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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:

enter image description here

Any help would be sincerely appreciated.

Simulate at falstad.com

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  • \$\begingroup\$ past Simulator link pls > File> Export link. Why 12V instead of 5V or 3.3V \$\endgroup\$ – Sunnyskyguy EE75 Jan 6 at 21:57
  • \$\begingroup\$ Tony, that's part of the spec. It is a 12V system, and this is only a very small part of it. I posted the Falstad file. It appears to work for me (but this is ugly). I can copy and paste it into a .txt. file and open it with Falstad. There has to be a better way of doing this. -sigh- \$\endgroup\$ – Rikki Jan 6 at 22:17
  • \$\begingroup\$ Dave, that would be great. But I didn't know (and don't see) how to save a file on their server. I tried to create a URL, but I get a "shortener error". I suppose I need to figure this out if I am going to come here and ask for help. Having a circuit simulated would really make things easier. I'll keep at it. \$\endgroup\$ – Rikki Jan 6 at 22:26
  • \$\begingroup\$ Rikki as I said, only comments allow Tiny URL's as they may become transient even google is getting out of goo.gl \$\endgroup\$ – Sunnyskyguy EE75 Jan 6 at 22:28
  • \$\begingroup\$ I went ahead and created the link for you. \$\endgroup\$ – Dave Tweed Jan 6 at 22:31
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The obvious answer is to reduce the value of your pullup resistors (R7 and R8) until the leakage current through the switches produces less than 0.6V of drop.

Anything less than 1 MΩ × 0.6 / 12 = 50 kΩ should work; use 33 kΩ or 22 kΩ for some extra margin.

You should reduce the series resistors (R5 and R6) to similar values.

BTW, it's R9 that regulates the current through the LED; the other resistors should have no effect on that.

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  • \$\begingroup\$ That wasn't obvious to me. I found that 15K worked, and it put the transistors well into the saturation region. I'll mark this as the answer. \$\endgroup\$ – Rikki Jan 6 at 23:19
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I really appreciate the help I've received out here. Like I said, I'm trying to learn while building something I really need. Bottom line is, Dave got me going on this one. Thanks Dave. Here is the revised version that seems to work well:

enter image description here

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