# NewBie needs help

Discussion in 'General Electronics Chat' started by Mic046, Apr 15, 2011.

1. ### Mic046 Thread Starter New Member

Apr 15, 2011
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I am Building a FM transmitter for the first time and have not had much experience in electronics so to some of you this may sound like a dumbass question. So here it goes. In my Schematic it show a capacitor straddleing a coil and one straddeling a transistor do they do this also on the breadboard?
I also have a short video on youtube about a 1:00 min long heres the like let me know what you think.

Mic

I know the FCC rules. just trying to learn something new.

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2. ### BillB3857 Senior Member

Feb 28, 2009
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You need to revise the breadboard layout. At the frequencies being used, lead length is a major item as to whether it will work or not.

Also do a search on this site for "FM Transmitter" . You may find many more suggestions.

Last edited: Apr 15, 2011
3. ### Mic046 Thread Starter New Member

Apr 15, 2011
5
0

Why is it no one can answer this question here or on youtube?

I didn't think it would be that difficult for the experts to answer.

4. ### BillB3857 Senior Member

Feb 28, 2009
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348
Lead length is how long the wires are for the components. You do understand that the frequency is controlled by the amount of inductance and capacitance in the circuit, right! even a straight piece of wire has both inductance within itself and capacitance with whatever it is close to. The way the wire is placed in a circuit makes a difference and the higher the frequency, the bigger the difference.
I guess I don't understand the question of, "... do they do this also on the breadboard?"

5. ### magnet18 Senior Member

Dec 22, 2010
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Generally things work best when as much grey area as possible is eliminated, so if it shows the capacitor straddling a transistor, place the capacitor as close to the transistor as possible with the shortest leads possible so as to eliminate possible inductances and such in the leads.
Also, the lines within the breadboard will act capacitively (or like antennas in some cases) and cause difficulties, I've never built one, but it might give you problems.

6. ### Mic046 Thread Starter New Member

Apr 15, 2011
5
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Yes I understand the inductance and capacitance of the circuit I built it on a breadboard to show what I was talking about and to see if I had the circuit built right to start with then I would transfer to PCB. Sorry for the Gray area Gents. if you look at link you can see what I am talking about.

Does the circuit look right to the schematic??????

7. ### Mic046 Thread Starter New Member

Apr 15, 2011
5
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What area of Indiana Magnet18 owned a 22 Acre Farm in Aurora near Lawerenceburg off of 350 wish I would of never of sold it....

8. ### BillB3857 Senior Member

Feb 28, 2009
2,400
348
The circuit looks right on the schematic. Really can't tell if your breadboard is built that way. There are a lot of un-necessary jumpers being used which not only confuses the issue, but contributes to stray capacitance and inductance.

9. ### Audioguru New Member

Dec 20, 2007
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896
A breadboard can be used to make a 1MHz transmitter.
But it cannot be used to make a 100MHz transmitter because the capacitance between rows of contacts and the inductance of each row and each piece of wire are too high.

A compact pcb is needed. I built my FM transmitter on a carefully planned compact stripboard with all the unused strips disconnected.

10. ### Mic046 Thread Starter New Member

Apr 15, 2011
5
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used jumpers to spread out circuit so it could be viewed better in pic. trying to lean to build from schematics. I understand about the circuit being to far spread out trying to learn to read schematics and making the transfer to a breadboard then to PCB not trying to get this to work on the breadboard. Breadboard is a standard breadboard the two wires you see at the end would be to power supply and the first 22n capacitor is between them.

11. ### Audioguru New Member

Dec 20, 2007
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896
The circuit should be about 2cm x 2cm, not 15cm x 15cm.

12. ### mbxs3 Active Member

Oct 14, 2009
141
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Per your schematic, 1 leg of the 10p capacitor should be tied to the collector of 2N3904. That same leg is also tied to one leg of the 10-40p component and one side of that choke. The other leg of the capacitor should be tied to the emitter of the transistor as well as one side of 330R.

13. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Hello Mic,

As has already been mentioned, breadboards are not very suitable for RF circuits; there are simply too many parasitics involved - and the long jumpers certainly don't help. A 10mm long straight piece of wire has around 14nH inductance; it adds up rapidly - and makes a big difference in RF circuits.

What I'd recommend that you try is the "dead bug" technique, where you epoxy the components to a plain copper PCB with the leads sticking up, and connect them together with the leads trimmed as short as practical. It may take you several times to "get the hang of it".
Some example pages:

Searching Google for "Dead bug wiring" and "point-to-point wiring" will net you a number of "hits".

The instructions on that I find are just a tad ambiguous; but let's roll with it.
The wire they said is 0.5mm, which translates to AWG-24 magnet wire. I can't tell for sure what gauge wire you used. However, I think it's AWG-22.
The diameter of the coil is supposed to be 6mm. The closest English size that works out to is 15/64" (0.2344"). Ideally, you would go to a hardware store and buy a drill that size - or better yet, a 6mm drill, and wind the coil on its' shank. If you can't find one that size, 1/4" (0.250") is the next larger size.

The coil length was not specified, which is a shame. You will need to start out with the coil wound as tightly as possible; ie: no gaps between the turns. If tuning is necessary later, you can use a razor blade or X-acto knife to very gently start spreading the turns apart - a very little bit at a time.

1 turn = one COMPLETE turn.
Scroll down to "The single layer coil" heading. On the right you'll see a single-layer inductor that is rather loosely wound. Note the bends at the ends of the coil for the leads; they make this inductor to be three complete turns.

So, you need one COMPLETE turn, then the antenna tap, and then 8 more turns.

You should start by soldering on the antenna tap to a piece of AWG-24 wire.
Then wind the eight complete turns on the drill shank in one direction as snugly as you can, with the windings as close together as possible. Then wind the other 1 turn on snugly, and leave the leads coming off the inductor as shown in the drawing I linked to.

I'm afraid that I won't be able to help very much more, as I'm about to be offline for what may be quite a while.

Last edited: Apr 16, 2011
14. ### magnet18 Senior Member

Dec 22, 2010
1,232
124
Fort Wayne area, gotta love this bipolar weather, eh?

And just to throw it out there, you should spend a few seconds and check the engrish in your posts... It goes a long ways towards getting helpful answers.

[EDIT]

The site will feel empty without you...
seriously, you handle like 50% of the posts, and contain more answers than google.
Hope you come back soon