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Piezo Transducer To Control Light Brightness

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Impathy

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Hi, I have a simple circuit using a Piezo Transducer to turn on an LED and it works fine. Squeeze the disc and the light gets brighter the harder you squeeze.

simplecircuit-jpg.9251

(Sorry, didn't know the diagram for a piezo transducer :) )

The voltage drops signifigantly after the transducer. (On a 9v it drops down to about 1.2v. Even with a 12v that's not turning on a light) So I tried setting it up with a simple comparator to make the transducer act as a switch (with the comparator - LM393) There are plenty of other ways to make it as a switch using low voltage to turn on the main 12v current. But here is my problem:

I want the end current to be directly proportional to the voltage output of the transducer as such:

The transducer outputs 0v... I want the light to output 0v
The transducer outputs 1.5v... I want the light to output 12v

Is there any part that can achieve this that I have not discovered?

If there is such thing as a voltage variable resistor, I'd be set. A resistor with 3 leads. A, B &C. Current runs from A to B light a normal resistor. It cuts down the voltage and such. But - if C is connected to a seperate current, it could work as a dynamic variable resistor dependant on a small voltage applied to it (0 - 1.5v) could be directly proportional to the ohms (47k ohms - 0 ohms). That would work for me if such part existed. Here's a simple picture if that didn't work for you.

fictitiousvariableresistor-jpg.9252


If there is any easy way to get my 1.5v signal to switch, amplify, change resistance or anything - to my main 12v current. Please let me know. Thanks in advance, Sorry if this is a silly question, I couldn't find an answer. ;)

-Andy
 

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Sorry, No.

I have the LED as a power indicator. It is just there to let me know that the piezo disc is working correctly. I am going to eventually attach it to a 12v light if I can get the current the way I have described above.

And I'm not adding a potentiometer as those take manual adjustments - with a knob or a screwdriver. I need something that is dynamic and requires no adjusments by me. Does that clear everything up?
 
Wow, no offense, but you're just mangling terminology and theory around left and right. So lemme start on the most basic misconceptions and work up to the trickier ones:

Read up on Ohm's law: V = I*R . This defines the voltage across a resistor. There are components which can (across all currents) drop 12V down to 5V, but they aren't resistors. For a LED though, you can calculate a resistor value which fits the equation for a given current.

Typically "voltage controlled resistors" tend to be called amplifiers. You can make/buy amplifiers ranging from a simple transistor to a more complex "opamp".

The symbol for a piezo transducer is the same one as a crystal and (since they're practically the same thing - though one is made of ceramic, and the other one out of quartz). When you apply force to a piezo, it will generate a charge, and when you release, it will generate an equivalent negative charge. This means a piezo transducer's output is basically a current vs velocity, and a piezo can't directly measure the position/force/deflection of it.

Now I don't exactly see how you are getting a constant voltage drop across a piezo by holding it, so you've got something else happening...

James
 
Yeah,
A piezo doesn't pass current. Maybe it is a piezo that is connected to a transistor that is at its terminals. Then it would pass current in a pulse each time the piezo is hit, but if you squeeze the piezo and don't let go then the current would still be just a pulse, not continuous.
 
HarveyH42 said:
One side of the piezo is positive the other negative, the path is through your fingers when you squeeze it...
No. A piezo doesn't have polarity. It is a speaker/microphone.
You might as well squeeze on a piece of wood and hope enough current goes through your fingers to light the LED (there won't be enough current unless your fingers are wet with salt water).
 
Sorry about the 12v to 5v in the picture - that was just a quick example, it wasn't ment to be accurate.

Let me explain what exactly I'm doing.

I'm making a circuit to attach to a drum on a drumset that can sense how hard the drum has been hit and turn on a 12v light accordingly to how hard the drum is hit. I am making a test circuit using a 9v and a LED.

I have a Piezo - just a raw disc with the soldered wires on it. It is attached to the bottom of the drum skin - bronze side touching.

Next I have the 9v battery's positive terminal connected to the red wire of the piezo and the black wire of the piezo is conncted to a resistor which inturn goes to my LED and finally goes back to the battery.

It works fine. I hit the drum lightly - the LED dimly turns on. I hit it hard - the LED is much brighter, but not fully bright.

I tested the circuit at a few points and found a drop in voltage after the piezo. I then assumed I must have mismeasured the voltage or something. So, I tried re-mesuring the voltage directly before and after the piezo.

Before the piezo, the voltage is 8.34v (coming straight from the battery of course) and directly after it I measured from 0 - 6.83v.

0v not doing anything to the piezo
6.83v after hitting the drum skin.

Is this normal? Am I going about it all wrong? I'm just experimenting and having fun, my apologies if I have mangled any more terminology or theories. :)

I think what I am looking for is a transistor. I misunderstood what a transistor did. The book I read gave me a different Idea of what it can do.

Thanks again. Hope this helps
 
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Hi,

Im trying something similar and having problems too. Heres my thread from a few days ago:

https://www.electro-tech-online.com/threads/biscuit-tin-drums.23301/?highlight=biscuit

I think (someone correct me if this wrong) you need a buffer to begin with, because the current produced by the disk is tiny. This would then feed an amplifier (voltage controlled resistor), which could control your light.

What sort of voltages does the piezo transducer produce (ie amplitude, shape, frequency)?
I want mines to trigger on a gentle tap of the hand rather than a hard hit with a stick.
 

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I'm thinking buffer, lowpass filter, comparator? Use a transitor emitter follower to light the LED. This wouldn't produce varying brightness though. I guess a piezo into an amplifier would do for that.
 
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A piezo has an output polarity depending on which side of it is hit. It produces voltage with hardly any current. It might have enough current for a transistor to amplify. Then the transistor can conduct current through the LED from a battery. If the current from the piezo is too small then use a darlington transistor that will have a current gain 100 times more.
 
Thanks for the help, I'm going to try it and let you all know how it worked out. Hopefully I can return the favor one day :)

I guess I was trying to make it harder than it really was
 
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