Difference between mechanical switch and a transistor switch?

user111_23

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?

Bob S

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

user111_23

Thanks for the reply. And if it isn't too much, how would an electrical signal alone switch a transistor?

╔(σ_σ)╝

I believe the answer to your last question can be readily found by considering the NMOS or PMOS transistor.

vk6kro

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?

Bob S

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