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I am following a very simple schematic which is a NOT gate with a push button as input. The output is used to drive the base of a second transistor that turns on an LED:

NOT Gate Schematic

This circuit works perfectly fine when two BC547A transistors are used:
If the push button is NOT pressed, the LED is on, otherwise off - like it is expected with a NOT gate.

If I switch the two transistors for 2N3904, it does not work anymore and the LED stays always off.

Both the BC547A and 2N3904 are simple NPN general purpose transistors. Up until now I was under the impression that they could be exchanged with each other pretty much always. Apparently, I was wrong, but would really like to understand what causes the difference in this very simple circuit to make one of them work and the other one not.

To be as precise as possible, here are the datasheets of the two transistors used during testing:

It's clear that the NOT gate could drive the LED directly, this is just a test setup to use the output of the gate to drive another transistor.

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    \$\begingroup\$ The first question is, did you built the circuit correctly? \$\endgroup\$
    – Justme
    Commented May 27, 2023 at 18:29
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    \$\begingroup\$ Pinout for transistors can differ from part to part. \$\endgroup\$
    – tobalt
    Commented May 27, 2023 at 18:30
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    \$\begingroup\$ (Plus the ratio of R2/R1 is unusually low, and R4/(R2+R3) very high.) \$\endgroup\$
    – greybeard
    Commented May 27, 2023 at 20:07
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    \$\begingroup\$ your schematic diagram is somewhat messy ... should be power at top, ground at bottom, input at left, output at right ... for your circuit, Q2 should be oriented same as Q1 ... R3 same as R1 ... D1 and R4 same as R2 \$\endgroup\$
    – jsotola
    Commented May 27, 2023 at 20:23
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    \$\begingroup\$ The BC547 has typical hfe of 200-800, while the 2N3904 is more like 100-400. So the resistance of R1 and R2 may be too high. The variation of current gain is rather large as well. If you really need the circuit to work at very low current, you need to allow for the minimum hfe. \$\endgroup\$
    – PStechPaul
    Commented May 28, 2023 at 7:43

1 Answer 1

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The transistors can be used interchangeably and exchanged, yes, but they don't have the same pinout.

BC547 has CBE and 2N3904 has EBC.

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    \$\begingroup\$ You are fully right, Justme, thanks for pointing this out - stupid me, simply overlooked that - what a beginner's mistake. ;-) Thanks again and topic closed! \$\endgroup\$
    – ab-tools
    Commented May 27, 2023 at 18:34
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    \$\begingroup\$ Also useful to note, Japanese 2SAxxxx through 2SDxxxx use BCE for their small plastic package transistors, and US manufactured RF transistors use BEC sometimes, which is better for Miller effect. \$\endgroup\$
    – Whit3rd
    Commented May 27, 2023 at 20:58

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