1. The short-circuit diode is added to the L ED voltage higher, the LED brightness is large, the diode is connected in series with a voltage drop of 0.7V in the circuit, the voltage applied to the LED is lower, and the brightness is small.
2. There is a certain limit to the voltage adjustment with PWM. It is difficult to achieve 0 to the maximum adjustment. Constant current regulation should be a more reasonable solution, but the circuit is relatively complicated.