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!
Best Answer
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 – Schematic created using CircuitLab
Figure 1. An NPN inverter.
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Figure 2. A double-inversion gives a buffer.
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.