Technical exchange: Sensors realize intelligent control of LED lamps

The biggest difference between LED lighting fixtures and traditional lighting fixtures is that LED lighting fixtures are completely electronic products, while traditional lighting fixtures are only electrical products. Therefore, LED lighting fixtures can be easily associated with various types of sensors to achieve various automatic control functions such as light control and infrared control. For example, the automatic switch of LED street lights can be easily realized with a photosensitive sensor; for community night walkways and courtyard lighting, infrared sensors can be used to collect human activity information and automatically turn on and off lighting fixtures.

LED lighting switch automatic control

Sensors are devices for signal acquisition and electromechanical conversion, and their electromechanical technology is quite mature. In recent years, the rise of MEMS (micro-electromechanical system) technology has made great strides in the miniaturization, intelligence, multi-function and low cost of sensor technology. Various types of sensors such as photosensitive sensors and infrared sensors can form an intelligent control system with LED lighting fixtures. The sensors convert various physical quantity signals collected into electrical signals, and can intelligently process the collected signals through integrated circuit AD (analog-to-digital) converters, MCU (microcontrollers), and DA (digital-to-analog) converters to control the opening and closing of LED lighting fixtures. Humans can use this to set various control requirements on the MCU to control the switching time, brightness, color rendering, and colorful changes of LED lights, thereby achieving the goal of saving power and energy. The system block diagram of sensors and LED lamps is shown in Figure 1. The current integrated circuit manufacturing technology can integrate AD, DA, and MCU in a 5mm×5mm or smaller package, which does not take up space and is very convenient to install in lamps.

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Combination of photosensitive sensor and LED lamp

Wind and solar LED street lights are highly intelligent and unattended road lighting fixtures that use wind and sunlight to generate electricity and use batteries to store energy. Therefore, automatic energy management is very important. Photosensitive sensors are ideal electronic sensors that can control the automatic switching of circuits due to changes in illumination during daylight and darkness (sunrise and sunset). Figure 2 shows the appearance of a photosensitive sensor. Figure 3 is a photoresistor board of a photosensitive sensor, which is very sensitive to the brightness of light. Figure 4 is a basic principle diagram of photoelectric conversion. The working principle of the light-controlled LED lighting system is shown in Figure 1.

Photosensitive sensors can automatically control the opening and closing of LED lighting fixtures in shopping malls according to weather, time period and region. By reducing its output power during bright daytime, power consumption can be reduced by up to 53% for a convenience store with a store area of ​​200m2 compared to using fluorescent lamps. The life span is also as long as about 50,000 to 100,000 hours. Generally speaking, the life span of LED lighting fixtures is about 40,000 hours; the color of the light can also be changed in RGB (red, green and blue) to make the shopping mall lights more colorful and the atmosphere more lively; compared with the original blue LED with yellow phosphor, the purple LED with red, green and blue phosphor has a higher color rendering performance.

Infrared sensor and LED lamp combination

Infrared sensors work by detecting infrared rays emitted by the human body. The main principle is: the infrared rays of about 10μm emitted by the human body are enhanced by the Fresnel filter lens and gathered on the pyroelectric element PIR (passive infrared) detector. When a person moves, the emission position of the infrared radiation will change, the element will lose the charge balance, and the pyroelectric effect will occur to release the charge outward. The infrared sensor converts the change of infrared radiation energy through the Fresnel filter lens into an electrical signal, that is, thermoelectric conversion. When there is no human movement in the detection area of ​​the passive infrared detector, the infrared sensor only senses the background temperature. When a human body enters the detection area, the pyroelectric infrared sensor senses the difference between the human body temperature and the background temperature through the Fresnel lens. After the signal is collected, it is compared with the existing detection data in the system to determine whether there is really someone or other infrared source entering the detection area.

