NE555 with piezo trigger

Discussion in 'The Projects Forum' started by tracecom, Feb 3, 2013.

  1. tracecom

    Thread Starter AAC Fanatic!

    Apr 16, 2010
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    The circuit below was posted in another thread, and is apparently a popular "project" for engineering students in India. Although the shematic is quite ubiquitous, it may have come from the attached report.

    I have no interest in building the complete project, but I have breadboarded the 555 portion as drawn, including the piezo trigger. My understanding of the claim made in the report is that the 555 latches high on the first trigger pulse from the piezo, but my breadboard does not do that. When I put pressure on the piezo, pin 3 of the 555 goes high, but does not latch high. As soon as the pressure on the piezo is relieved, the output of the 555 goes low.

    My question is whether the schematic does have a "latch" function designed in (which means I have a construction error) or not; if it does, would you please explain the latch function?

    Thanks.

    [​IMG]
     
  2. Audioguru

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    Dec 20, 2007
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    S2 conducts most of the time. Then C1 cannot charge to 2/3rds of the supply to make the output go back low.

    When the Mosfet conducts then the 555 is triggered and makes the output go high. It latches high until the pressure on the piezo is released and when S2 is opened. Then R3 charges C1 higher than 2/3rds of the supply and reset pin 6 resets the output to be low.

    Their poor English says Earth Quack! Hee, hee.
    Quack, quack is the sound from a duck, not from an earth quake.
     
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  3. tracecom

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    Thanks. I guess I have a construction error.

    What is the purpose of R4 and C5?
     
  4. praondevou

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    Jul 9, 2011
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    They hold the RESET pin low (and therefore disables the output) for a certain amount of time when closing S1.

    Very useful for other circuits too.
     
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  5. tubeguy

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    Nov 3, 2012
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    Make sure the voltage at pins 2/6 is sitting at half the supply voltage until the piezo is tapped.
     
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  6. tracecom

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    It wasn't. I had picked up a 1k instead of a 10k for R3.

    Thanks to all; I have learned something new about 555s.

    Here is a photo of my breadboard for the benefit of anyone who lands on this thread. Notice that I omitted S2 because I didn't have a NC pushbutton, and I "doctored" R3 to make it look like the 10k it was supposed to be.

    ETA: One final question: I removed C1 and the circuit continues to work. How can that be?
     
    Last edited: Feb 3, 2013
  7. tubeguy

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    @ETA: One final question: I removed C1 and the circuit continues to work. How can that be?

    I think that's basically for noise filtering.

    I learned something new too. Interesting way to use the trigger.
     
  8. tracecom

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    Thanks. I was actually wondering if it was a decoupling cap; that is why I removed it. Apparently, there's some capacitance inside the 555 that performs the function described in post 2.
     
  9. #12

    Expert

    Nov 30, 2010
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    C1 stops the internal voltage divider from picking up electrical noise. Your use of 1/2 VCC for your center point allows a lot of room for noise before it interferes with anything. That's why C1 can be omitted in this case.
     
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  10. tubeguy

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    Here's a block diagram:
    [​IMG]
     
  11. GRNDPNDR

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    Mar 1, 2012
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    Sorry, meant to start new thread
     
  12. tracecom

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    I have redrawn the portion of the schematic that interests me, and in the process, changed some of the component numbers. The new schematic is attached.

    I also looked for a pcb mount normally closed pushbutton switch to use for SW2 and didn't find anything. (SW2 is used to unlatch the 555.)

    It seems that the same reset could be accomplished with a normally open push button from ground to pin 4 of the 555, or a normally open pushbutton switch in parallel with R2 would also work. Maybe the switch in parallel with R2 should have a 1k current limiting resistor?

    Will either or both of these solutions be appropriate? Thanks.
     
    Last edited: Feb 3, 2013
  13. Audioguru

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    Dec 20, 2007
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    In your new schematic, pin 8 is not connected to the positive supply voltage.
     
  14. tracecom

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    Then, I guess I won't have to worry about resetting the unit, will I? :D

    Thanks. I fixed it.
     
    Last edited: Feb 3, 2013
  15. tracecom

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    Here's the last schematic for this circuit. As you may see, I did away with the NC pushbutton switch in favor of a NO pushbutton. The notes on the schematic tell the story.

    Thanks to all who helped with this. Additional comments are welcome.
     
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