) both have actuator lengths of 4.3 mm, 5.0 mm, or 7.3 mm and are designed for SMT termination.

Figure 6: STR series domed tactile switches (a) activate with 0.28 mm of travel while STS series switches (b) have 0.45 mm actuation travel. Both are IP67 rated. (Image source: CIT Relay and Switch)

Sealed STJ and STJV switches are designed with a stainless-steel cap and housing and through-hole termination for greater robustness. They have operating forces ranging from 130 gf to 500 gf.

Engineers choose switches with an IP67 rating for both external and internal use, such as those in vehicle doors exposed to the elements, and for those in the passenger compartment that may need to resist spilled coffee, crumbs, and other debris.

Conclusion

Board-level tactile switches such as those offered by CIT Relay and Switch use low-level currents to signal automotive microprocessors, whether they are manually operated in a vehicle’s passenger compartment or mechanically by an input such as steering wheel rotation. In the tight confines of an instrument cluster or sensor array, the switch height, shape, and PCB footprint are as important as its operating force and IP rating. Engineers designing the many control interfaces in modern vehicles can opt for the best combination of form and function when specifying these tiny switches.