Piezo Switch

Discussion in 'The Projects Forum' started by PulseLED, Feb 24, 2014.

  1. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    So what I have a need for is a piezo switch to trigger a camera flash, I've made the following:

    http://imgur.com/xASR1u1

    Which is working well a tap with my finger nail fires the flash, but it isn't quite sensitive enough. I'm wondering if there's any suggestions as to increasing the sensitivity or preferably having variable sensitivity. A hard poke on the piezo generates about 30v.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    yea u need an variable voltage amp between piezo and transistor
    Or you can bias the transistor to increase sensitivity
     
  3. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    So would a simple audio amp do the job?
    I'm not sure what biasing the transistor means, but is there any more suitable transistor then the 2n2222 that would increase sensitivity?
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Hmmm ! draw a better circuit can you ?

    It will be same for all transistors unless a darlington but still you would need more voltage at base to trigger it.

    You see the signal from the piezo is not enough for the transistor. It needs more to saturate.

    To vary sensitivity you need a simple voltage amp. like mic preamps might just do the trick
     
  5. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    I'll take a photo tomorrow. In the pic the transistor is from left to right C B E, the rest should make sense, red being +. It works well, I just want it to be touch sensitive rather then tap sensitive :p

    I just had a possibly silly idea, but couldn't I just add a battery through a potentiometer connected to the base of transistor so I could tune the current to just below its trigger voltage, then use the current from the piezo to push it over the edge?

    Its so simple already, I don't want to over complicate things.
     
  6. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Adding a battery like you said is like biasing.
    Two resistor will also do the same, less expensive and much simpler too.

    To be touch sensitive u need to google for capacitive touch plate switches.
     
  7. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    I've done it, I just bridged the positive wire from the camera flash (4.12V) through a 100kohm resistor onto the base of transistor and its working great, any tap on the desk of which the piezo is sitting fires the flash and that is what I wanted, it sitting on a hair trigger.

    I should have said vibration sensitive not "touch sensitive" so a capacitive plate wasn't an option. Thanks a lot though R!f@@ you put me on the right track.

    So I can correctly define what I just did as "biasing" the transistor?
     
  8. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    hmmm !

    What you did was a form of biasing.
    It's good that you figured it out on ur own.
    Helps to learn.
     
  9. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    Here's the finished product.

    First trigger with the 100kohm resistor

    http://i.imgur.com/DYTuOhv.jpg

    And second where I substituted it with a 47kohm resistor and 100kohm pot to give the variable sensitivity. Mounted in polycarbonate sheet, now when something is sat on the sensor and shot the flash will capture an image of it.

    http://i.imgur.com/a1mdrVg.jpg

    So its all working well, only thing that would now be good is a microsecond timer to delay the triggering after contact. Although I do have a camera axe (which this sensor will plug straight into and works well) that will do 10 microsecond increment delays, it might be nice to have it all as a complete unit.
     
  10. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    Here's the first photo, success for the sensor but fail for the air rifle! it failed to penetrate the bottle. I'll have to get out of the shed and use something more substantial.

    http://i.imgur.com/1hNY9R0.jpg
     
  11. KJ6EAD

    Senior Member

    Apr 30, 2011
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    For the time delay function, you can use a 555 monostable triggered by your existing circuit.
     
  12. PulseLED

    Thread Starter New Member

    Jan 24, 2014
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    Is there a schematic somewhere to help with this? it has to be triggered by a very short pulse, give a delay in the 0-10μs range, then close the switch.

    I've been thinking of using the piezo voltage through a resistor into a capacitor, the resistor slows the voltage rise in the cap giving the short delay.. is this at all a workable proposition? The problem is that the voltage from the piezo isn't always the same.
     
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