I have a need for an LED, driven by a constant current source, that can be smoothly dimmed and also semi-rapidly (say 2.5kHz) modulated for later bandpass filtering. I've designed a possible circuit, given below. The constant current source and PWM-dimming were already familiar to me but I seem to have found the switching solution through beginner's luck.
The circuit seems to work well in Spice and on a breadboard. Is there a compelling reason to edit or reject this circuit for the given purpose? Also, could someone help me understand how the switching via Q2 works?