Amplify signal from piezoeletric

[(*steve*)]

Here's a simple circuit for you:



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:



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]

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]

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*)]

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)

 

[Diogo]

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*)]

Can you explain what you're doing and why the negative excursions are important.

 

[Diogo]

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*)]

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]

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?

 

[Diogo]

Wait, won't the bridge rectifier output a DC signal?

 

[(*steve*)]

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]

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??

 

[Diogo]

Can you please diagram me??

No need afterall. What values of the series input resistor and the load resistor?