I built a simple circuit as an experiment. I thought it should have worked, but unfortunately, nothing happens. The goal here was to simply light the LED:



My logic here is this: the two resistors that you see act as a voltage drop. The bases of the transistors, after all, don't require 9V. At the same time, these resistors should lower the amount of current that flows through the LED so that I don't burn it. Since the bases of the transistors are always connected to the power supply, they should allow current to flow through them and light the LED.

However, nothing happens. The LED does not light.

Frankly, I'm not very experienced with a multimeter, so I don't know what to test and where.

Can anyone explain why nothing is happening?

 

I'm not sure what you trying to do with the transistors. If lighting the LED is your only goal, you only need one resistor and no transistors. A led will have a voltage drop (think, the amount of votage that it "consumes") and you need the resistor to take care of the rest of the voltage. Typical voltage drop for an led can be from 1.7 to 3+V. LEDs typically will run on 20 mA current so we select a resistor to limit the current to that. I'd just pick 2V. then use ohms law V = IR. Rerarrange so that R = V/I = (9-2)/.02 = 7/.02 = 350 ohms. a 390 or 470 ohm resistor would work fine (these are standard values). If the LED fails to light, you've probably got it in backwards.

If you are trying to get the transistors to work, I'd start simpler.

 

Well, being a newbie, I'm kind of experimenting with transistors. I also hoped to implement a sort of a logic AND gate here.

In the schematic, the two horizontal wires that lead to the bases of the transistors could have switches. I hoped that the LED would light only if both switches were closed. But the LED doesn't light at all, regardless of whether the hypothetical switches are open or closed. I really have no clue why. Perhaps lowering the resistance should work?

I think I understand transistors. I built a simple circuit to test its functionality, and it worked. I just tried expanding the idea.

My circuit is merely an experiment. I just can't understand the results (or the lack thereof).

 

were you trying to get the led to flash?

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Don't make me reach through this monitor to slap you a good one!

 

I was just trying to light it, albeit in a roundabout way.

But actually, I found a better way to do what I was trying to do here:



But this leads me to another question. Observe in the image above that there are two resistors of equal value, each connected to a base of one of the transistors. Why are they connected in parallel? Would it not be more senseful to use just one resistor?

In other words, other than the resistance on the LED (which differs), are there any other differences in these two schematics? (sorry, I mislabeled the battery as 1V, it should be 9V)

 

I re-drew your circuit so that the transistors are "the correct way up".
For the LED to light, one transistor must conduct to 0V which happens, but the other transistor needs to be at a higher voltage but cannot with its base connected to the base of the 1st transistor.

My circuit has both transistors completely separate. If one input goes high but the other input is low then the LED does not light. If both inputs are low then the LED does not light. If both inputs are high then both transistors turn on and the LED lights.

Attached Images

Transistor AND gate.PNG (17.7 KB, 45 views)

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Uncle $crooge

 

Thanks for the explanation. I think I understand...

So, despite that it's still a bit unintuitive to me, these two circuits are in fact different?



EDIT: And sorry for the "upside-down" transistors again. I'll have them fixed next time, promise

 

I Dunno. The circuit with the bases connected together might work. Try it and let us know.

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Uncle $crooge

 

Nope, the second one does not work. Which confuses me, but I'll give it some thought...

 

Did you have the transistors upside Down? I dont know why it wouldnt work, seems to me it would.

 

Same here, although he'll get more current if he moves the 820 to be in series with the LED.

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Ron

 

Can I ask, how did you come to those base resistor values of 470R? If we assume a transistor HFE of about 150ish, and 8.3V dropped across the base resistors (leaving 0.7V for the base-emitter junction) I make it 62K for a 20mA LED. Of course none of this is critical because the exact HFE value will never be certain, but 470R does seem quite a low value resistor.

Other than that, it makes better sense to have seperate base resistors because you should bias the transistors seperately. If you use just one base resistor, then the voltage drop across the base resistor will be different depending on whether one or two of the transistors are turned on. That means the biasing of the transistor will be different depending on whether you've got one or two of the switches depressed, and that's just not good practice - even if it does work.

Your drawings are really confusing when you draw the transistors that way. Try to get yourself into the habit of drawing them the right way up - that way you'll be sure that you're always on the same wavelength as everyone else.

Brian

 

Oh I see, you're using an 820R resistor for the LED. As RonH says, it's more common to put this resistor in series with the LED. But with the resistor value you've got there, you'll only get 8.5mA through the LED. That should still light it. Your base resistors still seem very low in value to me though!

Brian

 

By that, do you mean like this?



Can you explain how that makes a difference? Isn't this resistor and this LED still on the same circuit? Does their placement matter?

I'm afraid I don't see why...
V = IR
V = 9 volts
R = 820 ohms
I = V / R = 9 / 820 = 10.9756 mA

What's wrong with the work above?

Can you explain how you can calculate the voltage drop across a single resistor? Particularly in this example?

I'm not sure anymore. I remember I had a reason

Well, what should be my criteria for picking a resistor in this situation? It would seem to me that I should aim to provide enough voltage and current to the resistor base so that current can flow from the collector to the emitter, but at the same time to limit it so as not to burn the transistor. Is there any other criteria applicable? And, I hate to ask for so much work, but can you please take me through this step-by-step? Or alternatively, please provide a link to a good, beginner-friendly article on the matter.


Yeah, sorry, I'm still a complete newb

I rented a copy of Electronics for Dummies at the library, but that book does not make me comfortable with designing my own circuits at all. It has some cool projects, but it does not explain how they work.

I requested the classic (so I heard) Art of Electronics, so I'll have my hands on a copy within a week. By tomorrow, though, I should have Sams Teach Yourself Electronics (I think that's the name) in my hands. It's a nice and thick book, and that's probably a good sign.


Anyways, I'm sure I'll be back with more questions soon. Understanding electricity is hopefully a one-time deal. Once I get it, I won't bug you with such questions anymore

 

Just out of Curiosity, how old are you? If your college bound you can take electronics as your local college (I recommend a community college-cheap and effective)

I have a bunch of electronics book, but nothing has helped me more than a good education. If your not college bound, and are still in middle school, try getting into a local Tech High School.