The current fluctuations of several milliamperes flowing through the light-emitting diode may have a great influence on the life and light attenuation of the light-emitting diode. Therefore, the current flowing through the light-emitting diode should be considered in the design.

　　 Many circuits powered by dry batteries usually cancel the current-limiting resistor RA, so that the light-emitting diodes work directly under the output voltage of the battery, and the current is limited by the internal resistance of the battery. However, the internal resistance of the battery is related to the capacity, life and material of the battery. A battery with a large capacity has a smaller internal resistance.

　　The internal resistance of a new battery is usually small and will increase with time. Due to the internal resistance of the battery, its output voltage decreases as the current increases, and the voltage of a battery with a larger internal resistance changes greatly. The advantage of this circuit is that the circuit is simple and the cost is low; the disadvantage is that the current changes greatly, and the brightness of the light-emitting diode also changes greatly. Therefore, it is easy to burn the light-emitting diodes due to overcurrent.