I've created some home-made gates using discreet components onto strip board. So far I have an OR gates, an AND gate and 4 NOR gates. Logically they all work as expected but on only one of my 4 NOR gates I am getting some weird unexpected voltages on the 2 inputs and the voltage on the output is lower than on the other 3 NOR gates.

Let's start with my schematic. enter image description here

And the strip-board layout:

enter image description here

I've also built a testing circuit which basically consists of a pair of buttons which connect A & B to 5V. There is no pull down resistor on this basic testing circuit, which I probably should have but I don't think this is particularly relevant since I expect all 4 of my NOR gates to behave the same as eachother.

The buttons and the output line also drive LEDs via transistors but again I don't think this is particularly relevant.

For the 3 good NOR gates, when I connect them but don't press any buttons the 2 A & B inputs are at about 0.1V, relative to ground, which seems fine to me.

For the bad NOR gate, when I connect it but don't press any buttons, A & B are both at more like 0.5-0.6V, which definitly seems wrong to me.

Furthermore, on the 3 good NOR gates the output is about 4.5V, while on the bad one it is only about 3.5V

I have checked the obvious things like bad solder joins and shorts between tracks. I also swapped out the 2 transistors in case one of them had gone bad. I tested the new ones with a multimeter using this method (diode mode) before putting them in and they seem to be ok. The ones coming out also seem to be ok.

I imagine more information is required to make a full diagnosis but I don't know what else to check.

What am I missing? Should I actually be concerned about this or is this kind of variation expected?

Just for completeness, although I doubt it's helpful - here is the completed circuit:

enter image description here enter image description here

N.B. These 2N2222A331 transistors have their collector on the right when looking at the flat side.

  • \$\begingroup\$ (I remember burning three transistors in a row before suspecting the resistors. Turned out I had been sold 3k resistors as 1k which looked 1k (brown black red (gold) - guess what reminded me) - an off-colour orange…) \$\endgroup\$
    – greybeard
    Dec 30, 2022 at 16:22
  • \$\begingroup\$ How did you establish the voltages? Do you feel at ease with non-driven inputs? I might try adding BE resistors, 1-10 k-ish. \$\endgroup\$
    – greybeard
    Dec 30, 2022 at 16:25
  • \$\begingroup\$ (FWIW, I think the execution of the design rock solid.) \$\endgroup\$
    – greybeard
    Dec 30, 2022 at 16:29
  • \$\begingroup\$ @greybeard what is BE? The resistors all came from the same line but I'll double check \$\endgroup\$
    – DJL
    Dec 30, 2022 at 16:56
  • 1
    \$\begingroup\$ On both working and non-working the resistors are about 970 ohm, so we'll within gold tolerance \$\endgroup\$
    – DJL
    Dec 30, 2022 at 17:00


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.