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Is it possible to have "Normally closed" switch with NPN transistor when the voltage at the base is 0V? So far I only managed to achieve this result with NPN transistor when the base is HIGH. When the base is LOW it's "open" and outputting 1.8V as in provided image below and this is not what I'm expecting. Thanks!

enter image description here

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Is it possible to have "Normally open" switch with NPN transistor when the voltage at the base is 0V?

I suspect that you are confused with your terminology. A normally open switch has its contacts open and does not conduct. An NPN transistor, therefore, is normally open as it does not conduct when the base is held at 0 V.

An easier way to think of the NPN switch is that of an inverter. When the input is high the output is low. When the input is low the output is high.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. An NPN inverter.

schematic

simulate this circuit

Figure 2. A double-inversion gives a buffer.

schematic

simulate this circuit

Figure 3. A PNP inverting buffer.

This circuit works provided the logic input is working on the same V+ as the transistor.

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  • \$\begingroup\$ Yes, probably I got confused with the terminology. But I think you got an idea what I'm trying to achieve here with the transistor. Btw updated the question. \$\endgroup\$ Commented Mar 31, 2019 at 21:35
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Commented Mar 31, 2019 at 21:36
  • \$\begingroup\$ In my case logic input is working on a different V+ than transistor. Is this a problem? \$\endgroup\$ Commented Mar 31, 2019 at 21:40
  • \$\begingroup\$ Yes. The NPN will work because it is referenced to the ground. The NPN won't. That's why NPNs are so often used for level shifting between different supply voltages. \$\endgroup\$
    – Transistor
    Commented Mar 31, 2019 at 21:43
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    \$\begingroup\$ Your question doesn't explain the problem you are trying to solve. Hit the edit link. Its bedtime in Ireland. \$\endgroup\$
    – Transistor
    Commented Mar 31, 2019 at 22:05

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