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Smartphones and the Use of Capacitive Switches

Connector Geek
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When was the last time you looked closely at your smartphone? Many of us take our phone for granted, but the advances in technology that have come together to place such a powerful device in our hands are quite staggering. Smartphones are a blend of the latest developments in a wide variety of fields, from semiconductors and memory to cameras and batteries. However, one of the most impressive features of the smartphone is the interface. How we interact with smartphones is nothing short of revolutionary.

Startrek Capacitive Screen

Star Trek - Ahead of its time (as usual...)

When Star Trek: The Next Generation hit our television screens in 1987, it was a complete departure from the original series. Gone was the bridge of the original Enterprise, with its array of switches, lights and mechanical controls. Instead, the entire set was covered with touch-sensitive flat screens showing all of the systems that might be needed to operate a starship. Projected onto these screens was a huge amount of information, all in full colour.

Capacitive Touch Screens

We can compare the latest smartphone with older cell phones in the same way. Phones no longer use physical switches, instead replacing them with a completely dynamic display that changes both its function and its imagery according to context. In fact, this is a combination of two different technologies. The high-resolution display with its vivid colours and exception clarity sits below a capacitive, touch-sensitive layer. The phone’s software integrates both, providing users with a completely configurable interface system that can be customised in almost any way imaginable. The screen can respond to a simple touch, a swiping action or even the user’s fingerprint.

This interface is so intuitive that it seems natural to seek out other applications for this technology. However, the rise of the touch screen does not mean the end of mechanical switches. However useful the touch screen has become, it does not provide a universal solution. Anyone who has ever dropped their smartphone will know from bitter experience how easy it is to damage the screen, and therefore designers need to think carefully before using this technology in demanding environments.

Touch screens are also an expensive solution for any application that does not need a configurable control interface. For equipment that requires a limited number of discreet inputs, it is more cost-effective to employ conventional mechanical switches. It is for this reason that remote control handsets for devices such as televisions or air conditioning units still use tactile switches or membrane keypads. In many cases, if a device needs complex controls, it is often simpler to provide users with a software app that can be installed on the smartphone that they already have.

Mechanical Switches Are Not Dead...

There are other situations where a switch needs to be easily identified and operated. A prime example of this is the emergency stop switch for industrial machinery. The switch itself needs to be prominent and easy to see. It must work instantly, even in situations where a power failure would prevent a touch screen from operating. The switch must also work if the operator is wearing gloves or cannot see the switch clearly. In these circumstances, only a physical switch will be suitable.

Despite this, capacitive touch sensing switches have plenty to offer the designer. They work by sensing the change in capacitance created by a suitable object – normally the user’s finger – coming close to the surface of the switch. Unlike a mechanical equivalent, the operation of a capacitive switch does not employ any moving parts. This has important implications for the reliability of the device, as mechanical wear is one of the limits of a component’s useful life. In addition, it provides an ideal solution for vandal-resistant switches. The lack of moving parts prevents any damage caused by all but the most determined efforts, and the standard panel dimensions make them a drop-in replacement for push-button switches. Touch-sensitive vandal-resistant switches are available from manufacturers like Bulgin (204-7873) and RS Pro (175-8636) .

Bulgin Capacitive Touch Switches

Attractive Backlit Panels

Another feature of a capacitive sensing switch is its ability to work through a solid surface. To function correctly, the surface needs to be a non-conductive material such as glass or plastic. Mounting a keypad behind a clear glass panel creates a flat, wipeable surface that can be easily sterilised. This is ideal for equipment in high-traffic areas such as vending machines, or applications where the risk of infection is high including medical equipment and security entry systems. Manufacturer VCC has created a range of capacitive sensing modules (137-1131) with build-in lighting and standard markings. They are designed to be fixed to the rear of a transparent surface in an array, suitable for creating a control panel.

Capacitive touch switches mounted to a custom-printed panel.

This concept can be taken further with backlit capacitive switches like those made by TT Electronics (904-2696) . These are specifically designed to be mounted to the rear of a glass or Perspex screen that has been printed with a suitable design. The switch can be programmed with different characteristics depending on the context. In one application, the switch could function momentarily, remaining active only when pressed. In others, the switch could be latching, staying active until pressed again. The LED can even be programmed to emit different colours depending on the state of the switch.

In these examples, the design of the switch is rigid and flat, a limitation imposed by the construction of the electronics. Capacitive sensors are often made using indium tin oxide (ITO), a conductive alloy that can be deposited as a transparent film. However, ITO is inflexible and brittle, ideal for mounting onto a traditional printed circuit board (PCB), but not suitable for complex shapes due to its inflexibility.

Flexible Capacitive Sensors

Manufacturers like Molex have been experimenting with alternative designs to create a flexible touch sensor. They are working with organic polymers such as poly(3,4-ethylenedioxythiophene), more often known as PEDOT, that can be deposited onto inexpensive polymer substrates. This provides a more flexible solution that can be made to fit behind curved surfaces.

A flexible PEDOT-based capacitive switch from Molex

This new technology creates almost limitless opportunities for controls. Using PEDOT-based capacitive sensors, almost any non-metallic surface can be turned into a control device. This could be as simple as the volume control for an in-car entertainment system, or the safety switch for an industrial controller. In this application, a built-in capacitive switch could be programmed to sense when an operator releases the handle, cutting power to dangerous equipment to render it safe.

The technology that makes smartphones easy to use has many potential applications in the wider world. While the touch screen itself might be too complex for many devices, capacitive sensing is finding uses in a range of industries, from consumer goods to medical equipment.

The next time you pick up your phone, imagine how this technology might change your life too.

Connector Geek is Dave in real life. After three decades in the industry, Dave still likes talking about connectors almost as much as being a Dad to his two kids. He still loves Lego too. And guitars.

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