Last week, I posted a problem with a logic circuit (Need Help With Transistor Logic Circuit Design), and Analogkid was kind enough to solve it. However, at the time, I didn't realize I still had a couple of biasing issues to iron out in the rest of the design.
One of the issues that has cropped up is that the input switches have a 1M ohm residual resistance when open. This is causing Q1 and Q2 to forward bias when SW-1 and SW-2 are open, leaving LED-1 partially lit. I need to calculate the resistance values of R5, R6, R7, and R8 such that Q1 and Q2 are 30% of the way into saturation mode when SW-1 and/or SW-2 are closed, and in cutoff mode when they are open.
I can not change the resistance values R1 and R2. They are fixed. Note that R3 and R4 are simply to compensate for diodes D1 and D2 to bring the current draw across LED-1 to exactly 10 mA when SW-3 is closed.
Also, a "0" ('zero") in the truth table equates to a switch being open. Here is my schematic:
Any help would be sincerely appreciated.