Below is a circuit where S1 is the output of a 555 IC timer (S1 switch for simplicity). I need to invert the output which I have done so in another project using the circuit below.
Starting with transistor Q2 which is a 2N3904 (datasheet), how do you calculate the resistor R2 needed if R3 or "Load" is 180Ω. The source will be 5V. I'm having trouble understanding why Collector-Emitter saturation voltage is .3V where IC=50mA, when below it says for hFE4, VCE=1V, IC=50mA
Secondly how do I calculate R1 for Q1, to ensure Q2 is not driven.
I've have this circuit working with 12V and a 350Ω load. I used 1K resistors for both. I can't remember how I calculated it or if I even got it right.