Because the voltage across R2 is kept constant Ohm's law that the current through it remains constant so the current will vary between 0/33 and 0.7/33 or 0 to 21mA.
hi hero,
Are you sure about this, if the current thru the LED is to change from 0.7mA to 33mA, then the current thru R2 must also change by almost the same value.
I would also suggest you check the min/max currents you are expecting.
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