To reiterate my suggestion for the circuit about which I made my original comment (last one pictured) it would be insufficient to bias the transistor using a variable resistor and not change the coil connection nect to the "+" side of the battery. You can try the bias idea or not, but you must change the way your coil is connected.

I reiterate this comment so that it doesn't get lost in the background noise. Please ask if you have any further questions ( I mean the TS )

I do. I see now that I had the battery oriented backwards in my picture in post #42. Isn't the picture below what my circuit should look like, or am I still misunderstanding something? (The LED still is not lighting up.)

 

You're still missing something. From what I could tell, your coil connection, the tap between the 1st and 2nd winding was connected to the wrong side of the battery, but everything else was connected to the correct side. You only needed to move one connection. I would help to draw out he circuit you're trying to build.

 

I would help to draw out he circuit you're trying to build.

Here's the schematic:

I've tried to build my circuit to match this as closely as possible. The only differences are that I'm using a light blue LED instead of a white one, and I haven't wired a switch in yet, but neither of those should matter.

 

Turn your battery back around, connect the center tap of the coil to the "+" terminal, connect the transistor emitter to ground. Make sure your LED is connected correctly (which lead is the cathode, anode?) I can't tell from your picture if the transistor and LED are correct. You're gonna need to really get that right or somehow get a better picture.

PS: if you reposition the transistor and base resistor to more clearly show both of them in the picture, I can better tell if they are connected correctly. You may also test the transistor/LED if you have at least one more battery. I can tell you later how, gotta run.

 

connect the center tap of the coil to the "+" terminal

It is connected that way in the picture in post #81, right?

Turn your battery back around

I don't understand how that would help if I already have it wired correctly.

connect the transistor emitter to ground.

Sorry for my ignorance, but where is "ground" in this circuit?

Make sure your LED is connected correctly (which lead is the cathode, anode?) I can't tell from your picture if the transistor and LED are correct. You're gonna need to really get that right or somehow get a better picture.

I'll try to get a better picture. Thanks for your help.

 

I disassembled and reassembled the entire circuit. In doing so, I discovered that I was using a 10Ω resistor instead of a 10kΩ one , and I think my transistor was connected wrong. But after fixing both those issues, the circuit still doesn't work. I need to go now too, so I'll try posting a better picture when I get back.

EDIT: My multimeter showed that the output voltage (measured at the LED) is 0.7V less than the input voltage (measured at the battery). Might this have something to do with transistor biasing?

 

I've uploaded an updated photo which is hopefully clearer. Please let me know if you need more details.

 

There appears to be a wire from the "-" side of the battery to the 1K resistor. I'm not sure of that, but if it is, it shouldn't be there.

Ah, never mind. Looks like a stray wire that isn't connected.

 

There appears to be a wire from the "-" side of the battery to the 1K resistor. I'm not sure of that, but if it is, it shouldn't be there.

Ah, never mind. Looks like a stray wire that isn't connected.

Yes, sorry about that. It's just the end of the wire connected to the negative battery terminal, but I did notice that it looked somewhat confusing after taking the picture. Does anything else look wrong?

 

That actually looks pretty good. Are you sure the LED is conected right way around?

 

That actually looks pretty good. Are you sure the LED is conected right way around?

I think so. The longer lead is positive, so the shorter one should be closer to the negative terminal of the battery, correct?

I believe I've come to the same conclusion that I did in post #67:

I think the problem is likely the poor quality of my inductor.

@Brownout: Thanks for all of your help.

 

tj......

I can't tell if the circuit is good by the picture.......it's my eyes.

But if it's like Dick's print and still not working.........use smaller wire
size for coil.

Use many more turns and keep coil length under 2 inches.

I believe that you are not generating and cutting enough flux.

Look how many turns Dick has on his nail.

 

tj......

I can't tell if the circuit is good by the picture.......it's my eyes.

But if it's like Dick's print and still not working.........use smaller wire
size for coil.

Use many more turns and keep coil length under 2 inches.

I believe that you are not generating and cutting enough flux.

Look how many turns Dick has on his nail.

If that's the case, I'll need to get some more wire. I still think this seems odd:

My multimeter showed that the output voltage (measured at the LED) is 0.7V less than the input voltage (measured at the battery). Might this have something to do with transistor biasing?

 

I'm looking at your coil, and it looks like the secondary is wound in the wrong direction. I may be wrong, but when I apply the right hand rule to the primary and secondary currents, I get an incorrect winding direction for the secondary.

 

I'm looking at your coil, and it looks like the secondary is wound in the wrong direction. I may be wrong, but when I apply the right hand rule to the primary and secondary currents, I get an incorrect winding direction for the secondary.

I tried winding the secondary in the opposite direction of the primary, and now I get an output voltage reading of 0 V. I'm not sure if that's better or worse than before.

I now have the primary wound around the nail counter-clockwise and the secondary clockwise (from the perspective of looking at the point of the nail, not the head). I guess I should research how the right hand rule applies to inductor coils.

 

I now have the primary wound around the nail counter-clockwise and the secondary clockwise (from the perspective of looking at the point of the nail, not the head). I guess I should research how the right hand rule applies to inductor coils.

That's how it looked to me you had it before, in your last picture.

 

That's how it looked to me you had it before, in your last picture.

No, previously they were both wound counter-clockwise (again from the perspective of the nail point). Is that how they should be wound? Would it make any difference to wind them both clockwise?

 

I'm sorry, I'm just not seeing that in your picture. I see the primary wound CCW and the secondary wound CW.

 

When I say the primary wound CCW, I mean starting from the end that connects to the transistor collector. When I say secondary wound CW I mean starting from the center tap. I think you started winding the primary CCW, and then reversed the direction when starting the secondary.

 

When I say the primary wound CCW, I mean starting from the end that connects to the transistor collector. When I say secondary wound CW I mean starting from the center tap. I think you started winding the primary CCW, and then reversed the direction when starting the secondary.

No, the primary and secondary were both wound in the same direction: