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Activate transistor with piezo disc

Discussion in 'General Electronics Discussion' started by Drottson, Sep 19, 2015.

  1. Drottson

    Drottson

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    Sep 19, 2015
    So I'm basically trying to hook up the positive end of a piezo disc to the base of a BJT NPN transistor but it seems I'm not getting enough voltage out of the disc to actually push trough the base. I'm using a BC547B transistor which, according to the datasheet, has a voltage on rating for the base between 0.55 - 0.7 V. Now it's really difficult to actually see what voltage the disc supplies just with a multimeter since the pulse is really fast (don't have an oscilloscope). Can it be that the voltage is simply to low? If so, is there any solution to this? Can it be that the base is activated but that the pulse is so fast that the current I get out of the emitter is so brief that I don't see the effect (for example lighting up a LED)?
     
  2. Martaine2005

    Martaine2005

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    Hi Drottson,
    Transistors are quite fast at switching too.
    Have you tried applying pressure to the piezo disc? Like a quick tap?
    The Wiki on piezos is quite informative. Here

    Martin
     
  3. Drottson

    Drottson

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    Sep 19, 2015
    Yes, I have flicked it quite hard and that way I've actually managed to see a small spark in a LED if I just attach it to the disc alone. But it doesn't seem to be enough for the base of the transistor, for some reason. So there's my problem.
     
  4. Martaine2005

    Martaine2005

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    Well it shows the circuit is working. Did you read the link I posted for you?
    It may be a case of adding a storage device. A capacitor to keep the led on longer.
    What are you trying to do?

    Martin
     
    Arouse1973 likes this.
  5. Arouse1973

    Arouse1973 Adam

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    Piezos produce low current a high voltage. You may want to try a darlintong configuration with two transistors or a MOSFET. But limit the voltage by using a diode connected from the piezo to the supply.
    Adam
     
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  6. Drottson

    Drottson

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    Sep 19, 2015
    Yes, I've checked the wiki before and I've tried loading up a capacitor too. Same thing here, I can even push it a bunch of times to load up the cap more in order to light the LED longer, but it still doesn't activate the base of the transistor, even if I load the cap a lot. I'm trying to use my piezo disc as a switch. So when stress is applied to it I want the transistor to activate and let through my battery current of 9V to power the rest of my circuit. Right now the circuit is hooked up with a simple push button between the battery and the rest of the circuit. I want to eliminate that push button and have it working with the piezo disc instead.
     
  7. Drottson

    Drottson

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    Sep 19, 2015
    So what you're suggesting is that the voltage is actually high enough but the current is too low for a BJT?
     
  8. Arouse1973

    Arouse1973 Adam

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    Yes! Correct.
     
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  9. Drottson

    Drottson

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    Sep 19, 2015
    And thus should work better with a MOSFET since it's voltage activated?
     
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  10. Arouse1973

    Arouse1973 Adam

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    Yes thus should!
    Adam
     
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  11. Martaine2005

    Martaine2005

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    Arouse1973 likes this.
  12. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Actually, this requires a bit of clarification because when the Piezo is connected to the BE junction of the BJT the Piezo output will be heavily loaded. In other words the Piezo voltage will drop substantially. This however does not explain how the Piezo will light the LED directly but the transistor won't. If the Piezo provides enough voltage and current to light the LED directly it will certainly provide enough current to forward bias the base of the transistor. My guess is that you fried the transistor's BE junction when the piezo voltage swung negative. The reverse bias rating of most BJTs are typically only about 4 to 6 volts.

    Try another transistor with a protection diode connected between the base emitter junction. The cathode gets connected to the base. I'm not sure if a base current limiting resistor is needed too but it can't hurt.

    Chris
     
  13. AnalogKid

    AnalogKid

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    do you have a schematic schematic?

    ak
     
    Last edited by a moderator: Sep 20, 2015
  14. Martaine2005

    Martaine2005

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    I did find the schematic you are looking for.

    Martin:D
     
    Last edited by a moderator: Sep 20, 2015
  15. Arouse1973

    Arouse1973 Adam

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    Well done Chris I missed that bit about the LED. I was marinating the brain at the time, sorry :)
    Adam
     
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  16. Drottson

    Drottson

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    Sep 19, 2015
    When I say that it lit the LED what I mean is that I could see a brief, tiny spark in an entirely dark room when I flicked the disc. And I've been having a diod between the piezo and the transistor all along. Haven't had a current limiting resistor but I didn't have that when hooking up the LED either and I really don't think that the current is very high at all.
     
  17. CDRIVE

    CDRIVE Hauling 10' pipe on a Trek Shift3

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    Uh yeah, I'm in the process of doing that right now.

    I hope he posts a schematic.

    Chris
     
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  18. Drottson

    Drottson

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    Sep 19, 2015
    So basically I've been trying something like this:

    Note that I did NOT see even a spark in the LED in this setup. Only when I hooked the LED directly to the output of the piezo (after D2) and removed the battery and transistor all together.

    New-Project.png
     
  19. davenn

    davenn Moderator

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    OK you have no component values in there

    battery Voltage ?
    D1 and D2 types ?
    R1 value?
    Transistor type ?

    I cant see either of the diodes doing anything useful
    rather they are just causing unwanted voltage drops


    Dave
     
  20. Drottson

    Drottson

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    Sep 19, 2015
    Battery voltage is 9V and the resistor is 220 Ohms but I don't really see how that does any difference. If I hook up the LED with the resistor to the battery directly it will light up. The transistor is a BC547B typical NPN transistor and the diodes are both general purpose silicon diodes which are there to recifiy the signal from the piezo, since it delivers AC which would potentially destroy the components otherwise. Looking at it now I do realize that D1 is a bit unnecessary since the LED should do the same job anyway, but the setup is going to be used to power something else later anyway.
     
    Last edited: Sep 21, 2015
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