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Building a noise detecting circuit

  1. May 28, 2005 #1
    Hey,
    I never used to like electronics, but after doing a unit last semester it has really interested me.
    I've gone out and bought simple circuit components and have built a whole heap of simple circuits just because I wanted to learn more of how things behave.
    Anyway I still have the notes from that unit I did a while ago and found a pretty tough situation and wanted to at least be able to draw the circuit before getting the components (if I can ).
    I am pretty useless at electronics, but it's fun nonetheless and I would like to learn more.

    This is what I'm wanting to build:

    A circuit that will switch on a tape recorder when the input sound level is above a certain threshold and record for a preset amount of time (adjustable in the range 1 to 10 minutes). Include a filter to stop the recorder being triggered by 100 Hz wind noise. And you use batteries for this.

    So anyway I was thinking that you'd need a Microphone, maybe 12 V battery supply, A high-pass active filter, timing device, combinations of resistors and capacitors and this isn't a necessity but it'd be pretty cool to have an LED to let you know that it's recording or not.

    I know I'm probably missing some components that I'd need so I was hoping to find out what components and what exactly those components are doing exactly.

    I'm probably in way over my head with this one, but I'm determined to build it to satisfy myself that I'm not a complete retard haha :)
     
  2. jcsd
  3. May 29, 2005 #2
    Can someone tell me if this is along the right lines or not?
    I've attached a drawing of the circuit I've contructed for the above situation so far, but I don't know if all or any of the parts are correct really.
    Really appreciate any feedback.
    Thanks
     

    Attached Files:

  4. May 29, 2005 #3

    dlgoff

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    Well it really good that you are interested enough to experiment. For your 1 to 10 minute timer, you should consider the 555 timer integrated circuit. Look at the Monostable circuits here: 555 timer
    Regards
    Don
     
  5. May 29, 2005 #4
    help .. my file associations are messed up ..
    whenever i try to look at a .doc file the graphics never come out...
    i'm using XP ..
    thanks in advance..
     
  6. May 29, 2005 #5
    yeah I have the 555 timer in my circuit and I'm using a variable resistor to alter the length it allows the pulse through for.
    But like I said I think I'm missing some other vital components and I'm not sure if I've connected everything correctly.
    Thanks for the link by the way :)
     
  7. May 29, 2005 #6
    Sorry I don't know much about this, but I've uplpoaded the image here so you can look at it this way.

    http://nucite.net/isWext.php/4426/60351.jpg
     
  8. May 29, 2005 #7

    dlgoff

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    This doesn't work for me.
    I haven't been able to see the circuit.
     
  9. May 29, 2005 #8
    sorry about that
    Try right clicking on it and saving it to disk that works for me so it might work.
    so the word attachment isn't working at all??
     
  10. May 29, 2005 #9
    thanks Soilwork, that works for me...
     
  11. May 29, 2005 #10

    dlgoff

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    Your first op-amp will give you amplification but not high pass filtering.
     
  12. May 29, 2005 #11
    Ahh wait I need a capacitor there before R1 for it to act as a high pass filter don't I??
     
  13. May 29, 2005 #12

    dlgoff

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    You can use a passive RC circuit or an active low-pass filter with an op-amp. Check out the last circuit on page 15 of the .PDF data sheet.
    LM2900 ... data sheet
    Regards
    Don
     
  14. May 29, 2005 #13
    That high-pass filter is a lot different to the one that was given in the notes I have from that unit I did.
    Thanks for that though :)
     
  15. May 29, 2005 #14

    dlgoff

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    Using a quad op-amp (4 amps in a chip) make it ideal for your application. I like the LM324 quad amp. If you want a LED to see when it's on, just put it in series with a resistor on the 555 output.

    Regards
     
  16. May 29, 2005 #15
    what are you saying I would use the quad-amp for??
    Sorry for asking so many questions, but I'm just trying to understand this stuff :(
     
  17. May 29, 2005 #16

    dlgoff

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    Well I would probably use one amp as a mic pre-amp coupled to a op-amp high pass filter. Okay maybe just two amp for your application.

    Here's the LM124 series data and application sheet
    You can use these application circuits to make all kinds of analog devices. I once made a polyphonic music synthesizer.

    Regards
     
  18. May 29, 2005 #17
    You can't just run an analog signal into the 555. You need to break down into block diagrams and then make a simple schematic for each one. Then combine, then simplify where you can. You need a sound 'detector' block. You have a good start with a mic and amplifier stage. You want to add some high and low pass filtering to make the detector immune to things you don't want to trip the detector. These could be considered blocks within the blocks Then you need the timer block. I assume you want to make the recorder go as long as there is noise and for an adjustable amount of time after the noise stops. Or do you want a noise to trigger it and record for a preset amount of time and then stop regardless of whether the noise is still there or not? These are things that need to be decided before we start drawing schematics. Then there is the output block. A pretty simple part hardly worth mentioning. Basically a switch.
     
    Last edited: May 29, 2005
  19. May 29, 2005 #18

    dlgoff

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    "You can't just run an analog signal into the 555."

    Why not?

    "Then there is the output block."

    His looks good to me.
     
  20. May 29, 2005 #19
    If you look at the datasheet, the trigger pin of the 555 is a comparator checking if the trigger voltage goes above a certain percentage of the 555's power supply. So actually, if the analog signal is scaled correctly, and you want the 555 to trigger whenever the analog signal goes above a certain value, then you can connect the output of an op-amp directly to the 555. :-)

    The way he has it drawn, the transistor is connecting ground to ground. It needs to be connected between the power supply and the recorder instead. I would bet that just powering up the recorder wouldn't do what you want, anyway, since there would be a significant delay while it started up. You'd probably want to turn the tape motor itself on and off.
     
  21. May 29, 2005 #20
    (By the way, is there a built-in method for drawing diagrams, like there is for TeX math?

    I've used klunky for web diagrams in the past, and made my own version (it is public domain) with more symbols and such. It would be really convenient if something like this was built right into a forum like this, though. Save the hassle of screenshots and saving and uploading and so on.)
     
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