/diy/ - Do It Yourself

>>1616106
but do you know why 50uA?
likely: the dropper resistor for the LED is still in the way of the emitter, keeping your transistor from behaving like a switch. what you had created, more or less, is a switched current source that turns on just enough to keep the emitter one diode drop below the base. Ie = (Vb - 0.65)/Re and Ic = Ie-Ib so Ic = Ie*(1-(1/beta))
somewhat likely: the opamp is running well below its design voltage and can't push its transistors hard enough

>>1616120
often, you can use an op-amp in place of a comparator, but the opamp will generally be slower to respond and the outputs may not swing as tightly to the rails as you might like
anyway, the TBA222 is a terrible op-amp for this application. its input resistance is on the same order as your voltage dividers, it's only guaranteed to work with input voltages 3V or more inside the power rails, and it isn't even rated to run on any less than an 8V supply. you're very lucky that it works and is even close to accurate