I want to send a signal to uC and turn on a LED when there is light shining on phototransistor Q3. Currently I'm using stm32f4 (discovery board) as a signal receiver, but I would like to be able to replace it with an atmega at any time.
My problem is that base of Q1 is taking almost all of the current coming from Q3 and emitter-ground voltage after Q2 is too low for a uC to recognize it as a logical high.
I know I can add a resistor just before the base of Q1, but I don't know what is a resistance of the input of the uC (because it may be replaced with a different one)
How can I make sure that both the LED and the signal to uC have enough voltage to work properly?