Fan Speed controller with ''speedmeter''

Just simple circuit, but i found it quite entertaining and educating to do, figured it would make good post too.
So, idea is simple: Fan speed controller with ''speedmeter''. Fan is basic 12VDC computer fan without PWM I/O pins, only +/- wires. But it's still controlled with PWM fashion, made my 40106 schmitt trigger, connected as variable PWM controller. At alone, this isn't strong enough to drive fan, so N-fet, IRF540, takes care of this. Also, fet has necessary pulldown resistor on its gate, to shut it down during off-time.

Next stage is ''speedmeter''. Actually, it isn't speedmeter straightforward, but duty-cycle meter. First, there is low-pass filter, along with opamp,lm358 which converts PWM/frequency to voltage, cind of DAC (digital-to-analog?). After that, analog signal is fed to led-driver opamp, i used lm324 quad, which acts as comparator, different levels to different leds.

That's pretty much all to it, feel free to comment! .

scope settings were 10μs/div and 5v/div.