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Amplify signal from piezoeletric

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Here's a simple circuit for you:

upload_2017-7-8_12-35-40.png

You are unlikely to need any voltage gain, the piezo input will be a high voltage.

The 2 transistors Q1 and Q2 form a fast limiter to about ±8V. The power supply connections are not shown, but should be ±9V or greater

To connect it to an arduino (analog input?) you don't need any gain at all (most likely).

Use this circuit:

upload_2017-7-8_12-47-37.png

This is essentially a sample & hold for a piezo. You can read the value on the cap at your leisure, and then discharge the cap by forcing the pin to be a low output.

  • D1 protects D2 and D3 from reverse voltages;
  • D3 allows you to capture the positive peak;
  • D3 limits the voltage to Vcc + 0.6V (or less if you use a schottky);
  • R1 limits the input current (only an issue if someone connects something inappropriate (like a battery));
  • R2 limits the current through the input protection where the voltage on the capacitor is Vcc + 0.6V, and also the discharge current of the capacitor.
You can probably get by with just D2 and C1, but I wouldn't unless I was Chinese.
 

Diogo

Jun 28, 2017
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The fullwave rectifier circuit I posted should be fed from a low resistance to ground signal source like from a preamp opamp powered from a dual polarity supply. If an LM358 opamp is used for the fullwave rectifier and its input is capacitor-coupled from the output if the preamp then the input bias current of the LM358 will cause its input to float high and it will fail to work properly.
I'm afraid I didnt understand anything :\
 

Diogo

Jun 28, 2017
29
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Messages
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Here's a simple circuit for you:

View attachment 34996

You are unlikely to need any voltage gain, the piezo input will be a high voltage.

The 2 transistors Q1 and Q2 form a fast limiter to about ±8V. The power supply connections are not shown, but should be ±9V or greater

To connect it to an arduino (analog input?) you don't need any gain at all (most likely).

Use this circuit:

View attachment 34997

This is essentially a sample & hold for a piezo. You can read the value on the cap at your leisure, and then discharge the cap by forcing the pin to be a low output.

  • D1 protects D2 and D3 from reverse voltages;
  • D3 allows you to capture the positive peak;
  • D3 limits the voltage to Vcc + 0.6V (or less if you use a schottky);
  • R1 limits the input current (only an issue if someone connects something inappropriate (like a battery));
  • R2 limits the current through the input protection where the voltage on the capacitor is Vcc + 0.6V, and also the discharge current of the capacitor.
You can probably get by with just D2 and C1, but I wouldn't unless I was Chinese.
Hey! Thanks for your help :)
Whats the 1st circuit function?
The secund is the full wave rectifier, I suppose?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
25,510
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The first one is just a non-inverting buffer. What comes out is the same as what goes in. The difference is there limiter at the input. The resistor limits the current (which again, is probably only needed if you accidentally connect the input to something inappropriate life a battery) and the two transistors act as back to back zener diodes except they are faster.

The second circuit is simply a half wave rectifier charging a capacitor. (And I mention these as the essential components) The additional components are just protection.

There are alternative methods and topologies for these circuits but I chose some which use totally non critical, and/or easily obtainable parts.

Curiously enough, if you add an input capacitor to the second circuit, one half of the protection (D1) now works with the rectifier (D2) to make a voltage doubler. However, since the output from a piezo is already a high voltage, this is not something we need

Increasing R1 (in the second circuit) to values up to even several megohms will act as a sensitivity control by acting to increase the time constant. Continuing vibration at a fixed amplitude takes longer to charge the cap, and a larger amplitude vibration for a given duration will charge it more.

The second circuit is being used by a certain very colorful colleague of mine for something very similar to a drum simulator (where he does want to detect how hard the piezo is struck)

IMG_20170627_202302.jpg
 

Diogo

Jun 28, 2017
29
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The first one is just a non-inverting buffer. What comes out is the same as what goes in. The difference is there limiter at the input. The resistor limits the current (which again, is probably only needed if you accidentally connect the input to something inappropriate life a battery) and the two transistors act as back to back zener diodes except they are faster.

The second circuit is simply a half wave rectifier charging a capacitor. (And I mention these as the essential components) The additional components are just protection.

There are alternative methods and topologies for these circuits but I chose some which use totally non critical, and/or easily obtainable parts.

Curiously enough, if you add an input capacitor to the second circuit, one half of the protection (D1) now works with the rectifier (D2) to make a voltage doubler. However, since the output from a piezo is already a high voltage, this is not something we need

Increasing R1 (in the second circuit) to values up to even several megohms will act as a sensitivity control by acting to increase the time constant. Continuing vibration at a fixed amplitude takes longer to charge the cap, and a larger amplitude vibration for a given duration will charge it more.

The second circuit is being used by a certain very colorful colleague of mine for something very similar to a drum simulator (where he does want to detect how hard the piezo is struck)

View attachment 35006
Thanks for the detailed explanation!
Can you please addapt the rectifier in order to make it a full wave rectifier? I can't really lose my negative voltages!
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Can you explain what you're doing and why the negative excursions are important.
 

Diogo

Jun 28, 2017
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Can you explain what you're doing and why the negative excursions are important.
I'm doing a cough detector (detects the abdominal expansion) with the piezo (more detailts on the 1st post).
I can't have negative voltages because I intent to connect it to arduino, which doesnt support negative voltages
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Expansion will bend the piezo in one direction producing a voltage of a single polarity. It's only if the piezo it flexing both ways that you get positive and negative voltages.

There's no reason you can't feed the piezo into a bridge rectifier if that's what you want.
 

Diogo

Jun 28, 2017
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Expansion will bend the piezo in one direction producing a voltage of a single polarity. It's only if the piezo it flexing both ways that you get positive and negative voltages.

There's no reason you can't feed the piezo into a bridge rectifier if that's what you want.
I've inserted an epoxy ball in the piezo crystal zone and when I cough the elastic stretches and apllies force in the ball and therefore in the piezo.
Connecting the piezo directly to the bridge rectifier? No input resistance or capacitor, anything?
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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You could connect the piezo with a series resistance to the AC input of a bridge rectifier and connect the output in place of D1 and D2.

The output is full wave rectified, not dc
 

Diogo

Jun 28, 2017
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You could connect the piezo with a series resistance to the AC input of a bridge rectifier and connect the output in place of D1 and D2.

The output is full wave rectified, not dc
Can you please diagram me??
 
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