I'm fairly inexperienced with electronics in general, but I've learned quite a bit over the last many months of self instruction (and some really good help from forums like these). I have one question on transistors whose answer has eluded my normally excellent googling skills, so I figure I would just throw out the question on this forum in hopes of getting an answer.
With a regular 2n3904 transistor, what happens if my base voltage is significantly higher than the voltage that I am switching on the Collector/Emitter? 12V on the base side, 5V on the collector/emitter side.
I've looked at a few javascript simulators I've found online and it says my Ve=11.3V (allowing for the forward voltage drop b to e.). Is this correct? I was worried that the simulators might not handle this "reverse scenario" correctly.
Here's an example scenario:
I have a garage opener button in my garage just outside the back door. It operates on 12V. Close the circuit by mashing the button and the door opens/stops/closes.
I would like to hook up a MCU (Atmel) running at 5V to the button to detect when the button is pressed. All the inputs are max 5V, so I was thinking of using a transistor as a switch. But I worry I'll send ~12V to my MCU inputs, which is grossly out of spec and generally a bad thing.
So, is a transistor just the wrong part for the job if my base voltage is higher? I really want to understand when I can and when I can't use a transistor for such things. What about PNP?
I know there are other solutions like a relay or a solid state relay/optoisolator, which would protect the 5V side from the 12V side. This is more about understanding how a transistor works than figuring out this particular example. I've run into this situation on more than one occasion.