It might be a stupid question but i'm new to electronics, How do you know what kind or type of transistors you need to use in a certain circuit?

 

It depends on a number of factors:

#1. What technology of transistor is best for the circuit? FET, BJT, etc.
#2. If you are using BJT's What polarities do you use more often? This determines whether you should use NPN or PNP more often.
#3. Look at how the transistor is made. a BJT transistor contains two diodes back-to back. This greatly determines what kind of transistor you use.

What is the "certain" circuit you are referring to?

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-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-

 

Thanks alot?

 

Very appropriate punctuation.
Some of that was true, some was mumbo-jumbo. Unfortunately, he sometimes has a tendency to do a core dump when he sees something that he's had some experience with.
Your question doesn't have a simple answer. It takes a lot of education and/or experience to design a working circuit. If you're new, you will learn faster by copying proven designs and attempting to understand how they work.

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Ron

 

I had to make my point. Otherwise he could ask more.

Thats why I gave the three points.

Start with the easy designs, and when you understand with the easy designs, go with some more difficult ones. It does take research to be experienced.

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-=: The best low-priced components to troubleshoot with are the speaker and the LED :=-