How do i interpret this schematic?

Discussion in 'General Electronics Chat' started by Cretin, Apr 10, 2013.

  1. Cretin

    Thread Starter Member

    Dec 13, 2012
    68
    1
    Hey all take a look, i need to design a sound activated alarm sensor using a microphone...this seems like it would do the trick (and it has to use an lm741 btw).

    However...if you notice on the right hand side, the microphone trails off (with an indicator arrow) does this mean that i should connect it to ground or VCC?

    Also, what do i connect the speaker to? It also has that trailing indicator arrow.

    [​IMG]
     
  2. YokoTsuno

    Member

    Jan 1, 2013
    41
    9
    The schematic drawing has several errors.

    - 0-100V must be 0-100mV instead.
    - The 10K resistor at the output of the Opamp has to be connected to the gate of the SCR. An SCR has 3 terminals instead of the two shown:

    http://en.wikipedia.org/wiki/Silicon-controlled_rectifier

    - One side of the speaker (used as microphone, connected to the + input of the Opamp) is to be connected to the GND.
    - One side of the buzzer (A speaker won't work here) should be connected to the +15VDC

    The concept seems primitive. It might nevertheless work well though.
    - You should consider adding a potentiometer to have a bit of control over the sensitivity.
     
  3. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Maybe something like this:

    Edit: What he said, except the label is 0-10V
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,415
    3,354
    There are many things wrong with that circuit.

    1) The 741 op amp is a poor choice. This op amp requires a dual power supply. You might be better to use a single supply op amp such as an LM358.

    2) Your op amp has no feedback and hence you are using it with open-loop gain which is like 100,000. You should limit the gain to about 300-500.

    3) Most miniature microphones are of the electret type. These need to be biased properly with about a 1kΩ pullup resistor.

    4) The 1kΩ resistor in series with the SCR does not belong there. This is going to severely limit the current to the Sonalert to the point where it wouldn't work.

    5) You need to trigger the GATE of the SCR.
     
  5. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    He isn't using a microphone. He's using a speaker for a microphone.
    Not likely it will ever get to 10 volts, but that's what the sensitivity adjustment is for.
     
  6. Cretin

    Thread Starter Member

    Dec 13, 2012
    68
    1
    Thanks for your assistance everyone, if i wanted to find another similar circuit, what are the key words i can search for?
     
  7. YokoTsuno

    Member

    Jan 1, 2013
    41
    9
    Nice editing.

    What makes you think it's a he :D?

    http://en.wikipedia.org/wiki/Yoko_Tsuno

    Unless I have eyesight problems or a serious drug addiction, I still read 0-100 :rolleyes:.
     
    #12 likes this.
  8. #12

    Expert

    Nov 30, 2010
    16,248
    6,745
    Got it. 0-10 volts times 1k/101k = 99mv.

    It's a sound activated alarm. That's my first guess at googling.

    Edit: Welcome, female person.
     
    Last edited: Apr 10, 2013
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