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Im puting a project together that includes reading 12v signals form a tachometer and a reed-switch.

Testing the tacho signal I found out that the C549C-transistor works fine without resistors so I think ill use that.
I also have some BC337-transistors so my question is:

What is the difference between those two transistors, can I use either ?

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the land of sun+snow
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Did you look at the datasheets?
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Yes, but they dont tell me much since I dont have any real knowledge of electronics.
This is a one time project that is working fine and I just wanted some advice without reading all about transistors.
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you dont have to read all about transistors, how about some basics and when and why to use a base resistor
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OP, it would have been nice to include the links to the specific devices you were looking at.
I downloaded datasheets for BC549 and BC337,

http://datasheetz.com/data/Discrete%20Semiconductor%20Products/Transistors%20(BJT)%20-%20Single/BC549C-datasheetz.html

http://www.fairchildsemi.com/ds/BC/BC337.pdf

The most important parameters to consider are probably
BC549, BC337:
NPN, NPN
Ic max = 100 mA, 800 mA
Vceo max = 30V, 45V
hFE = 100-800, 100-250


For general use, they're fairly comparable transistors, and both should work in most ckts.
The BC337 has a better current capacity.

However, your comment about "works fine without resistors" leads one to wonder, as
you basically never see bipolar transistor ckts without some Rs in there.
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Thanks for a useful answer.
I will look up what hFE and ic max is.

The gadget is for my 91 Harley.
It will include a si4703 radio, a realtimeclock some buttons and a lcd.
It will be connected to the "old" tacho signal so I think thats ok (no resistor between the signal and the transistor).

Im not shure about the reed switch and will check that up when the temperarure outside is better (-10c now).

I will use the arduino mini pro but im testing things on a uno.
Its all built on "boards".
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Cumming, GA
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Quick Note about Hfe

Hfe is one of those things that often confuses someone just starting to ask the question "what is a transistor anyway?"

Another word for Hfe is β (beta)... these odd names are related to the mathematical models used to define how a transistor works... It can also be called "gain".

Gain, being the word least associated with a "math" formula,  is essentially how much does this particular device "transfer" what happens at the Base pin to the other 2 pins.  With "transfer" being one of the base words that make up the word "transistor"... maybe this starts to make more sense?

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Valencia, Spain
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Yes, but they dont tell me much since I dont have any real knowledge of electronics.
This is a one time project that is working fine and I just wanted some advice without reading all about transistors.

Read this: http://www.mcmanis.com/chuck/robotics/tutorial/h-bridge/bjt_theory.html

It has all you need to know and it's only two pages long.
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Thanks again for a useful answer.
Thats was an interesting tutorial for me and im goint to read it again tonight.
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Anaheim CA.
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That is also one of the best tutorials I have ever seen in 50 years of looking...
It Should be mandatory reading for all who use the Arduino. IMNSHO

Bob
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