I am starting to get beyond frustrated with gates and breadboards now in general because of this simple circuit (which is literally experiment #1 in my digital logic class).

Using only NAND gates, generate function F= (~A~C)+(~CD)+(ABC). Using only NOR gates, generate G= (~A+B)(A+~B)(~B+C).

I can’t get a consistent Logic level for each binary code value; ie it works except it’s off for 1111, or it works except 0001-0011, etc. I will try to post as many pics of the info I’ve done so far. I cannot get this to work. F worked for a little but I messed around with it and lost it. The teacher is no help; says my designs are fine yet gives confusing vague answers about literal connection junctions and how and why each affects the other based on gate voltages or whatnot and states no resistors at all are needed and that it is irrelevant whether or not empty inputs remain floating or empty etc. based on everything I’ve read my thoughts have become soup. I know how to theoretically do it; I’ve even simulated it out and the functions worked fine. I cannot even count the hours I’ve wasted killing brain cells just literally making every possible combination on the literal breadboard to no avail. Tried different gate sets, even bought an arduino power supply. Sorry for such an idiotic question but I’m literally losing my interest in college based on this kindergarten level issue I can’t tackle.

How does one know where inputs and the final output should be arranged? I.e., whether npn or pnp manufacturing requires specifically pull up or pull down resistors for the LED output, and whether or not you’re able to mix and match High Current models with low Schottkey.

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  • 2
    \$\begingroup\$ You haven't asked a question; you've just vented your frustration. Ask a question and keep it relevant like... do I need resistors - no, but you do need to tie unused inputs to logical zero (0 volts) or logical 1 (Vcc). Make this answerable and do yourself a favour. \$\endgroup\$
    – Andy aka
    Commented Apr 17, 2020 at 12:20
  • \$\begingroup\$ Do you have any bypass capacitors on your breadboard? Often they don't teach this in school, but most digital logic chips go nuts if there isn't a 0.1uF ceramic bypass capacitor with very short leads across each logic chip's power supply to ground pins. \$\endgroup\$
    – MarkU
    Commented Apr 17, 2020 at 19:22
  • 1
    \$\begingroup\$ I’m at a low level, I’m not sure what a bypass capacitor would be. Is that a specific micro Farrad that is put in parallel to something? Sorry and the first commenter was right, I never asked my question. Basically how does one know where inputs and the final output should be arranged, ie, whether npn or pnp manufacturing requires specifically pull up or pull down resistors for the LED output, and whether or not you’re able to mix and match High Current models with low Shotkey (sp?) \$\endgroup\$ Commented Apr 17, 2020 at 23:14
  • \$\begingroup\$ About bypass capacitors: see this answer for an example; electronics.stackexchange.com/a/247670/35022 \$\endgroup\$
    – MarkU
    Commented Apr 18, 2020 at 6:53
  • \$\begingroup\$ Normally we push the OP to edit their question with changes, instead of leaving them in the comments. Since you sound like you're about ready to boil over, I have done that for you. In future, keep in mind that Stackexchange, while a nice side, is weird in that rather than looking for a discussion, it wants you end up with a well-formed question, which can have well-formed answers. So editing is encouraged and expected (even sometimes unsolicited editing by other folks). If you don't like what I've done -- hit that "edit" button, and make changes. \$\endgroup\$
    – TimWescott
    Commented Dec 26, 2021 at 17:29

2 Answers 2


Your truth table, conversion to NAND gates and NOR gates looks correct, even the lower circuit looks correct,

At this point I would turn to basic troubleshooting, bread boards are not perfect devices, so your likely facing connection issues, and the contacts in each hole gets worse and worse as its used more, combine this with logic chips that have probably been used 10 times before, so there is a risk that one of the inputs on a chip could be damaged

From your circuit you can for the deviations from the truth table work out what the output of each logic gate should be, go through until you find the one in disagreement, then correct the issue, be it a wire, a faulty chip, or a breadboard hole

Floating gates can be an issue for CMOS based gates, so using say 10K resistors to ground for the 4 external inputs can make sure you know all inputs are 0 until you set them high, its not likely to be the issue, rather can in rare cases be.

Edit: I would not recommend your "OR" option on the schematic, it would short out the output of the logic gate through the LED, e.g. it tries to output 5V, the LED conducts somewhere around 1.5V, and the current increases, possibly pulling down the supply voltage of the logic chip.

  • \$\begingroup\$ Thanks for the response. Yea I figured half of what I’ve been doing is pretty much flat out wrong and dangerous with actual circuits in the real world. The OR drawing was just showing the amount of configurations I’ve wasted my time on trying. I have an old school multimeter with the very inconvenient prods so that also added to my frustration. At times the voltage outputs would be alright and then that gate stopped producing a logical output. My guess it has something to do with the open collector of the charges. I’m just frustrated how poorly taught these circuits are at the CSU level \$\endgroup\$ Commented Apr 17, 2020 at 23:10
  • \$\begingroup\$ All the logic gate models given have "push-pull" output stages, at no point is it an open-collector output, \$\endgroup\$
    – Reroute
    Commented Apr 17, 2020 at 23:17
  • \$\begingroup\$ Based on how hard things are to measure for you, it sounds like the chips are oxidized rather badly, this will be making things even harder, for the price of those logic chips I might suggest you go to your local store and just buy some new ones (about $1.50 for the 2), If you just want it to work, grab a small metal file and shine up the legs. If just to measure do on the top corner of the legs while in the breadboard. \$\endgroup\$
    – Reroute
    Commented Apr 17, 2020 at 23:23
  • \$\begingroup\$ Does the current or charge density actually even fully travel inside the gate? Or is it physically just somewhat a force barrier for allowing anything connected int that row to be passed on. It’s weird how complex these things are at the manufacturing level at times \$\endgroup\$ Commented Apr 17, 2020 at 23:23
  • \$\begingroup\$ Thanks Reroute, I haven’t had anyone to talk to about this since the lockdowns and what not and was starting to go awol on just systematically doing everything I’ve been taught and have been lazy and frustrated. Sorry for the vague non question everybody again, it’s my first question ever on here lol \$\endgroup\$ Commented Apr 17, 2020 at 23:26

borrow a volt-meter (or buy one from Harbor Freight) and measure each node, and the VDD and the GND voltages, while observing the output logic state using an LED+1Kohm in series as visual indicator.

If the voltmeter touches any point in the circuit and causes the LED to flicker, you probably have a floating node. Hang a 10Kohm or 22Kohm resistor to either GND or to VDD, so the node no longer is floating.

Aren't those white-boards fun? Buy your own, or borrow a different one, that has not been ruined by prior lab experiments.

Hang in there.

  • \$\begingroup\$ I bought a whole new set from amazon. It’s something I’m doing wrong. I think I’ve been wearing the gates down slowly but surely when I first started from finding mistakes then fixing to mess up something else etc.At one point I was so frustrated I took a 5A tv chord and literally just put 120V and 0 as the inputs and ducked like i was detonating c4. As stupid as that is that’s just how much this whole topic has gotten under my skin after teachers, internet electricians etc just vaguely explain only 1% of what’s going. The 7400 gate is a souvenir now, blew up 2A and 2B pins into dust \$\endgroup\$ Commented Apr 17, 2020 at 23:20

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