The power of proximity detection

What is proximity detection?

Proximity detection enables a system to make a decision based on the relative distance of an object to the proximity sensor. The purpose of proximity detection in most applications is to enable or disable system features when a system user is determined to be within a specified proximity range.

This specified range that determines a proximity event is a convenient way to categorize proximity detection applications. Short range applications are interested in seeing whether a user is within the range of a few centimeters, while long range applications base decisions on whether a user is within the range of about a meter. Short range applications are typically portable devices that rely on the user putting the device to their head, such as one would with a mobile phone when making a phone call. Long range applications, however are typically non-portable devices which rely on a user approaching them to activate.

Why proximity detection?

The primary benefit driving proximity detection is increased power efficiency. Correct use of a proximity detection system can increase battery run time in a portable device or minimize the power usage of a plug-in device.

Many government entities are enforcing higher power efficiency standards on consumer electronics. Additionally, many consumers have an increased expectation of power saving features within their electronic devices and often base purchasing decisions on the scope of these features. For these reasons, proximity detection can be a strategic, value adding feature to implement into many possible applications.

How does it work?

There are several methods for detecting the proximity of an object, but they are all based upon the same common idea. The idea, in this case, is that an emitter can produce energy which will be reflected off of an object, and that a sensor can then gather the reflected energy and pass it to a system that could then make a proximity decision. This high level system description describes how sonar, radar and even human vision can detect the proximity of an object. The variables that differentiate these technologies are the type of energy being emitted and reflected, and the complexity of the analyzing and deciding system.

This system also describes proximity detection as it relates to an optical system. In this case, the energy emitted is light of a particular wavelength from an emitter. This light is reflected off of an object and is read by an optical sensor, where it is then converted to a signal, which can then be interpreted to signify whether or not a proximity event has occurred.

TAOS themselves have developed integrated ALS/proximity detection devices, which check for object proximity between ALS integration cycles. These sensors have an integrated approach that allows a system to make decisions based on the ambient lux measurements as well as whether or not a proximity event has occurred.

To accomplish this, the device has two modes of operation: ALS mode and proximity mode. During ALS mode, the device gathers data across two channels (a broadband channel and an IR channel). The ratio of these channels indicates the photopic ambient light (lux). Please see the TAOS application note "DN26: Ambient Light Sensing" for more information regarding TAOS' ambient light sensing solution.

In proximity mode, the device outputs a pulsed signal which drives an IR emitter. If an object is near the emitter, the IR energy will be reflected toward the sensor. The returning signal contains both the reflected IR energy as well as ambient light, which the sensor converts to a signal. The device subtracts the IR energy from the ambient light and charges a capacitor towards a threshold. The device determines the strength of the proximity event by counting the number of emitter pulses that are required to reach that threshold. This proximity strength information is then made available for the user to access.