Buzzer vodoo

Discussion in 'General Electronics Chat' started by Chillum, Feb 4, 2015.

  1. Chillum

    Thread Starter Active Member

    Nov 13, 2014
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    Hi,

    I'm using a strong bar magnet the size of those white plastic boxes with small magnet inside for the reed switch. First I need to explain, we leave the security gate open during the day, and have a piezo buzzer 555 system with magnetic reed switch to trigger it, to alert us when the gate is opened or closed... and it works beautifully; but the gods do not favour us, every 1 to 2 hours or so we get a false positive, and it can't be the magnet, it's way above specs, and when it was still in breadboard form we wrote off the false positives as bad connection on the trigger capacitor, but now (buzzer7.mees.intuitief) I put it on vero (strip) board, so it can't be a bad connection on the vero board side, and it can't be a bad magnetic connection, so why do the gods not favour us with false positives? Any ideas?

    PS buzzer7.pdf is the scematic for this project (my first)
     
  2. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,517
    555s are prone to glitches if you don't place a strong capacitor between their Vcc and ground pins. I'd suggest one with a value of at least 10 or 22µF
     
  3. Reloadron

    Active Member

    Jan 15, 2015
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    Why does it look like your cap for debounce is installed with the (+) to ground? Is that just a typo?

    Ron
     
  4. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,517
    BTW, reed switches are quite sensitive to vibration. If your gate is not firmly anchored and the wind (or some other influence) makes it vibrate, then it might trigger false positives. You could replace the reed switch with a hall sensor and that would pretty much solve things, if that's what's happening.
     
  5. Chillum

    Thread Starter Active Member

    Nov 13, 2014
    546
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    its typo in buzzer7.pdf, you'll see it's correct on the veroboard mask (buzzer7.mees.intuitief.pdf)
     
  6. Chillum

    Thread Starter Active Member

    Nov 13, 2014
    546
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    Hall sensor? It's a very strong magnet, I shaked the gate vigorously and no false positive hope its the filter cap, I used 22uF, is that enough?
     
  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Maybe your cap is connected incorrectly. It should be connected between R2 and pin 2, and not between the switch and pin 2.
    Check the figure on the right:

    Mono-Stable-One-Short-with-555-timer-circuit.jpg

    EDIT: You can find the circuit's explanation here.
     
    Last edited: Feb 4, 2015
  8. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,517
    What might be happening is that you're leaving the cap's negative "floating" and it's charging over time with electric static, until it triggers the 555
     
  9. cmartinez

    AAC Fanatic!

    Jan 17, 2007
    3,555
    2,517
    Check this other configuration, it clearly shows you can't just leave one of your cap's legs floating:
    555-T.gif
     
  10. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    BTW, that last circuit toggles the relay, it's just an example.
    If you implement these changes, please let me know how it went... I'm curious... :D
     
    Last edited: Feb 4, 2015
  11. Chillum

    Thread Starter Active Member

    Nov 13, 2014
    546
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    Check my veecad file please. There shouldn't be any dangling capacitor ends to my mind.
     
  12. Chillum

    Thread Starter Active Member

    Nov 13, 2014
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    Hopefully I won't need to but if I have to I will implement it per your sugesstion, thanks!
     
  13. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    You need to change the cap's connection as I told you i post #7, otherwise I doubt you will get your problem fixed...
     
  14. Chillum

    Thread Starter Active Member

    Nov 13, 2014
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    but its the same, except for the extra resistors and diode
    edit: are you referring to the 01uF cap, the one between the resistors, do I need those?
     
  15. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    I'm saying that your circuit should look like this:

    Capture.JPG
     
  16. Chillum

    Thread Starter Active Member

    Nov 13, 2014
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    If I'm going to redo this board, which design would you recommend as best?
     
  17. Chillum

    Thread Starter Active Member

    Nov 13, 2014
    546
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    Oh OK, thx!
     
  18. Chillum

    Thread Starter Active Member

    Nov 13, 2014
    546
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    On the veecad layout the polarity of CsinglePulse is correct, this is how it is on my vero board (buzzer8.1.pdf)... do I still need to move that wire (The polarity was *sigh* a typo, but I wired the polarity correctly on VeeCAD -- veroboard cad for veroboard masks)
     
  19. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Yes you do... I didn't correct the polarity when I posted that last image
     
  20. Chillum

    Thread Starter Active Member

    Nov 13, 2014
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    but then the negative of the electrolytic goes straight to live? (well through R2, but it goes to live)
     
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