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FM Transmitter?

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Nigel Goodwin

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Most Helpful Member
nerosrevenge said:
BC109C Transistor I was able to get BC107b.
4.7pF Capacitor. No luck got a 4pF and a 5pF. Will either do?

A BC107 will be fine, a BC109 is simply a higher gain, lower noise version, intended for high quality microphone and tape preamplifiers.

The capacitor isn't at all critical, I've often made bugs like this and simply twisted two pieces of insulated wire together for the feedback capacitor - about half an inch long, if you wanted to know.

Also I want to add an on/off toggle switch and an LED to indicate switch position. Green being on. I assume I will have to use a resistor ahead of the LED as a 9v battery will likely pop it. Here's the problem, I don't know the rating of the LED. I am pretty sure I need that to calculate the resistor I require correct? Any suggestions (short of going back to the store and looking it up among the several hundred LED's they have)? :shock:

I also got a proto board to make sure it all works.

LED current limiting resistors are easy, it's just ohms law - V=I*R. Rearrange it to give R=V/I, and fill in the blanks. You have a 9V battery, and the LED drops about 2V (2 is a nice easy figure!), so the voltage dropped across the resistor is 7V. Then decide on your current, it's a compromise between brightness and current consumption - lets say 10mA.

So we have 7/0.01 ohms, or 7/10 kohms - so 680 ohms would do nicely, but so would anywhere near - 560, 470, 820, 1K - it's not at all critical.

A proto-board probably isn't a good idea, this is a VHF oscillator, the stray capacitance in a proto-board could well upset things enough to stop it working at all - or at least shift it outside the VHF radio band.

I do have another question though. How do I tune the transmiting frequency of the circuit?

Simply move the windings of the coil apart or together, it's a lot easier to do than it sounds.
 

panic mode

Well-Known Member
Yep,

he is right :D
 

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nerosrevenge

New Member
I had intended to use a printed circuit board for this project. I got the proto board to test it out. If it's a bad idead I won't use it. Maybe I'll save for the voltage reg. project.

I have completed the layout for the PCB and was wondering if you guys wouldn't mind looking at it to see if I have it right. This is my first go at it so I may have misread the circuit diagram or something.

here it is:
114751575DBHdMa_ph-1.jpg


I haven't added the LED. I also forgot to mark the + and - inputs. + is on the left of the diagram.

Thanks a million! :D

Steve
 

nerosrevenge

New Member
I had intended to use a printed circuit board for this project. I got the proto board to test it out. If it's a bad idead I won't use it. Maybe I'll save for the voltage reg. project.

I have completed the layout for the PCB and was wondering if you guys wouldn't mind looking at it to see if I have it right. This is my first go at it so I may have misread the circuit diagram or something.

here it is:
114751575DBHdMa_ph-1.jpg


I haven't added the LED. I also forgot to mark the + and - inputs. + is on the left of the diagram.

Thanks a million! :D

Steve

Addition - I was just going through the parts I picked up and I fear I may gotten a wrong capacitor. The drawing calls for a 22nF and I think I have a 2.2nF (if I'm reading it correctly) the code is 222J which is 2200 tolerance of +/-5%. Is that right?? From indications thusfar it would seem that certain variances in the parts are ok. I hope this one of them!

Bye for now.
 

vaineo

New Member
I'm not sure if you've stopped checking this, but a 222 is a 2.2nF. If you want to try it, I have a little program on my site that will convert the valuse back and forth and you can enter something like 222 and it'll give you a value.

If you want to check it out, its called Capacitor Converter, my site is https://www.misdomain.vzz.net/ just goto the downloads section.
 

nerosrevenge

New Member
Thanks so much vaineo. I guess I will have to head back to the shop and get a different one. I'm glad they're only like 80 cents! :D
 
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