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Transistor question

  1. Mar 21, 2004 #1
    I want to build this circuit http://www.uoguelph.ca/~antoon/circ/fmt1.htm. [Broken]

    It requires transistor 2N3904. Is this any NPN transistor or do I have to get a specific one? I have some transistor but it just labeled NPN (from science kit). Can I used that?
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. Mar 21, 2004 #2


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    I believe 2N3904 transistors can be found at radio shack. Your link does not work for me so I cannot tell you anything more specific. The 2N3904 is a VERY generic and common transistor. More than likely it could be substituted with about any NPN transistor as long as you don't exceed the maximum current and emitter base reverse breakdown voltage. A 2N2222 is a likely match if you cannot find the 3904.
  4. Mar 21, 2004 #3
    You should have a 2N2222 in your kit, I know we do. :smile:

    Averagesupernova: remove the dot at the end of the URL: http://www.uoguelph.ca/~antoon/circ/fmt1.htm [Broken]
    Last edited by a moderator: May 1, 2017
  5. Mar 21, 2004 #4


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    Who are you going to be spying on?
  6. Mar 21, 2004 #5
    nobody:wink: . It is just good to make something other than blinking lights from the science kit. uh.:smile:
  7. Mar 21, 2004 #6
    I wired up the circuit and it didn't work :(.

    Would it make any different if I got a condenser microphone instead of an electret microphone?

    As for the coil (L1), is it necessary to have a pencil in the coil?

    For C5, I used a ceramic cap that has .473 on it. Could this be why it doesn't work?

    I don't really know how this circuit works so my troubleshooting is sh**. I just follow direction.
    Last edited: Mar 21, 2004
  8. Mar 21, 2004 #7


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    Typically a condenser mic IS an electret mic. One of the problems is definitely that you have the wrong value for C5. It's value is supposed to be 4.7 picofarad. What you used I would guess would be either a .47 microfarad or a .047 microfarad. Are you sure there is a decimal point in front of the 473?

    Usually, the 473 means this: a 4, then a 7, then 3 zeros. This is the value in picofarads. 47,000 pF, which is .047 uF. A LONG way from 4.7 pF.

    You will also find that the placing of leads and there length will have a large factor on how this thing tunes. Just the placement of your hand when tuning the variable trimmer capacitor will change the tuning.

    Good luck. You will need it. Trust me, I've been there.
  9. Mar 21, 2004 #8
    yes there is a ".". It saids .473 on the Cap. So what is .473?

    The period throws me off.

    So as for the tunning, the lower the var. cap, the lower the freq. that is going to be broadcasted on FM radio?

    What is the different between different types of NPN transistors? U know like 2n3906 and 2n2222 ...etc
    Last edited: Mar 21, 2004
  10. Mar 22, 2004 #9
    I bought some 2n3904 and 5pf cap and installed it and it worked. The tunining is unstable though. I am able to tune it using radio that has a knob but not a digital one. why? Anyway, it is too unstable to be useful. It supposily have a range of 1/4 mile but it wasn't the case. Am i doing something worng? How do I make the tuning more stable and increase the range?
  11. Mar 23, 2004 #10


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    I really don't want to tell you this, but it is not a surprise that it didn't work out for you. Generating a stable signal around 100 Mhz is no easy task. The circuit you built is full of design flaws. The first being that the antenna is directly coupled to the main oscillator. If you take some electronics classes you will find out about coupling and isolation. The oscillator is definitely NOT isolated from the antenna in this case. In my opinion about all this circuit does is show how a simple RF oscillator can be made. It is in no way useful on its own.
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