Smart Sensing with Contactless Optical Proximity Sensor Solutions

In response to consumer demand for better user experience and intelligent automated control, optical proximity sensors have been widely used in various technologies with face detection, hand motion and distance detection, such as smartphones, LCD TV displays, computer and keyboard backlight displays, digital camera viewfinders, automatic light switching and bathroom faucet control.

This article will introduce the working principle of proximity sensors and the challenges faced in signal amplification, filtering and signal conditioning when using them. In addition, it will also discuss how design engineers can ensure the robustness and performance of optical proximity sensors by using a comprehensive proximity sensor solution to solve problems such as long-term high current protection of LEDs, sunlight and ambient light elimination, etc.

Introduction and working principle of reflective optical proximity sensor

An optical proximity sensor is basically composed of an infrared LED emitter and a PIN light detector. Figure 1 is a functional block diagram of a typical proximity sensor.

Figure 1: Functional block diagram of a proximity sensor.

As an electrical to light converter, the LED transmitter emits infrared pulses and senses the infrared pulses reflected by the obstructing object or surface through the photodiode detector, which provides the conversion of light back to an electrical signal. Please refer to the functional block diagram in Figure 2 for illustration.

Figure 2: Schematic diagram of the operating principle of a proximity sensor.

The basic working principle of the proximity sensor is very simple. The infrared pulse is emitted by the LED transmitter, reaches the obstacle or surface at a specific detection distance from the sensor, and penetrates, scatters or reflects back to the photodiode detector. The photodiode then generates a photocurrent that can be converted into an output voltage through an external load resistor. The size of the output photocurrent is determined by the detection distance and the driving current of the LED. Under specific LED light output conditions, the closer the object or surface is to the sensor, the higher the reflected light intensity, so the photocurrent output provided by the photodiode sensor is also greater.

Challenges of Proximity Sensor Signal Amplification, Filtering, and Signal Conditioning

In actual design, proximity sensing circuits can be very complex. Most current designs are implemented using discrete solutions. Signal conditioning circuits are usually added to the input and output of the optical proximity sensor to enhance the sensor's capabilities and phase ratio so that objects can be detected at the farthest possible detection distance and provide a reliable and appropriate output signal to the microcontroller.

At the input end, the intensity of the light pulse generated by the LED transmitter is basically determined by the power supply size of the LED. Usually, the microcontroller that generates the electrical pulse signal cannot provide enough current to drive the LED, so a current amplifier circuit such as a transistor is added.

Importance of long-term high-current protection for LEDs, high PSSR: To avoid shortening the life of LEDs due to long on-times, we can add long-term high-current protection circuits, which can avoid unnecessary long pulse widths on the LEDs.

The power input circuit of the proximity sensor must also have high ripple suppression capability to avoid fluctuations caused by input voltage changes.

Importance of sunlight and ambient light elimination: Ambient light and artificial light sources, such as incandescent and fluorescent lamps, may affect the sensitivity of photodiode detectors. Any stray sunlight or bright backlight sensed by the photodiode detector will produce a significant continuous DC or low-frequency spike voltage. In addition, since most sunlight contains a certain amount of infrared light, ordinary filtering circuits cannot effectively reduce noise.

A clean photocurrent output is usually desired at the output of a proximity sensor, so a complex filtering circuit with ultra-narrow bandwidth characteristics and matching the target noise wavelength must be designed at the output circuit for sunlight elimination, see Figure 3.

Figure 3: Schematic diagram of the proximity sensor with sunlight cancellation circuit added.

Importance of signal amplification: To be reliably and properly read by a microcontroller, the typically tiny output photocurrent must be further amplified by an amplifier circuit.

Importance of proper signal form interface: After amplification, the output photocurrent signal can be connected to a current-to-voltage conversion circuit to provide a voltage output signal. Similarly, other circuits such as hysteresis comparators and Schmitt triggers can also be added in the form of the desired control function.

Importance of printed circuit board space and implementation cost: Usually LED driver circuits, amplifier circuits and narrow bandwidth filter circuits are designed using discrete circuits, resulting in expensive PCB expenses and implementation costs.

