Detection of common light-emitting diodes:

(1) The pointer type multimeter with a ×10kΩ block can be used to roughly judge whether the light emitting diode is good or bad. Normally, the forward resistance of the diode is tens to 200 kΩ, and the value of the reverse resistance is ∝. If the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easily damaged. This kind of detection method can't see the illuminating condition of the illuminating tube in reality, because the ×10kΩ block can not provide a large forward current to the LED.

(2) It can accurately measure the light and electric characteristics of LEDs with 3V regulated source or two series of dry cells and multimeter (pointer or digital). To do this, connect the circuit as shown in Figure 10. If the measured VF is between 1.4 and 3V, and the brightness of the light is normal, it can be said that the light is normal. If VF=0 or VF≈3V is measured and does not emit light, the light-emitting tube is broken.

Detection of infrared light-emitting diodes: 

Due to infrared light-emitting diodes, it emits infrared light of 1 to 3 μm, which is invisible 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 the luminous intensity of different types of infrared LEDs is also different. The forward voltage drop of an infrared LED is generally 1.3 to 2.5V. Because the infrared light emitted by the human eye is invisible, the detection method of the visible light LED can only determine whether the positive and negative electrical characteristics of the PN junction are normal, and it is impossible to determine whether the illumination condition is normal or not. For this reason, it is best to prepare a photosensor (such as a 2CR, 2DR type silicon photocell) as a receiver. Use a multimeter to measure the change in voltage across the battery. To determine whether the infrared LED emits infrared light after adding an appropriate forward current. Its measurement circuit is shown in Figure 11.