New trend of firelight sensor active integration v

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New trend of optical sensors: active integration vs passive separation

there are a variety of optical sensors with small packages, and the price is reasonable. For decades, passive components have been active in the market. As photoresistors, photodiodes and phototransistors become more and more common and cheap, they are widely used in many consumer products, from night lights to digital cameras. However, the integrated device that came into being soon integrates a phototransistor or photodiode with a current amplifier to achieve on-chip calibration, filtering and higher resolution. When the resolution is high, the low light ability, power suppression and other functions play a role. Such devices are expanding the application range and effectiveness of ambient light sensors

types of optical sensors

as shown in the table, these sensors are arranged from left to right according to the complexity. The comprehensive situation of performance and cost will be discussed below

the simplest optical sensor is a photosensitive resistor, which can be identified through the channel between two terminals. The low-end version is made of CDs (cadmium sulfide), while the more expensive type is made of GaAs. GaAs has a small energy band gap, which enables it to absorb low-energy photons in infrared light and make electrons transition to the conduction band. The data of the reference element shows that its illumination range is 1~100lux, but it has various resistance values

various available sensors provide a variety of cost-effective options

photodiodes are more complex. Photons bombard semiconductor junctions to produce electricity. In order to fully play its role, the photodiode should be reverse biased. The bias voltage is directly converted into the operation quality, because the large reverse bias voltage can improve the speed and linearity, and can also improve the dark current and shot noise. Light will generate a forward current and reduce the reverse bias current. An external circuit can be added to the photodiode to linearize the I-V curve to amplify the signal and allow the failure function to be achieved

the general characteristics of phototransistors, such as Beijing's bird's nest, water cube, Guangzhou New Baiyun Airport, Japan's EPSO, in order to make the test piece level n in the experimental process, the gate of Dubai Bay, Abu Dhabi central market, the Russian Federation building, Germany's Frankfurt air railway center and other characteristics, like photodiodes, have added amplification characteristics. It requires a larger bias current, but the noise associated with the current forces the sensitivity of the sensor to a higher lux range, that is, 1K ~ 100klux (rather than 7K ~ 50klux). The response time is similar to that of photodiodes, which can be adjusted by using bias current. The current can also vary with the detected signal level. Phototransistors can roughly determine the level of ambient light, such as indoor/outdoor, day/night and bright light/shadow. Such devices also need external circuits to calibrate the output signal and include an enabling function

the shrinking size of components enables people to design integrated devices, such as el7900. The device places a photosensitive resistor and a mutual resistance amplifier in a package. This combination can shorten the lead length and minimize the parasitic capacitance in the input of the amplifier, which is an ideal condition for achieving minimum noise, high frequency response and convenience. The low noise characteristic can expand the sensitivity of the sensor to 1Lux, while keeping the upper limit at 100lux. Power consumption still depends on the amount of light induced, and the current is 0.9ma at 1000lux. In order to save power, a power-down pin is used. In addition to digital cameras, this device is suitable for many occasions

further integration is embodied in devices such as isl29001, which is a packaged solution for optical sensing and calibration. This temperature compensated optical sensor is a pin diode. Before entering a high pass filter, the output of the sensor is calibrated and sent into an amplifier to eliminate 60Hz noise. After passing through the filter, an analog-to-digital converter (ADC) and I2C interface provide the output signal

using ADC has two advantages: one is that the power consumption can remain unchanged, and the other is that it can achieve a resolution of 15bit. In fact, its current consumption is lower than that of all other active devices in the table. ADC has an internal 327.6khz clock, and the response time of the device is set at 100ms. Even when the delay increases, the serial 15bit output signal makes the sensor suitable for a wide range of applications

application of optical sensor in portable products

isl29001 can achieve a resolution of 15bit

optical sensors are widely used in. Some applications use the reflected light of optical detection to sense the position, including bar code readers, laser printers and automatic focusing microscopes. Portable electronic products such as digital cameras and laptops use light sensors to measure the amount of ambient light. We will further discuss the second kind of applications mentioned above

ambient light sensor in notebook computer is used to adjust the backlight of the screen to make it look comfortable. The comfort range of the backlight depends on the light condition of the room. It is conceivable that the brightness of the screen needs to increase with the brightness of the ambient light, while in the case of weak light, in order to look comfortable and prolong the battery life, the brightness of the screen needs to decrease

in the design of notebook computers, the ambient light sensor is usually next to the acoustic device with obvious relaxation characteristics, rather than on it. The machine has an opening in the position of the speaker, so that light can enter the interior. These audio ports are usually covered with shapes to protect the speakers. Due to this protection, the light is blocked, which reduces the amount of light that can be measured. Therefore, a solution with good dark light accuracy needs to be adopted. In order to obtain the required accuracy under low light conditions, it is best to choose an integrated photodiode with ADC. At the same time, the high pass filter can reduce the power supply noise to the lowest level and prevent its coupling into the backlight

another common application is ambient light sensing in. In this application, every milliampere saved means longer battery life and increased customer satisfaction. Using the light sensor to adjust the backlight illumination, the battery life will be extended by at least 6 times (assuming that when there is no light sensor feedback, the current experimental data and the average value of the experiment can be printed, and the backlight will remain in the full power state)

taking el7900 as an example, it senses the intensity of ambient light in and outputs a current proportional to the light intensity, and the output current is injected into the feedback input of white LED driver. In a bright environment, the optical sensor sends more current to the feedback node. Therefore, it reduces the output current and output light intensity of white LED, so as to achieve the purpose of power saving


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