I am currently diving deeper into electronics and have been researching silly to find an answer to the one question that is not clear to me.

I am still unsure how a transistor's input is changed from on to off and vice versa without a moving part or a physical action such as a keyboard key click or a mouse click. I think this due to the fact the computer is still running background programs and tasks without in physical action.

The only way that I may of thought of is that current is constantly running throughout the computer and it uses logic gates to change the input signal but in my mind this contradicts the idea of a transistor being that it has an "input" which can be changed.

I have noticed that all examples of this online that I have found whilst researching are always using physical switches of some sorts but this doesn't explain how a computer can still work without a physical action.

Any clarification to clear my confusion or links to where I can find out more on how transistors receive an input without a physical action would be much appreciated!

  • \$\begingroup\$ Please explain this part: "but in my mind this contradicts the idea of a transistor being that it has an "input" which can be changed". It sounds wrong, but I don't understand what you mean well enough to explain. \$\endgroup\$
    – Justin
    Nov 4, 2020 at 15:35
  • \$\begingroup\$ Initially how I understood the concept of transistors is that the input of the transistor would be created by a physical action like the flick of a switch causing the input to change to on or off but why this confuses me is the fact that there isn't always a "physical" input. \$\endgroup\$
    – dotmp3
    Nov 4, 2020 at 15:37
  • 3
    \$\begingroup\$ You're asking two questions here. One is how a transistor works as a switch, and the other is how a computer uses transistors to do anything. Both are big questions. How transistors work is commonly available, if a bit confusing and in depth. Start by looking up depletion regions and charge carriers work inside a diode. Then move onto the same thing with BJTs and MOSFETs. Here's a starting point: ittc.ku.edu/~jstiles/312/handouts/Doped%20Silicon.pdf. After this, move onto depletion regions, then diodes. \$\endgroup\$
    – DKNguyen
    Nov 4, 2020 at 15:37
  • \$\begingroup\$ The part about the computer is this: electronics.stackexchange.com/questions/455897/… As for the "contradiction", think about it this way. After I turn over a car engine to start it, what keeps it spinning on its own afterwards? After all, I'm not cranking it any more, and yet it continues to run. The answer is obviously that it keeps itself spinning; It can accept a turnover from you, and from itself. A transistor can accept inputs from you, or from other transistors in a self-perpetuatating loop \$\endgroup\$
    – DKNguyen
    Nov 4, 2020 at 15:38
  • 2
    \$\begingroup\$ There are millions of transistors in a computer, and almost all of them get their input signal from another transistor. Very few (probably 0 on the CPU itself) are controlled by physical switches. \$\endgroup\$
    – Justin
    Nov 4, 2020 at 15:40

2 Answers 2


You're going from very low level (how does a transistor switch on) to very high level (processes running in the back ground of a PC) in a single leap. This is not really advisable. If you want to study electronics then best not to start with understanding a whole PC.

If your question is really "how does electronics keep doing different things when there is no external input" (this is a legitimate question - "how does it move from one state to the next entirely on it's own - surely at some point it would have to stop...").

Then this might be what you want:

All chips like microcontrollers / processors etc, which keep 'running' have one thing in common (besides being powered) - and that's a clock source.

This is a circuit that, when voltage is applied, keeps changing from 1 to 0 periodically, without any outside intervention. This is like a heart beat which allows all sequential processing right up to the background process running on your PC.

If you look up all the low level sub systems of a 'computer' (counters, shift registers, latches etc) you'll find they all need a clock to run.




I think you're missing the concept of a "clock", which provides a continuous series of on-off pulses at high speed to drive digital logic. Counting clock pulses allows for periodic actions - everything from the displayed "real-time clock" which tells human time, to the repeating series of video signals sent to the display to produce an image.

The main input for transistors in a computer is of course other transistors.

You might want to have a look at an "astable multivibrator" for the simplest possible example of two transistors which drive each other. The attached capacitors provide a time delay.


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