555 monostable multivibrator circuit

Discussion in 'The Projects Forum' started by steven334, Aug 22, 2011.

  1. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
    I am trying to build a 555 one shot monostable multivibrator circuit, to sound a buzzer for about 2 second when a door opens and closes a N.O. (normally open) switch.
    It does not work there is no sound when I close the switch.
    Can you help me find my problem?
    Thank you
    steven
     
  2. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
    drawing attached
    thanks steven
     
  3. Wendy

    Moderator

    Mar 24, 2008
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  4. Sparky49

    Active Member

    Jul 16, 2011
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    Hi there Steven,

    it looks like you haven't used enough resistors. Either to protect the 555 timer or the piezo, they aren't there.

    I've attached a schematic which should help you.

    Make sure that the piezo is the correct way 'round as well.

    Sparky


    P.S. I'm being a little nitpicky, but try to draw your schematics 'correctly'. Remember to show connections with a dot, and try to set everything out 'logically'. Keep up the good work!:)

    P.P.S. I really should learn to draw these things quicker, you're too fast Bill! ;-)
     
  5. steven334

    Thread Starter Member

    Aug 9, 2011
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    ok R1 is 10k ohm ,,R2 is 10k ohm,,
    whats the value of R3,R4 and C1

    thank and i'll do better on my next schematic.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Hello Steven,
    One big problem is the red jumper that you have between the 555 pin 2 and pin 4. When your switch closes, you not only pull the trigger to ground, you also pull the reset input to ground. The reset input being grounded prevents the circuit from working.
    Un-plug the end of that jumper that is connected to pin 2, and connect that same end to pin 8; that way the reset pin 4 will always have Vcc on it.

    Sparky indicated you need a 10k resistor from the reset pin to Vcc, but unless you are otherwise planning on using the reset input, you can simply connect it to pin 8.

    The other big problem is that you have your power rails marked +Vcc and -Ground, but you have the red lead from the battery pack plugged into the -Ground bus, and the black lead connected to the +Vcc bus. Unless you have re-wired the battery pack itself, the red lead is positive, and the black lead is negative. Connecting the circuit in reverse polarity will likely destroy the 555 timer and the electrolytic capacitor in very short order. However, try correcting the battery connections and see if it works; you might be lucky, as you are only using 6v for the supply.

    As a side note, I also have one of those breadboards that I bought many years ago. One quirk about those boards is that the power rails at the top and bottom are not connected across the wide break in the center. I have put jumpers across those breaks, and leave them in there permanently.
     
  7. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
  8. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
    i tried this but still no sound.
     
  9. JingleJoe

    Member

    Jul 23, 2011
    185
    10
    As far as I can tell, that circuit diagram is wrong; pin 4 is the reset input and pin 2, the trigger. The circuit is being both triggered and reset by the same pulse. Also those inputs are "active low", meaning they should ordinarily be connected to +V and to ground the trigger them. So the diagram's representation of a positive trigger and pin 4 is incorrect.
    To clarify, pin 4 should connect to +V for regular operation.


    Additionally one could use the monostable to activate a 555 astable; astable 555 circuits can be activated and deactivated using pin 4, as you may have gathered from Sgt Wookie, connect it to ground to stop the circuit and +V to activate it, this can be done with a resistor between pin 4 and ground and a switch or npn transistor between pin 4 and +V.


    Just to add, your latest circuit diagram seems like ti should be working, although the non-standard positingion of ground and +V have thrown me off a little :p
     
  10. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Did you make certain that your battery was originally connected properly by using a multimeter or an LED with a resistor?

    I don't see a switch in your circuit.
     
  11. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
    I tried this “one shot” circuit design and the buzzer is buzzing constantly (switch open or closed).
    Also the 555 is running hot.
    (Sgtwookie you were right and I did correct the batterie)
    Thanks
    steven
     
  12. JimWright

    New Member

    Aug 14, 2011
    8
    0
    If the power was wired backwards, you may have damaged the 555, which would explain it running hot. You might want to pick up another.
     
  13. Sparky49

    Active Member

    Jul 16, 2011
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    417
    I'd buy a few more, they are as cheap as chips and having a few spare saves you having to buy more every time something goes wrong, which is often.:D
     
  14. steven334

    Thread Starter Member

    Aug 9, 2011
    37
    0
    thank work great with new 555.
     
    Last edited: Aug 23, 2011
  15. JimWright

    New Member

    Aug 14, 2011
    8
    0
    Glad to hear it. I was doing a circuit recently and had a chip plugged in backwards, by the time I noticed it was too late. Never even got hot, just plain died, but I picked up more than I needed. Like Sparky said, plan on things going wrong.
     
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