"Normally closed" switch with NPN transistors
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.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. An NPN inverter.







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Figure 2. A double-inversion gives a buffer.







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Figure 3. A  PNP inverting buffer.

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