LED Intelligent Lighting System Based on STC Microcontroller

1 Introduction

LED is called the fourth generation of lighting source or green light source. The light-emitting device of LED is a cold light source, which has the characteristics of energy saving, environmental protection, long life and small size. The light efficiency of incandescent lamp and halogen tungsten lamp is 12-24lm/W, fluorescent lamp is 50-70lm/W, sodium lamp is 90-140lm/W, and most of the power consumption becomes heat consumption. LED can reach 50-200lm/W, and the single light has good monochromaticity, narrow spectrum, no need to filter, and can directly emit colored visible light. Under the same lighting effect, the power consumption is about one-tenth of incandescent lamp and one-half of fluorescent lamp. A fluorescent lamp with the same effect is more than 40 watts, while the power of each LED is only 8 watts. The average life of LED is 100,000 hours, with strong safety and reliability, and does not contain mercury, sodium and other substances that may endanger health. It is beneficial to environmental protection and is called "green lighting source".

2 Intelligent lighting control solution design

Photoresistors are used to detect the intensity of indoor light, and passive pyroelectric infrared detectors can detect the characteristics of the human body. The sensor transmits the detection data to the control core - the single-chip microcomputer, which controls the opening, closing and illumination of the lighting equipment according to the processing results. Figure 1 is a block diagram of the principle of the intelligent lighting control solution.

The system mainly consists of three parts: sensor part, controller part and LED driving circuit and lighting system, see Figure 1.

3 System Hardware Design

3.1 Sensor part

3.1.1 Passive pyroelectric infrared detector

The detector has three key components: Fresnel filter chip, which acts as a bandpass filter through a filter chip with a cutoff wavelength of 8 to 12μm, so that environmental interference is significantly controlled; Fresnel lens, which has a focusing effect, that is, refracting (reflecting) the pyroelectric infrared signal on the pyroelectric infrared sensor. The second function is to divide the warning area into several bright and dark areas, so that moving objects entering the warning area can generate changing pyroelectric infrared signals on the pyroelectric infrared sensor in the form of temperature changes, so that the pyroelectric infrared sensor can generate changing electrical signals; the pyroelectric infrared sensor converts the changes in infrared radiation energy passing through the filter chip into electrical signals, which is called thermoelectric conversion.

The human body has a constant body temperature, generally at 37 degrees, so it emits infrared rays with a specific wavelength of about 10μm. Passive infrared probes work by detecting infrared rays of about 10μm emitted by the human body. The infrared rays of about 10μm emitted by the human body are enhanced by the Fresnel filter and gathered on the infrared sensing source. The infrared sensing source uses pyroelectric elements. When this element receives a stable change in infrared radiation from the human body, it will lose its charge balance and release the charge outward, which will produce a change in the level after detection and processing.

Based on this principle, a stable infrared module is used. When someone is walking, the module outputs 3.3V voltage, and when no one is around, it outputs a low level. The module has an adjustable delay, which can reach up to 18 seconds.

The core component of this sensor module is the photoresistor. A photoresistor is a resistor made by the photoelectric effect of a semiconductor, and its resistance value changes with the intensity of the incident light; when the incident light is strong, the resistance is small, and when the incident light is weak, the resistance increases. Photoresistors are generally used for light measurement, light control, and photoelectric conversion (converting light changes into electrical changes), see Figure 2.

3.2 Control part

STC12C5628AD series MCU is a single clock/machine cycle (IT) MCU produced by Hongjing Technology. It is a new generation of 8051 MCU with high speed/low power consumption/super strong anti-interference. The instruction code is fully compatible with the traditional 8051, but the speed is 8 to 12 times faster. It has an internal integrated MAx810 dedicated reset circuit. 4-channel PWM, 8-channel high-speed 10-bit A/D conversion, for motor control and strong interference occasions.

The main reason for choosing this type of microcontroller is that it has PWM and AD conversion functions, which greatly simplifies the peripheral circuit. At the same time, the generated PWM signal can be directly connected to the driver chip, which greatly reduces the cost of the system.

Since PWM dimming is adopted, in order to reduce unnecessary peripheral circuits, the selected driver chip can directly input PWM square wave through the DIM pin.

The LM3407 is a pulse width modulated floating buck converter with integrated N-channel power MOS field effect transistors, designed to provide precise constant current output to drive high power light emitting diodes (LEDs). The LM3407 features a pulse level modulation (PLM) control scheme that ensures a constant current output accuracy of better than 10% over the entire input voltage and operating temperature range when an external 1% current setting thick film resistor is used. Another feature of the converter is a DIM pin that can receive standard logic pulses to control the brightness of the LED array, making the LM3407 an ideal device for precision power LED drivers or constant current sources.

4 Software Design

This program adopts modular design concept, with the main program as the core and two functional module subroutines set up, so that some functions can be implemented in the subroutines, simplifying the design structure. During the operation, the main program calls the functional module subroutines.