AN442 Datasheet, PDF (1/22 Pages) STMicroelectronics – The aim of this note is to show how to connect

AN442
Si1102 AND Si1120 DESIGNER’S GUIDE
1. Introduction
The Si1102 and Si1120 are low-cost, high-performance, active-optical reflectance-based proximity sensors. Both
drive an LED to illuminate a target, then measure the reflectance from the target to determine its proximity.
Both the Si1102 and Si1120 use short-duration strobe pulses to measure reflectance. This keeps the average
power consumption in the microwatt range. Both devices cancel background dc ambient before making a
reflectance measurement. The reflectance measurement is the difference between the dc ambient and dc ambient
plus reflection from the target illuminated by the LED.
The Si1102 is a stand-alone, dual-port proximity sensor driving a single LED. It uses an internal analog wakeup
controller that is controlled through an external resistor to set the time interval between measurements. The PRX
output is maintained between measurements allowing the Si1102 to behave as a proximity on-off function. The
Si1102 is well suited for applications, such as electronic toys, powering transmitters for RF alarm sensors, and
saving energy in homes or offices.
Although the best performance is achieved with 850 nm LEDs, the Si1102 can also be used in short-range
applications where red visible LEDs are useful, such as hand washers or paper towel dispensers.
The Si1102 sensitivity adjustment sets a fixed proximity threshold, a reflectance level at which it will detect
proximity. This absolute level threshold may drift around 20% or more depending on temperature, LED supply
voltage, LED aging, and other environmental factors. Consequently, although it is possible to set a threshold that is
less than 10% different than the absolute reflection, it may not be consistent. It is, therefore, good system practice
to allow for some system programmability, such as using potentiometers.
The Si1120 is designed to operate with a microcontroller. Rather than an on/off output as in the Si1102, the Si1120
encodes the reflectance measurement as a pulse-width-modulated output where the pulse width is directly
proportional to the measured reflectance.
Silicon Laboratories offers a wide range of microcontrollers that are well-suited for use with the Si1120. Most
Silicon Laboratories microcontrollers offer the PCA (Programmable Counter Array) that can easily be used to
measure the pulse width output. With the addition of a microcontroller, higher level functions can be added.
The microcontroller can be used to control multiple LEDs, enabling position determination through triangulation. In
addition, unique human interface concepts, such as gestures, can be implemented to enhance the user interface
for your product. Such an interface can provide product differentiation, resulting in additional product revenue. The
microcontroller can also be used to control multiple sensors and a single LED for applications that are particularly
power-sensitive.
The Si1120 can be programmed to drive a 400 mA or 50 mA pulse. When used with a microcontroller, it is possible
to dynamically change the LED current drive to either optimize for range or lower overall system power
consumption. With the microcontroller, the reflectance measurement frequency can be customized based on the
current usage state.
Another feature made possible by a microcontroller is the ability to improve the SNR of the reflectance
measurement through pulse averaging. When used with IR filters and lenses, it is possible to use the Si1120 and
microcontroller to detect human-sized objects one meter away.
Rev. 0.1 1/15
Copyright © 2015 by Silicon Laboratories
AN442