How to choose a reflective integrated proximity sensor

The most important benefit of integrated optical proximity sensors is that they do not require contact. Since there is no physical contact between the sensor and the object, contamination can be avoided. Optical isolation with robust shielding can bring almost zero optical mutual interference, lower power consumption, smaller size and optimized detection distance, making the market acceptance of integrated reflective sensors higher. However, the detection range of integrated proximity sensors is fixed, so the choice must be determined according to the form of application.

Accelerate time to market with comprehensive optical proximity solutions

Avago Technologies' optical proximity sensor solutions provide intelligent sensing for a variety of applications. The solution includes proximity sensors and signal conditioning chips. The complete solution provides the following important advantages:

* Improved performance and robustness

* Suitable for low power applications

* Accelerate time to market

* Improve design flexibility

Avago's APDS-9700 is an ASIC that enhances the performance and robustness of optical sensor circuits by providing appropriate signal conditioning, such as driving the emitter with sufficient current and strengthening the sensor output to provide a proper and reliable connection to the microcontroller. In addition to adding intelligence to the object detection system, the chip can also handle ambient light interference issues, and its small and compact QFN package (2x2mm) effectively reduces board space and saves external components.

Figure 4 shows the functional block diagram of the APDS-9700 signal conditioning chip.

Figure 4: Functional block diagram of the APDS-9700 signal conditioning chip.

Proximity Sensing Application Circuit Using HSDL-9100 and APDS-9700

Figure 5 shows a proximity sensing application circuit reference design using the Avago APDS-9700 signal conditioning chip and the Avago HSDL-9100. In this design, the proximity sensor transmitter sends serial pulses in the form of a pulse train signal, a chirp signal, or a pseudo-random signal, passes through a specific detection distance, and is reflected back to the receiver by an obstacle or surface.

Figure 5: An optical proximity sensing design using the Avago APDS-9700 signal conditioning chip and the HSDL-9100 proximity sensor.

In this design example, the pulse is generated by a pre-programmed microcontroller and then fed into the LEDON pin of the APDS-9700. For proper operation, the pulse width should be greater than 1μs.

When the switching pulse on the LEDON pin changes from a logic high level to a low level or from a low level to a high level, a spike voltage may be generated on the power supply Vcc. The main reason is that the built-in infrared LED drive circuit operates at a high current. This high current will be affected by the inductance to form a "bounce and rebound" effect, causing a spike voltage during the fast switching process. The induced inductance value may be generated by the internal bonding wires of the chip, the external test probes, or even the wires connected to the power supply. Since the spike voltage may cause errors or even damage to the chip, CX1 and CX2 decoupling capacitors are added to absorb these spike voltages. In this application, it is recommended to use 100nF CX1 and 6.8μF CX2.

LEDA is the output pin that drives the infrared emitter, and R1 is a current-limiting resistor used to control the current flowing through the infrared emitter. The higher the resistance value of R1, the smaller the current flowing through the infrared emitter. For some applications with a short object detection distance, high current is not required. Reducing the current flowing through the emitter helps to reduce the peak voltage of the power supply voltage.

The cathode of the photodetector is directly connected to the PD pin of the APDS-9700.

Resistor R3 and capacitor CX3 are connected in parallel and connected to the PFILT pin to form an integration circuit that generates the output voltage VPFILT. The current provided by the internal voltage-to-current converter passes through this integration circuit to perform charging and discharging operations with a specific time constant.

The PFILT analog output pin can be connected to a microcontroller's analog-to-digital converter to convert the continuously changing voltage into binary digital form. These binary codes can be used to display the detection distance on a PC, LED or LCD panel.

In addition to providing the output voltage of the PFILT pin, the integrated voltage VPFILT is also connected to the input of the hysteresis comparator. When the input of the hysteresis comparator reaches the preset reference threshold voltage VTH, a change from a logic high level to a low level will occur at the output, otherwise a change from a low level to a high level will occur. Therefore, the change in the output of the photodiode detector will be presented as a digital output on the DOUT pin. Since DOUT is an open-collector pin, a pull-up resistor R2 needs to be connected to the power supply Vcc on DOUT.

The DOUT digital output pin can be connected to a microcontroller, LED, or switch to provide status of whether an object is sensed.

APDS-9700 has an ENB pin that allows users to shut down the chip to save power when the device is not needed. When the ENB pin is high, the device will be shut down. When the ENB pin is low, the device will start and return to normal operation. These actions can be implemented using a state machine, and the order of actions can be controlled by microcontroller programming.