Passive infrared sensors have three key components: Fresnel filter lens, pyroelectric infrared sensor and matching low noise amplifier. Fresnel lens has two functions: one is focusing, that is, refracting the pyroelectric infrared signal on the PIR; the other is to divide the detection area into several bright and dark areas, so that the moving objects/people entering the detection area can generate changing pyroelectric infrared signals on the PIR in the form of temperature changes. Generally, a low noise amplifier is also matched. When the ambient temperature on the detector rises, especially when it is close to the normal body temperature of the human body (37°C), the sensitivity of the sensor decreases, and the gain is compensated through it to increase its sensitivity. The output signal can be used to drive the electronic switch to realize the switch control of the LED lighting circuit. Figure 5 shows the appearance of the infrared sensor, and Figure 6 shows the internal structure and internal circuit diagram of the infrared sensor. Figure 7 shows an LED lighting fixture with an infrared sensor. This is an E27 standard screw-type lamp. Its power supply range is AC180V-250V (50/60Hz). The infrared sensor detection range is about 3m~15m. Its standard product IFS-Bulb 3W lamp reaches 80lm, and 5W lamp reaches 140lm. The infrared sensor is embedded in the central part of the LED light source module. Once the infrared sensor detects the body temperature of a person, the LED bulb will automatically turn on and off within 50s. It is suitable for any indoor application, such as corridors, storage rooms, stairs and hall entrances.

Technical exchange: Sensors realize intelligent control of LED lamps

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Ultrasonic sensors, which are similar to infrared sensors, have been used more and more in the automatic detection of moving objects in recent years. Ultrasonic sensors mainly use the Doppler principle to emit high-frequency ultrasonic waves that exceed the human body's perception through crystal oscillators. Generally, 25~40kHz waves are selected, and then the control module detects the frequency of the reflected wave. If there is an object moving in the area, the frequency of the reflected wave will fluctuate slightly, that is, the Doppler effect, which is used to determine the movement of objects in the lighting area, thereby achieving the purpose of controlling the switch. Figure 8 is an application solution for the combination of ultrasonic sensors and microprocessors.

The longitudinal oscillation characteristics of ultrasound can propagate in gases, liquids and solids, and their propagation speeds are different. It also has refraction and reflection phenomena. It has a lower propagation frequency and faster attenuation in the air, while it has less attenuation and propagation farther in solids and liquids. Ultrasonic sensors use these characteristics of ultrasound. Ultrasonic sensors have the characteristics of a large sensitive range, no visual blind spots, and no interference from obstacles. This technology has been used in the commercial and security fields for more than 25 years and has been proven to be the most effective method for detecting the movement of small objects. Therefore, the system composed of LED lamps can sensitively control the switch.

Due to the high sensitivity of ultrasonic sensors, air vibrations, ventilation, heating and cooling systems, and movement in the surrounding space can cause false triggering of ultrasonic sensors, so ultrasonic sensors need to be calibrated in a timely manner.

Temperature sensor for over-temperature protection of LED lamps

Temperature sensor NTC (negative temperature coefficient) has been widely used for over-temperature protection of LED lamps. If LED lamps use high-power LED light sources, they must use multi-wing aluminum heat sinks. Since the space of LED lamps for indoor lighting is very small, the heat dissipation problem is still one of the biggest technical bottlenecks. If the heat dissipation of LED lamps is not satisfactory, the LED light source will cause early light decay due to overheating. After the LED lamp is turned on, the heat will also be concentrated on the lamp head due to the automatic rise of hot air, affecting the life of the power supply (Figure 9). Therefore, when designing LED lamps, an NTC can be placed close to the aluminum heat sink near the LED light source to collect the temperature of the lamp in real time. When the temperature of the lamp cup aluminum heat sink rises, this circuit can be used to automatically reduce the output current of the constant current source to cool the lamp; when the temperature of the lamp cup aluminum heat sink rises to the limit setting value, the LED power supply is automatically turned off to achieve over-temperature protection of the lamp. When the temperature drops, the lamp is automatically turned on again (Figure 10).

Conclusion

Because LED lamps are complete electronic products, with the diversification of LED lamp structures, the expansion of applications, and more creativity and innovation in the design of LED lighting fixtures, more sensors will be combined and applied in the systems of LED lighting and lighting projects. A new era of intelligent LED lighting is coming, and human lighting life will become brighter and more comfortable.