Only a few technologies are suitable for underwater applications or those involving extremely high humidity.
Roland Barth •SCHURTER AG
Whether you are lighting up your swimming pool, sprinkling it with music, or making a whirlpool bubble, you need a switch to make these functions happen. All these applications are characterized by a proximity to moisture. There are several switch technologies able to manage such uses. Before discussing these candidate devices, it might be helpful to briefly review the standards that generally come into play in applications where moisture is a possibility.
Switches designed for use in the presence of moisture often carry an IP67 rating. This label refers to the IP Code, or Ingress Protection Code. The IP rating classifies and rates the degree of protection provided by mechanical casings and electrical enclosures against not just water, but also intrusion, dust, and accidental contact. It is published by the International Electrotechnical Commission (IEC). There is an equivalent European standard, EN 60529.
The point of IP standards is to provide users more detail about performance than is signified by vague marketing terms such as “waterproof.” There are up to four digits in every IP code. They indicate the degree of conformity with specific conditions. The first digit signifies protection from solid particles; the second denotes liquid ingress protection. There can also be one or two additional digits that signify other protections. But that vast majority of IP ratings have just one or two digits.
The IP rating that is found most often on parts designed for use around moisture is IP67. The six in the first digit indicates the part is dust-tight. The seven in the second digit indicates the part is waterproof when immersed in one-meter-deep water. There are two other more stringent water ratings. An eight in the second digit (IPx8) indicates the part is waterproof when subjected to more severe conditions—such as a deeper immersion, a longer immersion time, or temperature cycling during immersion—than used for IPx7. An IPx9K rating means the part can withstand close-range, high-pressure, high-temperature wash downs.
For both general-purpose applications as well as those near moisture, the most widely used technology is that of mechanical switches with stroke. We encounter them every day as when we switch room lights on or off. They are characterized by an availability in a large range of actuation pressure points, high reliability, and use on an enormously wide range of products.
For mechanical switches used outdoors, an IP67 rating is a must. The reason is simple: Mechanical switches working on the stroke principle have moving parts. Water can penetrate the space between the moving parts. In the presence of freezing temperatures, ice on the actuator can prevent contact closure. Ditto for dirt, dust, vapors or even spilled liquids.
In the case of keyboards and other user-interfaces, it may be possible to employ membrane switches when moisture is a problem. These are special mechanical switches made with silicon-rubber and conductive carbon pills or non-conductive rubber actuators. Through the compression molding process, an angled web is created around the keypad that collapses whenever a user presses a key, making a conductive contact between internal layers of keypad material. The outer layer of the keypad is one continuous piece that can be sealed to keep moisture out of the layers that implement the mechanical switching.
But all in all, mechanical switches lacking an IP67 rating aren’t particularly suitable for wet areas.
Capacitive switches are currently experiencing a mushrooming growth thanks partly to their use in smartphones. There is no stroke, no moving parts. A capacitive touchscreen panel consists of an insulator, such as glass, coated with a transparent conductor, often indium tin oxide (ITO) or silver. Because the human body is also an electrical conductor, a finger touch on the surface of the screen distorts the screen’s electrostatic field, measurable as a change in capacitance. Different technologies may be used to determine the location of the touch.
But capacitive touch switches are not a first choice for every application. Some capacitive touchscreens cannot be used to detect a finger through electrically insulating material, such as gloves. High air humidity or water droplets, for example, can also disturb the touchscreen electrostatic field. Thus capacitive switches are usually unsuitable for use near a swimming pool or whirlpool.
Particularly robust and durable are switches based on the piezoelectric principle. A piezoelectric element (usually ceramics that include lead zirconate titanate or PZT, barium titanate or lead titanate) generates a charge when under stress. The compressive pressure of a finger push causes the (typically disc-shaped) piezo element to bend slightly like a drumhead.
Thus piezo switches produce a single, brief “on” pulse that varies with the amount of pressure applied. The pulse is typically used to turn on a semiconductor such as a field effect transistor (FET). After the voltage pulse dissipates, the FET turns off. A capacitor may be used to store the generated charge to increase the time constant of the gate circuit and lengthen the generated pulse.
In contrast to a mechanical switch, a piezo switch has no moving parts. It can be hermetically sealed with IP protection up to IP69K. This feature predestines it for use under the most adverse conditions.
That brings us to pneumatic switches. For decades, these switches were the first choice of pool and spa builders because they don’t handle electrical current. They generally consist of a spring-loaded plunger that opens or closes an air passage when the operator pushes the button. One drawback of pneumatic pushbuttons is that their internal mechanics must be relatively precise, which is reflected in the price.
As with mechanical switches, pneumatic switches also have moving parts which can eventually wear out. And because they handle compressed air, pneumatic switches require special attention to seals. It should also be mentioned here that optical feedback via point or ring illumination is not used with these types of switches.
Increasingly, pool and spa designers have recognized the advantages of piezoelectric switches. These devices are comparatively inexpensive, extremely robust and durable. They can handle the aggressive chemicals frequently used in wet areas. DW