AN580 Datasheet, PDF (1/10 Pages) Silicon Laboratories – INFRARED GESTURE SENSING

AN580
INFRARED GESTURE SENSING
1. Introduction
Touchless user interfaces are an emerging trend in embedded electronics as product designers seek out
innovative control methods and more intuitive ways for users to interact with electronics. Active infrared proximity
motion sensing can solve this challenge. Silicon Labs Si114x proximity and ambient light sensor products are
ideally suited to touchless gesturing applications such as page turning on an e-reader, scrolling on a tablet PC, or
GUI navigation. The Si114x features up to three LED drivers and has the ability to sense gestures within a 7 to
15 cm product interaction region, assuming a hand as the detectable object. This document will discuss in detail
how Silicon Labs implements motion sensing using infrared technology. There are two primary methods used for
gesture sensing – position-based and phase-based. Position-based gesture sensing involves finding gestures
based on the calculated location of an object while phase-based gesture sensing is based on the timing of the
changes in signal to determine the direction of an object’s motion.
2. Hardware Considerations
This application note focuses on detecting gestures made by a user’s hand. It is important to recognize that the
concepts introduced in this application note can be applied to targets other than the hand, as long as the hardware
is designed appropriately. The end application and individual system constraints will each dictate the range
requirements for IR gesture sensing. Since object reflectance is the main measurable component for touchless
gesturing, a hand is presumed to be the detectable object for the examples in this document. Whereas a hand can
achieve gesture sensing up to 15 cm away from the Si114x sensor, a finger, with dramatically lower reflectance,
can achieve gesture sensing at a range of < 1 cm for thumb-scroll type applications.
The general guideline for designing a gesture sensing system with multiple LEDs is to make sure that there is no
“dead spot” in the middle of the detectable area. When a target is placed above the system and is not detected, the
target is in a reflectivity dead spot. To avoid dead spots in the system, the LEDs must be placed such that the
emitted infrared light can reflect off the target and onto the sensor from the desired detection range. Figure 1
shows systems designed to detect a hand or a finger. The most susceptible area for a dead spot is directly above
the sensor in between the two LEDs. The two LEDs are placed as close to the edge of the target as possible to
provide feedback in the middle while also keeping enough distance between the LEDs so that it can be detected
when the finger or hand moves to the left or right.
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Copyright © 2015 by Silicon Laboratories
AN580