humidifier circuit

Discussion in 'The Projects Forum' started by clintonb, Apr 12, 2015.

  1. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I've been trying to drive a piezoelectric transducer to form a mist from a body of water. I bought the transducer from audiowell and basically is just a high frequency speaker.

    I'm using a 555 timer that can operate at 2mhz. I started by getting a frequency of 16hz and connected the 12 volt source to a pc speaker. When the circuit was closed it gave a buzz. After this it appeared everything worked and I replaced the speaker with the transducer but got nothing. I changed the resistors and capacitors to get a higher frequency but still nothing.

    Is there anyway to test the transducer? I tested for connectivity, resistance across the terminals but its possible these don't apply to the transducer.
     
  2. Dodgydave

    AAC Fanatic!

    Jun 22, 2012
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    As the piezo is a capacitive device, it needs to be driven by a push pull transistor output for best results, try changing the frequency to 3.5 to 4Khz,
     
  3. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I was googling a push pull transistor setup but only come up with push up and pull down resistor type topics. Are you referring to using 2 transistors with diodes that use the syncing and sourcing of the 555 timers to reverse the polarity of the two points? Do you perhaps have a schematic or example - I'd really appreciate it
     
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    I doubt 12V will be enough. Also, the transducer needs to be driven at its resonant frequency for maximum output. Try searching for a 'self-resonant' driver circuit.
     
  5. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I worked on 12v because its the figure that is given in the very poorly written datasheet. I was looking at making sure it was working before diving into troubleshooting the circuit.
     
  6. DickCappels

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    Aug 21, 2008
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    Just curious, but do you know the frequency response of the transducer?
     
  7. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    It
    It Quotes a reonant frequency of 1.6Mhz +-0.05
     
  8. DickCappels

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    Aug 21, 2008
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    Then you have to drive it at 1.6 Mhz, and you won't be able to hear or feel it vibrating.

    One way to see whether or not it is working is to place another matched transducer near it and monitor the transducer's output with a scope. Otherwise you can build a local oscillator and down-converter so you can hear the oscillator beat against your receiving transducer's output.

    There are links to "Bat Detectors", which is what you need, in the page for which the link is given below. Some adaptation will be required to shift it up to your 1.6 Mhz.

    http://www.clivethomson.com/2013_09_01_archive.html
     
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  9. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Just feature the transducer as the feedback filter in a power oscillator configuration as per the design of most commercial ultrasonic atomization products (think 'Pierce oscillator on steroids' ;)) --- BTW production of the desired effect will require Ca 50v peak applied to the transducer...)

    Best regards
    HP

    PS: Please don't 'test' the circuit via touching the transducer! High intensity ultrasonic vibration at that frequency will heat the osteology of your finger such that injury will likely precede discomfort!
     
    Last edited: Apr 12, 2015
  10. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I was doing some homework on the transducers in the circuit and I noticed a variety of ways in which the transducer is connected. My connect is much simpler than what I keep seeing. I basically just connected vdd to the transducer and the transducer then to the drain pin(being an N-channel). Is there a right and a wrong way of doing this.

    I tried to simulate this diagram but cannot for the life of me figure why it was done this way.
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    That is a push-pull drive which ensures the average current in the transducer is zero, i.e. there is no standing current.
     
  12. Brownout

    Well-Known Member

    Jan 10, 2012
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    Then why would you drive it at 16Hz?

     
  13. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I might have been a bit ambiguous. Since the device runs at 1.6mhz and I can neither see or hear it, I setup the circuit for 16Hz with a normal Pc speaker. By doing this, I can establish a starting point. After getting this right, its just a matter of a few substitutions to change to the piezo transducer.

    I'm famous for careless mistakes and I tend to look past the basic mistakes and it takes longer than it should.
     
  14. Brownout

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    Jan 10, 2012
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    Me too. When working with piezo devices, there are many circuit characteristics you need to be aware of. For example, the device will have a particular impedance profile. Typically, the impedance will be very high at frequencies other than the resonant frequency, falling to a fraction of an ohm at resonance. This is something you will need to consider in your design - will your driver provide a matching low impedance to the device. This is one reason to use a differential driver. Also, when supplying a rectangular waveform to the device, the component of the fundamental frequency will be lower than the P-P of the square wave signal. This is because the square wave is composed of a range of frequencies, many at harmonics of the fundamental. Thus, a higher voltage signal will be need to get the input voltage to spec. Other things to consider - you'll need a way to directly measure the output frequency of your generator as well as the output from your device. Setting up an oscilloscope to measure the input voltage and current will give you an idea of exactly where the device is in resonance. From the discussion, resonance will correspond to maximum current draw. If you can get an equivalent circuit for your device, it is possible to simulate, which I strongly recommend.

    I have to disagree with your definition that the piezo is just a high frequency speaker. A piezo will typically have very high Q-factor, which means the bandwidth around resonant frequency will be extremely narrow. By considering device Q, impedance profile and frequency, you should be able to get closer to operational conditions, but you really need a way to measure input and output signals. You really need an oscilloscope. Good news is a medium performance 'scope will be sufficient.
     
  15. Brownout

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  16. Hypatia's Protege

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    FWIW -- A good 'starting point' is to think of unloaded piezo transducers (and, indeed, piezo resonators in general) as very high 'Q' "tank circuits". -- Of course, in the case of a transducer, the effective 'Q' will be, in large part, dictated by the dynamic load...

    Best regards
    HP
     
  17. clintonb

    Thread Starter Active Member

    Mar 6, 2011
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    I think bottom line is that I need to get my own oscilloscope and step through the circuit.

    I'm basically testing in software simulation and then building and praying for a result without any real way to measure the output(not clever). I'll have to shelf this one for a while and revisit at a later time.

    When I said high frequency speaker it was the best model on which to base it at the time. Seems that has changes in the last week ;).
     
  18. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Indeed! - Reality always 'trumps' simulation in the final analysis:D --- One caveat, however, use caution with expensive test equipment on this project, piezoelectric devices operating at high power can generate some nasty EMFs!:eek:

    Best regards
    HP
     
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