selecting speaker output transformer

Discussion in 'The Projects Forum' started by ungar, Jun 20, 2013.

  1. ungar

    Thread Starter New Member

    Apr 25, 2013
    2
    0
    I'm building this simple circuit ...

    http://www.eleccircuit.com/wp-content/uploads/2008/04/annoying-high-pitch-noise-generator-by-555.jpg

    taken from this page (near bottom) ...

    http://www.eleccircuit.com/555-timer-audio-alarm-circuits/

    I have it on a breadboard and it works fine -- except I have not yet purchased the speaker transformer and speaker.

    The schematic says only "miniature audio transformer" and it appears to be a step-down type but how do I choose the specs for the transformer?

    Would it depend on the speaker chosen? If so, how do I determine what transformer I would need?

    Any help would be appreciated -- thanks.
     
  2. #12

    Expert

    Nov 30, 2010
    16,261
    6,768
    A crystal speaker is very high impedance. Probably a piezo type. The transistor has a 4.7 k resistor on the base, so we can call the transistor 470 ohms. I remember audio transformers for modems rated at 600 ohms. That might work if it has a higher impedance winding you could use for an output.

    Warning: This is only estimating for a project that is not life threatening if it fails. Start with this and see how it goes...unless somebody corrects me and says a crystal speaker is low impedance.
     
  3. Ramussons

    Active Member

    May 3, 2013
    557
    92
    Why a transformer coupling for a crystal speaker?

    This should work(replace the transformer, do as per RED)

    Ramesh
     
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  4. ungar

    Thread Starter New Member

    Apr 25, 2013
    2
    0
    ^ Thanks! I will try it.
     
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