Make an LED switch on or off depending on the state of an NPN transistor.You will need:
1 Breadboard
7 Solid - Core Wires
1 NPN Transistor
2 AA Batteries
1 Green LED
1 LED (Any color!I used a white one that glows an amber color.)
That is all!Now let us get started!

Step 1: Step 1

These wires are were you wil connect the battery! (The left wire is positive,the right wire is negitive.)

Step 2: Step 2

Then add this blue wire wich will connect to the NPN transistor's collector.

Step 3: Step 3

Then connect the yellow wire from the base of the NPN transistor to positive.Also,you can see the blue wire and where it connects from the last step.Then,connect one lead of the LED of any color to the NPN transistor's emitter.After that,connect the dark blue wire to the LED's other lead and connect that to negative. 

Step 4: Step 4

Last but not least, add a green LED to tell you if your machine is on.Connect one lead to positive with the dark blue wire and the other to negative with the orange wire.

Step 5: Lastly

Here is a pic. and a video of the final product:
Oh,and just to note that as long as the yellow wire is connected to positive the LED of any color will stay on.
If you take it out, the NPN transistor will become switched off and the LED will turn off.Also,there is an on and off circuit schematic!
Do you have a schematic drawn up of this circuit? It is easier for me to see what is going on in a schematic than photographs of the circuit, and a text description. Transistor switching can be handy to use.
<p>Reach <a href="http://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=11&cad=rja&uact=8&sqi=2&ved=0CE4QFjAK&url=http%3A%2F%2Fwww.dummies.com%2Fhow-to%2Fcontent%2Felectronics-components-use-a-transistor-as-a-switc.html&ei=QQ1aVNC7M4OwuATUsILoAQ&usg=AFQjCNEG4-avEO0cDDuD9ujUjZFkDbXwGg&bvm=bv.78677474,d.c2E" rel="nofollow">Here </a>for Better Understanding !! :)<br>Best of Luck !! (y)</p>
No problem at all :) , I see that the transistor tester is just a circuit for using the transistor as switch, and the transistor that I was using worked fine, it looks like that the problem was that I had to give the base (+ voltage) FROM THE + of the circuit and not from an external battery, but I didn't know why??
<p>I apologize for not getting back to you earlier again, I was on a trip in Washington D.C. But, it is good to hear that you figured it out.</p>
This is the circuit that I'm trying on, as you can see, i've tried several voltages I also tried reversing the connection of the transistor but still same result, is there a problem in the circuit? And by the way, can you tell me an easy method to test the transistor and see if it's working, in my multimeter I don't have the transistor socket, but I have diode function.
<p>Try using another NPN transistor that you know works on other <br>circuits. By doing this, you will be able to see if it is the transistor <br> or the circuit itself that is not working. You will be able to see if it is the circuit that is not working if the new transistor does not work. You will be able to see if it is the transistor that was not working if the new transistor works. As for a transistor tester look at this instructable: <a href="https://www.instructables.com/id/Transistor-Tester-1/" rel="nofollow">https://www.instructables.com/id/Transistor-Tester-...</a></p><p>PS: I apologize for not getting back to you earlier.<br></p>
So +3 volte ( positive ) to the base of the trans. Will close the circuit?
So I am getting at that you are trying to use two AA batteries to try and generate three volts of voltage. And you want to know if the NPN transistor will be opened or closed if you add these three volts to it's base? If this is what you are trying to ask then my answer is that in the instructable I used a nine volt battery. As you saw in the video, it opened the NPN transistor. So, I would recommend you just use a nine volt.
Sure,I will put that on!

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More by Amazing Soldering Iron:Use an NPN Transistor as a Switch! (No Soldering!) Make Your Own Plastic! 
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