How to Design Capacitive Touch & Proximity Sensing Technology into Your Application

The phrase "Human Interface" has suddenly become part of the lexicon of the technology industry. At its most basic level, this phrase is used to describe not only the aesthetics of a device, but, more importantly, the process by which a device responds to human interaction. While not a new concept, the availability of enabling technology that can revolutionize the way we interact with consumer electronics products and an urgency to incorporate this technology demonstrates the increased importance consumers are placing on interface design.

What defines good human interface design, and how can system designers implement a smarter, friendlier and more intuitive solution? To begin answering these questions, it is helpful to view a human interface simply as a set of functional interactions with end users and their surroundings. These interactions can be subdivided into two logical groupings: inputs and outputs. Input events are those in which a user causes, either directly, indirectly or even inadvertently, a specific action to be performed. Examples of input events are:

• Touch detection – single finger touch, multi-finger touch, finger slides, taps, etc.

• External stimulus detection – proximity, motion, hand waving, voice, etc.

• Environmental detection – ambient light, temperature, etc.

• Physical detection – rotation, inclination, shock, vibration, etc.

With advances in fields such as touch sensors, proximity sensors, ambient light sensors and accelerometers, the ability and sophistication of devices to accept inputs has dramatically changed the entire human interface landscape. However, it is equally important to tie input events to a tangible output event because the output event informs the user of an action that has taken place as a result of the input provided.

Examples of output events include:

• Switching items on or off – screens, speakers, lights, safety features, etc.

• Adjusting controls – volume, backlight, brightness, stabilization, etc.

• Providing tactile feedback – auditory ("hear"), visual ("see"), haptic ("feel"), etc.

Human Interface Process Flow

The types of input events and output responses desired will vary greatly as they depend on the type of device being built. However, upon the detection of a touch that exceeds a certain pre-defined pressure threshold, the handset will turn on the screen, provide an auditory confirmation over a speaker and turn off the screen locking feature. In this example, a single input event has been directly tied to three different output events affecting two different subsystems. The good news for designers is that technological innovation has dramatically improved the ability of a device to offer a wide variety of creative input and output choices that can add a significant amount of appeal to an end product.

Challenges

The sheer numbers of input and output event possibilities are challenging even for the most experienced designer, and it is becoming exceedingly difficult to predict what users may find appealing not just today, but in the future as well. From an implementation perspective, what is really needed is the ability to create an interconnected framework that enables a tight coupling of these input/output interactions while still leaving flexibility to adjust for ever-changing market requirements.

To better understand the role of the microcontroller as the orchestrator of a human interface system let's review each of the technologies that comprise the block diagram shown in the picture above.

Ambient Light Sensor (ALS) At the core of an ambient light sensor is the photodiode, which is a semiconductor element that produces a current proportional to the incident light when a bias voltage is applied. The transfer function might be linear or logarithmic depending on the sensor. The produced current might be then digitized by an ADC and fed its output to the MCU or measured by the sensor and converted into a data format that a processing unit can easily interpret, such a PWM, I2C or a simple change in the state of an I/O line indicating the presence or absence of light.

Therefore, a device with both a photodiode having only visible light sensitivity (380 to 750 nm) and another photodiode with infrared spectral sensitivity (750 to 2500 nm) can be used to take a measurement from each and find out the type of the source from the ratio of these readings.