Transistor questions

[skysurf76]

I'm building an 8 bit adder on a breadboard, and my college education isn't to the point where I've been formally introduced to transistors so I'm kind of stuck on the following in my project.

Here are the datasheets for the devices I'm working with...

Texas Instruments XOR gate: http://www.ti.com/lit/ds/symlink/sn74hc266.pdf

Fairchild Semiconductor NPN Transistor: http://www.fairchildsemi.com/ds/MM/MMBT3904.pdf

Kingbright LED: http://www.us.kingbright.com/images/catalog/SPEC/WP7104SRC-E.pdf

My problem is that according to the datasheets the XOR gate only has 4mA output current and I am trying to figure out how much current is needed to fully turn on the transistor.  I've tried reading several beginning electronic books about this, but to be honest I'm just getting buried in the acronyms and technical jargon, and reading the datasheets just leaves me scratching my head.  I understand for every transistor there is a relationship between voltage and current on the base, and the voltage and current between the collector and emitter.  I just can't figure out how to find that out with the information I have.

My plan is to power the whole thing with 5 volts going into dip switches, their outputs go into the logic gates arranged as an adder, those outputs then activate the transistors whose collector's will be on the 5 volt rail, which activate the LEDs.

I guess what I really need to know is what values on the datasheet show this information and how do I interpret it?  I really think I was doing ok until I got to the transistor data sheet.  I really just don't understand the information it has.


Thank you in advance for spending your time de-noobifying me. 

[TerminalJack505]

I think the only real transistor parameter (other than maximum ratings) you need to worry about in this case is the hFE parameter.  This is the DC current gain.  If you have a base current of Ib then the collector current (Ic) will be hFE * Ib.

In other words:

  Ic = hFE * Ib

You're going to need 10 to 20 mA of collector current for the LED.  If you look at the table, the transistor will have an hFE of at least 100 for a collector current of 10 mA.

So, if you need to drive the LED with at least 20mA then you will need to have a base current of at least:

  Ib > Ic / hFE

  Ib > 0.02A / 100
  Ib > 0.0002A

Just to be sure you should multiply this by 3 or more.  Let's multiply it by 5.

So:

  Ib = 1 mA
 
Now all that you do is choose a resistor for the base that would allow for 1 mA of base current. 

You're using 5V so:

  Rb = (5V - 0.7V) / Ib
  Rb = 4.3V / 1 mA
  Rb = 4300

A 4.7k resistor should work.

Note that the 0.7V in the above equation is to factor-in the voltage drop from the base to the emitter.  The 4.3V will be dropped across the resistor.  The 0.7V will be dropped between the transistor's base and emitter.

That should be all you need to worry about in this case so far as transistors go.

[skysurf76]

Thanks for your help.  It was just what I needed.

[TerminalJack505]

Thanks for your help.  It was just what I needed.

No problem.  Transistors are fairly simple when you are just using them as a switch.