Designing a Sense of security

Capacitive sensing is making its way into more and more applications everyday. From mobile handsets to computers, point-of-service terminals to home electronics, medical devices to security applications, capacitive sensing is showing up in applications everywhere. Robert Jania, Global Marketing Manager, Cypress Semiconductor Corp. explains what capacitive sensing can offer the security market.

What is capacitive sensing?

Capacitive sensing is a form of proximity sensing. It is an alternative to traditional mechanical buttons and sliders, working without mechanical components that can suffer from physical wear and tear. In addition, capacitive sensing can be used to create touch screens, touchpads and proximity-sensing systems.

Security systems are an emerging market for capacitive sensing with several application areas. For example, a security keypad can comprise capacitive buttons or even a touch screen, replacing its mechanical counterpart, alternatively proximity sensing can be implemented to monitor the status of a door, window or person.

 

 

Capacitive sensing

So how does capacitive sensing work? To illustrate how a capacitive sensor works, Figure 1 shows a cross section of a single capacitive-sensing button. Under an overlay material there are conductive copper areas and conductive sensors. Whenever two conductive elements are within close proximity to each other, a capacitance is created, (C_P in Figure 1) for the coupling between the sensor pad and ground plane. C_P is the parasitic capacitance and is typically in the order of 10pF to 300pF.

The sensor and ground planes also create a fringe electric field that passes through the overlay. Because the tissue of the human body is basically a conductor, placing a finger near fringing electric fields adds conductive surface area to the capacitive system.

 

 

This additional finger capacitance (C_F in Figure 2) is, however, in the order of 0.1pF to 10pF. Although the presence of a finger induces change, the scale of the change in comparison to the parasitic capacitance is quite small.

With no finger present, the sensor’s measured capacitance (C_X) is basically equal to C_P. When a finger is present, then, C_X is a combination of C_P and C_F.

 

Security interface

A security system control unit typically consists of buttons and a display screen. These mechanical buttons often have a cover that closes to hide and protect the buttons.

The plastic cover and mechanical buttons can be replaced with capacitivesensing buttons. In this design the button is just a copper pad printed on a circuit board with a plastic overlay. This can be taken one step further by replacing the display with a touch screen. Now, not only can mechanical buttons be eliminated to save space, but the buttons are no longer fixedfunction, opening up options in interface design and functionality.

 

Proximity

A capacitive-sensing button is simply a fine-tuned proximity sensor. A proximity sensor can not only be used to detect the location of an object, such as a door or window, but could also detect presence of a person. If someone walks through a doorway, a proximity sensor could be used to detect this.

In this application, it is necessary to increase the sensitivity of the sensor compared to that required for buttons. Increased sensitivity is realised by acquiring data from the sensor for a greater time. Obviously, when the acquisition time is increased for such applications, the update rate is slower. However, proximity detection applications require that sensors only detect presence, not fine, rapid movements.

There are also some changes that can occur in hardware to increase the sensitivity of a proximity-sensing apparatus. Larger sensors have greater sensitivity to larger conductive objects, such as a hand. Increasing the space between the sensor and the surrounding ground plane, or removing the ground plane, also increases the sensitivity, but allows the field lines to direct themselves toward the user, rather than toward a ground plane.

 

CapSense™

Because sensitivity is key to implementing capacitive sensing in security applications, it is useful to consider with a programmable solution. One such programmable solution has been developed by Cypress Semiconductor with its PSoC-based CapSense™ technology, which is optimised for dealing with the design challenges of capacitive sensing.

This PSoC solution allows designers to implement buttons, sliders, touch screens, touchpads and proximity sensors at the same time, using a single chip. PSoC devices also offer various analogue and digital resources, giving extra options for security designs. PSoC development utilises pre-defined user modules, reference code and calibration tools to make designing these capacitive-sensing security applications fast, easy and effective.

 

 

The PSoC® FirstTouch™ Starter Kit CY3270 is available to members of the Future Board Club. To apply for the development board, and membership of the Board Club, go to www.my-boardclub.com/manf-offers.htm This offer is free and subject to qualification.

 

Cypress Semiconductor/CapSense