Digital photo frames (DPFs) are starting to appear in many places where traditional photo albums and frames have been used for decades. The growth and proliferation of digital cameras has rendered hard-copy photos from analog cameras all but obsolete. On top of displaying the photos, DPFs can transfer them to printers and other devices, play videos, and offer remote control over all these functions without a computer.
The basic DPF uses a multimedia chipset to handle image storage and conversion. The frames often provide video and audio output with flash memory storage for the images and files. They also have an LCD, usually integrated in a modular form, to display all images (see the figure). As DPFs get more complex, their user interfaces are growing more complicated as well.
Users need a simple way to control the frame and access all of its features. Mechanical buttons present problems, though. When they’re on the back of the frame, they can be hard to access, especially if the frame is hanging on the wall. Buttons on the front likely will harm the aesthetic appeal of the frame, which is often made to be as attractive as possible. Now, designers can eschew mechanical buttons in favor of capacitive buttons and proximity sensing.
Capacitive buttons work very well for digital photo frames for a variety of reasons. First, they generally bring a significant cool-factor to any product. Second, they can be placed on the front of the frame for easy access without ruining the aesthetics. And, capacitive buttons can decrease some system costs by eliminating the mechanical buttons and simplifying some manufacturing processes.
Using capacitive buttons for the user interface of a digital photo frame does not solve all problems. One issue that still remains is that capacitive buttons that are “invisible” can be somewhat unintuitive. This is where proximity sensing is desirable. Proximity sensing can be used to bring up a menu on the screen and light LEDs to show where the capsense buttons are located.
Adding capacitive and proximity sensing to a digital photo frame will increase its consumer appeal and avoid the wear and tear of the mechanical buttons. Fortunately, there is a simple method for adding capacitive and proximity sensing to a digital photo frame, as adding one device to the existing circuit will make it happen.
CAPACITIVE SENSING
Adding capacitive sensing to a digital photo frame or other display involves embedding a printed-circuit board (PCB) behind or within the frame. Capacitive elements are laid out on the PCB in the form of copper pads. There is a very small capacitance between these pads and a grounded mesh on the PCB that surrounds them.
When a user “pushes” these capacitive pads, or buttons, there is an increase in the capacitance between the copper pad and ground. This is due to the electrically conductive nature of a human finger. Therefore, detecting these capacitive button presses becomes an exercise in detecting small changes in small capacitances.
There is a variety of ways to accomplish this. Noise immunity, resolution, accuracy, and measurement time are important factors in choosing a capacitive sensing method. It is also very beneficial to choose a method and device that permit a great degree of flexibility. For example, being able to dynamically trade off resolution for measurement speed (and vice versa) is quite useful. It is often best to choose an IC that has all of the necessary hardware on-chip to do capacitive sensing. This is a much simpler approach than designing a unique circuit from the ground up.
A digital photo frame typically requires at least five buttons to implement the user interface. These buttons may include an on/ off switch and several navigation buttons. However, another advantage of using capacitive buttons is that it is not difficult to add more once the first few have been added.
If five capacitive buttons have been added to the frame, there is no reason not to have 10 buttons on the frame. There is very little added cost to adding more capacitive buttons. This is not the case with mechanical buttons. Therefore, a typical digital photo frame with capacitive buttons may have five to seven buttons and a “slider” control created out of five to seven more capacitive elements. This makes a total of about 12 to 15 capacitive elements.
Additionally, it is important to ensure that the device that measures these elements can scan each element and process the data within about 15 ms. This corresponds to an update rate of about 60 Hz and allows for an acceptable latency between button pushes and the execution of their corresponding functionality.
Continue on Page 2
Reprints
