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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.

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  • \$\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
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    \$\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

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