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Use the Si1102-EK kit for simple, low-power proximity sensing

For simple, short-range, ultra-low-power proximity sensing, Silicon Labs’ Si1102 active-optical reflectance-based sensor and accompanying Si1102-EK evaluation kit provide a practical, easy-to-use solution. The kit comes with a board, battery, LEDs, power switch, and potentiometers, but a little background on the nuances of infrared-based proximity sensing and design implementation will help optimize a design.

Active-optical, reflectance-based proximity sensors drive a visible-light or infrared LED that illuminates a target and then measures the reflectance to determine proximity (Figure 1).

Figure 1: The Si1102 uses a dual-port approach to active-optical reflectance-based proximity sensing to avoid overload and transmit and receive signal interference. (Image source: Silicon Labs)

It’s a deceptively simple concept that is popular in applications that include electronic toys, powering RF alarm systems, and energy-saving systems. These tend to use infrared light with optimal performance at a wavelength of 850-nm range, but visible-light red LEDs are often desired for applications such as hand washers, soap dispensers, and paper-towel dispensers.

Reflectance proximity sensing contrasts sharply with passive-infrared receivers (PIR) that detect motion by sensing changes in illuminated or naturally emitted infrared. Also, unlike other time-of-flight (ToF) proximity-sensing technologies, such as LiDAR, radar and even sonar, that measure the time differential between transmitted and received pulses, reflectance proximity sensors use the signal level and optical techniques to determine the range.

While it sounds simpler, it’s actually quite difficult to accomplish reliably and repetitively. The dc ambient light must be accounted for, along with LED degradation over time, LED supply-voltage variations, temperature variations, and other environmental factors. Even if the right resistors and capacitors are chosen for correct biasing and threshold control up front, time and environmental effects in the field can combine to affect the output power of the LED, as well as the sensitivity of the sensor.

Also, while it may be tempting for a designer to shrink the footprint of the sensor by choosing a single-port design – where the same window is used for both transmit and receive – this requires that significant time and attention then be given to avoiding overload and transmit/receive interference conditions (see Figure 1 again).

Overloading happens because the sensor has no way of differentiating between reflections (bounce-back) from the window itself, or the object being detected. The bounce-back, or internally reflected signal, can be up to 100 times that of the light signal reflected from the object, even if it is only a few centimeters away.

Si1102-EK: Simple and flexible

For an experienced designer, it’s not a big deal to factor in all the requirements and create the optimal implementation of the Si1102, but if it’s already been created, there’s no need to reinvent the wheel. That’s the purpose of the Si1102-EK, an interestingly simple solution that takes the Si1102 dual-port sensor and wraps it in an easy-to-use kit that saves time for both experts and newbies alike (Figure 2).

Figure 2: The Si1102-EK provides all the necessary components for a full implementation, including potentiometers for flexible control of sensor threshold and LED strobe frequency. (Image source: Silicon Labs)

The kit comprises a board, a CR2032 battery, a pin connector, a battery power switch, an infrared LED, the Si1102 sensor, and all the passive components required for basic operation. Critically, the kit includes two potentiometers that are used to provide a level of programmability through active control of the proximity threshold and the IR LED strobe frequency. This helps account for, and offset, any changes over time. For example, the sensor’s threshold can drift by up to 20% or more, depending on the temperature.

A look at the schematic provides insight into the kit’s operation (Figure 3). To play with the design and use proprietary variables, designers can access the schematic online here at Schematics.com.

Figure 3: The schematic of the Si1102-EK shows both the simplicity of the final implementation as well as the flexibility provided by potentiometers R1 and R2. (Image source: Silicon Labs)

The Si1102 itself is a standalone, dual-port sensor that drives a single LED and which provides a simple on/off output. It has in internal analog wakeup controller that is controlled through an external resistor to set the time interval between measurements.

As shown in Figure 3’s schematic, the Si1102 strobes the IR LED at a frequency defined by the R2 potentiometer. The Si1102 then measures the amount of light reflected onto the package. If it exceeds a threshold set by the potentiometer, R1, the Si1102’s output latches low, thereby turning on the blue LED (D2). S1 connects or disconnects power from the battery, while the header, H1, gives access to the Si1102’s pins.

In total, the Si1102 is designed to keep the average power consumption in the microwatt (µW) range. This is accomplished through simplicity of design and function, such as the simple PRX on/off output. The Si1102-EK extends that simplicity of design into simplicity of implementation.

Again, for further exploration of the Si1102-EK’s capabilities, play with the schematic.