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Difference between mechanical switch and a transistor switch?

  1. Oct 17, 2009 #1
    Doesn't a transistor switch need a mechanical switch? Otherwise, how does the transistor switch turn on and off? Does the switch have a "mind" of it's own?
  2. jcsd
  3. Oct 17, 2009 #2
    A transistor switch, e.g., an npn transistor, can be triggered by a sound, light (photodetector), an electrical signal, or a radio signal (e.g., garage door opener).
    Bob S
    Last edited: Oct 17, 2009
  4. Oct 17, 2009 #3
    Thanks for the reply. And if it isn't too much, how would an electrical signal alone switch a transistor?
  5. Oct 17, 2009 #4
    I believe the answer to your last question can be readily found by considering the NMOS or PMOS transistor.
  6. Oct 17, 2009 #5


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    Science Advisor

    What did you have in mind?

    Bipolar junction transistors draw very little collector current if they have no base current and draw a lot more current if they do have a base current.
    So, to make them draw collector current you just need to produce a base current from some event.
    If the event is very short (in time) then there are ways of producing a much longer time to turn a switch on or off for a longer time.

    If they do draw a collector current, this can be used to do something useful. You might turn on a LED, for example.
    The transistor has to be used in certain ways to keep it safe and so it works at all, so it is not as easy to use as a mechanical switch but it has many advantages.
    It can be triggered by electrical signals.
    It can switch much faster than a mechanical switch.
    It is silent.
    It never wears out.

    A good example.. Suppose you want to turn on a light when the sun goes down.
    You have a light detector driving a transistor switch which turns on the light. It isn't as simple as that, but you get the idea.
    How would you do that with a mechanical switch?
  7. Oct 17, 2009 #6
    I've got a little black box im our sunroom, about 5 or 6 cm on a side. It plugs into the wall socket. It has a photodetector on it, and a knob labeled OFF ON 1 Hr 2Hr 4Hr 8Hr.
    I use it to turn on a 120 Vac lamp for 4 hours after sunset every day.
    Very convenient.
    Bob S
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