Intercom Help: 3 pin mic?

Discussion in 'The Projects Forum' started by mjelic, Mar 24, 2008.

  1. mjelic

    Thread Starter Member

    Mar 11, 2008
    21
    0
    Hi Folks,

    I'm looking to make an intercom that works using my network cabling in my house (with patch panel) and so far I have made two kits I have purchased from Jaycar with not much success: One worked really well as a radio :) and the other picked up a lot of hum and crackle, but the voices were not very load at all.

    So, after having a quick search on this forum, I found a link to this project:
    http://www.mitedu.freeserve.co.uk/Circuits/Misc/intercom.htm

    I have a couple of questions before embarking on this:

    1) Do you think this will work over cat6 ethernet cabling throughout a residential house. I would estimate the typical run of cable would be anything up to 20m.

    2) He uses a 3 pin electret mic... I have only ever seen 2 pin and that is all that I seem to be able to find on Farnell or Jaycars website. How would I make this circuit work with a 2 pin mic?

    Thanks in advance of your expertise.

    Cheers,
    Mark
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    1. It might. Use one of the 4 twisted pairs.

    2. Eliminate the line to the 2k2 resistor (and that resistor). If the sensitivity range is ok, you may get good result.
     
  3. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    Ad. 2) The point where the 2k2 resistor goes to the mike (it is for mikes with 3 terminals) should still be connected. On a 2 wire mike, it should be connected between the mike and the capacitor - it is the bias voltage and it won't work without it.
     
  4. mjelic

    Thread Starter Member

    Mar 11, 2008
    21
    0
    >On a 2 wire mike, it should be connected between the mike and the capacitor

    OK, that's easy to do. Thanks for that. :)

    Next question: Why are the capacitors on either side of the relay drawn differently? Are they two different types of capacitor (but the same value) or did he just draw them wrong?

    Lastly: The capacitor before the 8h speaker says it is 1000u... Doesn't that mean it is supposed to be a whole Farad ???!!! Isn't that going to be gigantic? Has he made a mistake with that value, or am I reading it wrong?

    Thanks, as always, for your anticipated help. :)

    Mark
     
  5. Søren

    Senior Member

    Sep 2, 2006
    472
    28
    Hi,

    He made a blunder, (not bad enough to be a real error).
    The left 10µ is an european symbol and the right 10µ is a US-symbol - they're both the same value and all, just two different symbols.


    1F = 1*10^0
    1mF = 1*10^-3
    1µF = 1*10^-6

    Some US based people confuses m and µ, writing mF but meaning microFarad.
    The capacitor before the 8R (8 Ohm) speaker is 1000µ = 1mF = 0.001F.

    1F is quite large, unless it's a SuperCap - the 1F SuperCaps I have is about the size of (perhaps slightly less than) 3 CR2032 lithium primary cells stacked, but then it has a working voltage of max. 5.5V.

    Just look at the caps they sell for automotive stereo - even if you strip the neon tubes and LED's which seems unavoidable today, it's still huge.
     
  6. mjelic

    Thread Starter Member

    Mar 11, 2008
    21
    0
    Ah, thanks. :)

    So I was reading it wrong. :) Thanks for clearing that up.

    And is it important what sort of capacitor I use there? Should it be a greencap or a ceramic? (I never got an understanding what the differences in those capacitors are, except greencaps are prettier ;-) )

    I assume that if the capacitor symbol has a + or - near it, then it should be an electrolytic?

    <lol> Yeah, what is the deal with the neons on those? Wouldn't that make more noise? <lol>

    Thanks for all the help, Soren.

    Mark
     
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