During the decade from the 1960s to the 1980s, SMT LED light-emitting diodes were only available in red, yellow, and green colors, with low luminous efficiency (only about 1 lm/W), low brightness, and high price. At that time, they were only used as indicator lights for electronic products. From the development and application history of SMT LED light-emitting diodes, it can be seen that at that time, SMT LED beads were only used as indicator applications for LEDs. Application scope of SMT light-emitting diodes: 1. AC power indicator light. As long as a 220V/50Hz AC power supply line is connected, the LED beads will be lit to indicate that the power is on. The resistance value of the current limiting resistor R is 220V/IF.

2. The AC switch indicator light uses SMT LED light-emitting diodes as the circuit for the incandescent light switch indicator light. When the switch is disconnected and the bulb goes out, the current flows through R, LED, and bulb EL to form a circuit, and the LED beads light up, making it convenient for people to find the switch in the dark. At this point, the current flowing in the circuit is very small, and the light bulb will not light up. When the switch is turned on, the light bulb is lit and the LED beads are turned off.

3. The AC power socket indicator light uses a dual color (common cathode) SMT LED as the circuit for the AC power socket indicator light. The power supply of the socket is controlled by switch S. When the red LED bead is on, the socket is without power; When the green LED bead is on, the socket is energized.

4. The fuse socket indicator light is mounted with LED light-emitting diodes and used as the circuit for the fuse socket indicator light in the factory equipment distribution box. When the fuse is intact, the LED beads do not light up; When the fuse is blown, the LED beads will be lit to indicate to the user which fuse has been blown for replacement. This is very convenient for porcelain core fuses that cannot be observed with the naked eye.

4. The fuse socket indicator light is mounted with LED light-emitting diodes and used as the circuit for the fuse socket indicator light in the factory equipment distribution box. When the fuse is intact, the LED beads do not light up; When the fuse is blown, the LED beads will be lit to indicate to the user which fuse has been blown for replacement. This is very convenient for porcelain core fuses that cannot be observed with the naked eye.