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mosfets + piezo film vibration sensors

Discussion in 'Electronic Design' started by Leon Sorokin, Dec 21, 2004.

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  1. Leon Sorokin

    Leon Sorokin Guest

    http://rocky.digikey.com/WebLib/Measurement Specialties Inc/Web Data/LDTO Vibration Sensor.pdf
    http://rocky.digikey.com/WebLib/Measurement Specialties Inc/Web Photos/0-1002794-0.jpg
    i just got these LDT0 MSI piezo film tabs that i'm hoping to use in a design
    i'm working on. the idea is to get an LED to flash on physical impact of the
    entire PCB. but the LED is a Luxeon emitter, so it requires a good drive
    current to flash it. i'm using the NDT451AN N-channel mosfet. it has a high
    maximum VGSS voltage since the tab can generate up to 70V through a 90
    degree deflection. but i looked at the graphs and i shouldnt ever see more
    than 15V given that the impact would never deflect it more than 5mm. also i
    needed it to trigger on fairly light impacts, so i chose the mosfet because
    the typical gate threshold voltage is pretty low...1.6V, but i'm not sure
    how much voltage is needed to completely saturate the gate. i connected the
    piezo film tab between the gate and GND (rather the negative batt contact.)
    i also have the gate grounded through a 169K resistor to discharge any
    residual gate capacitance. i'm using a CR123A lithium battery for good
    temperature range and flat discharge characteristics...so 3V for LED power.

    how does the tab work?...does it generate +V with deflection in one
    direction, and -V with deflection in the other? does that mean it will only
    open the mosfet when deflected in one direction but not the other?? do i
    need 2 tabs in opposite directions to generate the +V that the gate needs to
    open it regardless of the deflection direction?

    am i missing something critical that will surprise me?

    if i understand anything, and i hope i do. this should work and not blow out
    my mosfet? and will trigger fairly easily.

    thanks,
    Leon
     
  2. Guest

    You could use an op-amp to amplify the piezo's output, and perhaps a
    lowpass filter (made from another op-amp) to smooth out the signal you
    feed to the mosfet. If you use a single supply op-amp, you probably
    won't need to worry about the negative voltages generated by the piezo.

    Or you could use a microcontroller. I worked on a car alarm that took
    readings from a piezo via an analog to digital converter. The micro
    kept track of max and min readings. If the difference between max and
    min readings was above some threshold value, the micro sounded an
    alarm. If your micro has built in eeprom, you can teach it the level of
    shock that you consider to be an alarm condition. And you can do all
    sorts of filtering that it would be tough to do with analog
    electronics.

    Mark
     

  3. Ahoy mate: Leon!

    This is an interesting project you have there. I've never played with
    piezoelectric tabs, however...

    I think your basic circuit concept is workable, but you will likely be very
    disappointed by the sensitivity of your system if you build it as you have
    proposed. The piezoelectric tab is a capacitor. It can only supply AC
    current. You bias the gate of the MOSFET to some DC level, and then if you
    flex the piezo tab it will produce an AC voltage about that bias point. In
    your case you have a 169k resistor from gate to ground, so the DC voltage
    level on the gate will be 0V, but flexing/not flexing the tab will get it to
    move above and below this point (but on the average it will always be at
    0V).

    Unfortunately you have selected a rather large MOSFET for this application.
    Also, the 169k resistor is much to low in value. As a consequence only the
    most powerful and rapid physical shock forces will be able to energize the
    LED.

    The datasheet you provided for the LDT0 says it can provide up to 7.2nC of
    charge if it gets deflected 5mm. Suppose for a moment your 169k resistor
    was infinite in value instead. Now look at the MOSFET datasheet:

    http://www.fairchildsemi.com/ds/ND/NDT451AN.pdf

    Look at figure 10, the gate charge characteristics. 7.2nC deposited on the
    gate would probably just barely get it up to the end of the plateau region
    (where the MOSFET is actually turning on). Below the plateau the MOSFET is
    off. During the plateau the MOSFET is in the active region and is turning
    on. After the plateau the MOSFET is fully on and represents a low value
    resistor. So basically in your application (if we make the 169k resistor
    almost infinitely large) the MOSFET will just barely be able to turn fully
    on for a deflection angle of 5mm.

