Due to the infrared light-emitting diode, it emits infrared light of 1~3μm, which is not visible to the human eye. Usually, the emission power of a single infrared light-emitting diode is only a few MW, and the angular distribution of LED luminous intensity is different for different models. The forward voltage drop of infrared LED is generally 1.3~2.5V. It is precisely because the infrared light emitted by it is invisible to the human eye, so the detection method of the above-mentioned visible LED can only determine whether the positive and reverse electrical characteristics of the base PN junction are normal, but can not determine whether its luminescence is normal, for this reason, it is best to prepare a photosensitive device (such as 2CR, 2DR silicon photocell) as a receiver. Use a multimeter to measure the change of voltage at both ends of the photocell. to determine whether the infrared LED emits infrared light after adding an appropriate forward current.