1. The identification of ordinary light-emitting diodes (LEDs), abbreviated as light-emitting tubes, is mainly used in electronic products as an indicator light for power supply or working status. According to the different colors of the light emitted, light-emitting diodes generally distribute several types of light, such as red light, green light, and yellow light. According to the number of pins, there are two types of light emitting diodes: two pin type and three pin type. The two pin type light emitting diode only has one light emitting diode, while the three pin type light emitting diode has two light emitting diodes with different colors, as shown in Figure (a) below. The circuit symbol is shown in Figure (b). The working current of a light-emitting diode is generally from a few milliamperes to several tens of milliamperes, and the luminous intensity of the light-emitting diode is basically linearly related to the forward current of the light-emitting diode. The LED only operates in a forward bias state. Under normal circumstances, the forward conduction voltage of light-emitting diodes is mostly 1.5-3V, and the common conduction voltage of light-emitting diodes is mostly around 1.8V. Reminder: If the conduction current flowing through the LED is too large, it may cause overcurrent damage to the LED. In practical applications, a current limiting resistor is usually connected in series in the LED power supply circuit to prevent it from being damaged by overcurrent. 2. Polarity discrimination of ordinary light-emitting diodes: The pin (electrode) can be confirmed by checking the volume of the chip inside the cap. The pin connected to the larger chip is the negative electrode, while the pin connected to the smaller chip is the positive electrode. Good or bad detection: Place the digital multimeter in the "diode" detection position, and the detection method is shown in the following figure. Firstly, connect the two probes to the two electrodes of the light-emitting diode. If the measured conduction voltage drop value is 1.766V and the light-emitting diode can emit weak light, it indicates that it is good, and the red probe is connected to the positive electrode, while the black probe is connected to the negative electrode; If the LED still cannot emit light after replacing the probe and the displayed overflow value is "1" or too small (such as "0"), it indicates that it is damaged. 3. Identification of infrared light-emitting diodes. Infrared light-emitting diodes are light-emitting diodes that directly convert electrical energy signals into infrared light signals. Although they are made of gallium arsenide (GaAs) material, they also have a semiconductor PN junction. Infrared light-emitting diodes are mainly used in infrared remote controllers of devices such as color televisions, VCDs, and air conditioners. The physical appearance of a common infrared light-emitting diode is shown in the following figure, and its circuit symbol is the same as the light-emitting diode. 4. Detection of infrared light-emitting diodes. Place the digital multimeter in the "diode" position, connect the red lead to the positive pole of the infrared light-emitting diode, and connect the black lead to its negative pole. The measured forward conduction voltage drop value is about 1.019