    If you want more sensitivity you either need an amplifier/buffer, or you
    need to use a lower gate charge MOSFET. What kind of LED current do you
    need? Hopefully you don't need that much, and some very low gate charge
    MOSFET such as the BSS138 would work for you (though something tells me you
    want lots of LED drive since you are using a Luxeon and have already
    selected a very large MOSFET to begin with):

    http://www.fairchildsemi.com/ds/BS/BSS138.pdf

    See the gate charge curve of figure 7.

    Otherwise you will need to buffer the output of the piezo somehow...

    The other problem is your 169k resistor. Suppose for a moment the shock
    impact is robust and fast enough to deflect the piezo tab 2mm in 5ms. The
    total charge output available would be 3.4nC. The definition of current is
    charge (in coulombs) per unit time (in seconds). One coulomb per second is
    1 amp. So, 3.4nC in 5ms is 680 nanoamps. Hmm... Suppose a MOSFET
    threshold voltage of 1.3V. In order to get the MOSFET to 1.3V, how much
    current would flow through the 169k resistor (ignoring gate charge current
    for a moment)? V=IR so 7.7microamps. Uh oh. 7.7uA >> 680nA... This
    obviously isn't going to work but for the most robust of impacts. In order
    to get any kind of decent sensitivity (without buffering the piezo tab
    output first) you are surely going to need a resistor in the tens of megohms
    or more range.

    You may also consider placing a 1N4148 (or perhaps a low capacitance/leakage
    zener [IE: highest suitable voltage/lowest wattage you can find] if you are
    still concerned about gate overvoltage) between gate and source of the
    MOSFET (anode to source, cathode to gate).

    By the way. I'm very curious. What is this for?
     
  4. Leon Sorokin

    Leon Sorokin Guest

    thanks for the advice. i now realize i have a lot of reading to do.

    the project is for my girlfriend, she does track and cross country and goes
    out running at night and needed some sort of bright flashing indicator. and
    being an active Luxeon LED flashlight modder on CandlePowerForums, of course
    i decided to do some research into the possibility of building a low-cost,
    light-weight, good temp range (for winter running), vibration based flashing
    circuit to avoid using a switch and the annoyance of turning the circuit on
    and off and just have it flash on impact. technically all i need is a
    vibration CONTACT switch but it needs to be really small and springy, light
    and "wearable", i dont need to generate a voltage, just to close a
    circuit..if i can find a spring contact switch i can drop the mosfet, the
    LED usually is driven around 320ma-350ma at 3.42V when constant on, but that
    would drain the CR123A battery in 2 hours if regulated...i have a flashlight
    that has a regulation circuit on a Luxeon LED like this that proves it.
    BUT...flashing at say 1.5hz or 2hz, maybe 10% or 15% duty cycle, it can go
    much much longer. i guess the alternative would be to just use a <50% duty
    cycle 555 timer circuit with an on/off switch and keep the mosfet. i'll be
    underdriving the LED by quite a bit as it is. 3V max instead of 3.42V typ,
    so i dont really need any current limiting resistor for the LED itself. if
    anyone knows a good springy contact switch so i can drop the whole
    piezo/mosfet idea, i'd appreciate it. looking to make it fairly simple and
    inexpensive. i might make a few more for some of her runner friends. i got
    the idea from those yo-yos that flash when they spin, they have springy
    contacts. technically i can make one but i'm sure they exist i'm just not
    sure where to find them. digikey and google didnt yeild much. but i'm
    probably not searching the right terminology.

    thanks,
    Leon

     
  5. Rich Grise

    Rich Grise Guest

    What ever happened to those flashing shoes? One of those with some really
    lightweight wire would make a pretty good trigger. And I've seen
    bicyclists with a hazard flasher on their helmet, but that'd be pretty
    heavy.

    Good Luck!
    Rich
     
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