Proximity Sensing Applications Using Infrared Sensors

Proximity sensing is becoming increasingly accepted as a method of detecting the presence of a user's body or hand in consumer electronics. The technology can also be used for motion sensing, such as detecting user gestures. User gestures as an input can be applied to many devices, such as mobile phones, computers, and other home electronics.

To understand the theoretical basis of motion sensing system design, it is necessary to understand the difference between infrared (IR) and visible light, explore how proximity and motion sensing systems operate with a single LED, and how motion sensing works when multiple LEDs are used for multi-proximity measurement.

When we talk about "light", we usually mean visible light from the sun or lamps, however, visible light only accounts for a small part of the spectrum. We define visible light as all light that can be recognized by the human eye, usually the wavelength of light that can be recognized by the human eye is 380-750nm. So, what about non-visible light that the human eye cannot recognize (such as light with a wavelength of 850nm)?

IR radiation has a wavelength of 750nm-1000μm. IR light has the same properties as visible light, such as reflectivity, and can be generated by special bulbs or light-emitting diodes. Because the human eye cannot see IR light, we can use it to complete some special human-machine interface tasks, such as proximity detection, without the user having any direct contact with the system.

IR proximity sensing systems are able to detect the presence of nearby objects and react based on the detection results. Applications for IR proximity detection are everywhere. For example, mobile phones can use proximity sensing technology to detect if the phone is close to your face while you are on a call. When you put the phone to your ear, the phone will detect the presence of your head and automatically turn off the screen to save power. Other examples of proximity sensing systems include soap dispensers and water dispensers, where you can place your hand near the sensor (usually near a soap dispenser or faucet) to obtain soap or water in a "non-touch" and hygienic way.

On high-end cars, proximity detection is also used in exterior collision avoidance systems to alert the driver when the car is too close to another car or object. Some vehicles can also use in-car proximity sensing systems to detect the presence of passengers so that safety devices (such as airbags) can be adjusted. Proximity detection is achieved using specially designed IR LEDs. The counterpart to IR LEDs is a photodiode, which is generally used to detect the IR light emitted by the LED. When the IR LED and photodiode are placed in the same direction, the photodiode will not detect any IR light unless an object is in front of the LED and reflects light back to the photodiode. The intensity of the light reflected back to the photodiode is inversely related to the distance of the object from the photodiode.

Figure 1: Action detection in one dimension

A single LED combined with a photodiode can detect some actions, such as whether an object is approaching or moving away from the photodiode, which is only a one-dimensional spatial detection. Assuming a system with the layout shown in Figure 1, the single LED system only uses LED1 and the IR sensor. Figure 2 shows the output value of the Silicon Labs Si1120 sensor after sensing the IR LED during three gestures, where the Y axis is the reflected IR light intensity and the X axis is time. The three gestures include sliding from left to right along the X axis of Figure 1, sliding from bottom to top along the Y axis, and reciprocating along the Z axis from far to near and then from near to far. Figure 2 shows that the single LED system cannot distinguish these gestures. Using a single LED, the system can only detect that the object is approaching or moving away from the sensor, but cannot determine its direction.

Figure 2: Single LED system performance analysis

Two-dimensional sensing consists of two LEDs at different locations and a single photodiode. A measurement is taken from LED1, then another measurement is quickly taken from LED2, and the two measurements are used to calculate the position of the object in two dimensions. One dimension is close to LED1 (left) or close to LED2 (right), and the other dimension is close to or away from the photodiode. Figure 3 shows the same three gestures as Figure 2, where the white line represents the data read from LED1 and the red line represents the data read from LED2. During a left-to-right swipe, the white line rises, followed by the red line. As the hand slides from left to right, LED1 reflects IR light to the sensor, followed by LED2.

Figure 3: Gesture performance analysis in two-dimensional space

The three-dimensional motion detection consists of three LEDs and a single photodiode. LED3 is not in the same straight line as LED1 and LED2. As shown in Figure 1, the line between LED1 and LED2 can be regarded as the X-axis, the line between LED1 and LED3 can be regarded as the Y-axis, and the line from the photodiode and LED to the object to be measured can be regarded as the Z-axis. Figure 4 shows the same measurement process as Figures 2 and 3, where the blue line represents the measurement data of LED3. When the hand slides from left to right, because the hand passes over LED1 and LED3 at the same time, the LED1 and LED3 data lines rise at the same time, followed by the LED2 data line. When the hand slides from bottom to top, because the hand first encounters the IR light from LED3, the LED3 data line rises, followed by LED1 and LED2. When the hand moves back and forth, because the hand reflects the same amount of LED light throughout the process, the three LED measurement values ​​are the same.

Figure 4: Analysis of motion performance in three-dimensional space after adding LED3

When IR LEDs and IR sensors are applied to products, these components are usually not placed outside for decorative purposes, and the end product requires at least an opening or a transparent window to let the IR light pass through.

The IR LED shines out of the window, reflects off external objects, and then passes through the window into the Si1120 sensor. The main disadvantage of a single window configuration is that the window will cause some light to be internally reflected into the Si1120, and a large amount of reflected light may cause the sensor to output even when there are no external objects within the detection range.

The dual window design uses one window for the IR LED and the other window for the sensor. By providing proper isolation between the LED and the sensor, the design eliminates the problem of internal reflections, providing the system with better sensitivity and detection range.

The choice of IR LED is a very important decision for IR proximity sensing system design. The IR LED viewing angle has a great impact on the maximum detection distance and range. The IR light emitted from the LED forms a cone, and the top angle of the cone (where most of the LED energy is output) is called the LED viewing angle.

Figure 5: Difference between narrow and wide viewing angle IR LEDs

All LEDs have a specific viewing angle, a narrow viewing angle LED means the energy emitted is more concentrated and irradiates farther than a wide viewing angle LED. This means that using a narrow viewing angle IR LED will result in a longer detection range in a narrow detection area, Figure 5 illustrates the difference between narrow and wide viewing angle IR LEDs.

When designing an IR system, the characteristics of the objects being detected in the system are also important to consider. In addition to detecting gestures, IR proximity sensing systems can also be used to detect inanimate objects such as garage doors (open or closed). When detecting larger objects, the detection distance will be greater because more IR light is reflected. The color of the object is another factor to consider because IR light has the same properties as visible light, and light-colored objects reflect more light than dark-colored objects. The darker the color of the object, the closer it must be to the IR system because only a small amount of IR light from the IR LED is reflected to the IR sensor.

Many electronic systems in consumer, industrial and automotive applications benefit from contactless reflection. IR proximity sensing provides an optimal method for systems that need to detect the presence of an object. Proximity sensing can also be used to detect motion and even gestures in up to three dimensions, making the human-machine interface of the next generation of electronic products more advanced and intuitive.