My electronics is limited.
[correction, see following posts: PWM follows RPM, not vacuum power or current]
As I understand it, the PWM (pulse width modulation) signal is a logic level square wave switching the motor voltage, so motor current is a mirror of it, smoothed some by motor inductance, again mirrored in very low voltage across the ammeter shunt resistor. All the same, equally affected by battery drift (see below}. I guess you can smooth the PWM with a capacitor easier than amplifying shunt voltage, but don't have specs. When such pulses go through an LED brightness varies as the eye smooths the pulses.
PWM might simplify as a higher voltage directly (smoothed) driving a comparator without battery compensation or closed intake subtraction, but I've room for the opamp and like subtracting closed intake for display. Not needed just to set a trigger point (and just a resistor can position the trimmer at an offset).
Comparators just like high voltages, inaccurate near ground.
The battery drift is limited to around 10 per cent over many months, so could be ignored over short periods, but will significantly shift air flow readouts over long periods given twenty per cent differences etc. in filter states observed (see "Electronic Filter Monitoring" thread http://www.robotreviews.com/chat/viewtopic.php?f=20&t=15981
). The extra opamp is already in the dual 8-pin package, so used it. The same thing occurs over a run, but you can only read the filter at the beginning before the bin fills anyway.
) looks the best; includes the comparator besides power readout. Not sure how to filter pulsing; maybe easier, on the high voltage shunt signal (on high side of motor instead of ground, getting voltage with current) -- or filter power readout. Tinker, tinker (but no tailor, soldier or spy).
Don't have an oscilloscope for more precision -- too expensive just for this.