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Piezo Control voltage source

darren adcock

Sep 26, 2016
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Hi I tend to use piezo's quite alot. I would like to use them as control voltage sources for CV ins on my synth.

I have been trying to work out how I make them more sensitive, does making them more sensitive mean getting them to give out a larger voltage range? or is the issue to modify the input on an op amp?

When I measure the contact mics i have they get to around 3.6v when i tap them rather fiercly.

Thanks in advance

Darren.
 

kellys_eye

Jun 25, 2010
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Their sensitivity is fixed by their physical construction so amplification of anything they produce will be required to make them 'more sensitive'.
 

Audioguru

Sep 24, 2016
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A tapped piezo has an output voltage that is "loaded down" by a load. Then the resistance of the load should be very high for the output voltage to be high.
A tap causes a voltage with a very short duration that might be difficult for a meter to show (maybe an oscilloscope will show) and maybe is a duration too short for an electronic circuit to respond.
I think a tapped piezo resonates and causes a high peak voltage then a fading AC signal at resonance.
 

darren adcock

Sep 26, 2016
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Everything here helped. i was getting odd readings off my meter, and couldn't understand the voltage fade down. Sometimes it ended up sitting in minus and wouldn't resolve itself.

I Initially tried to make a variety of voltage amps, but couldn't get them to offer up the sensitivity i needed. I also couldn't quite understand why sometimes the readins would be + voltages and sometimes -.

Next I thought i could put two contact mic's in series. Nope.

The two links AG sent have made me now want to build a drum synth. Whoop. The circuit here http://devices.sapp.org/circuit/piezo/ solved ll my needs. Here is a quick video. It's so sensitive it respons to air pressure. And a small piece of very flexible wire. When I have it complete in a circuit would it be usefull to take some measurements and post here off the circuit?

 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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That circuit is similar to what I've recommended a local musician use to interface a large number of these devices to an Arduino.

You can get by with something significantly simpler than that if you only want to hold a peak voltage for a short amount of time before reading it.

In my case, we used the Arduino to first read the voltage from the cap before using the same pin to reset the voltage to zero. A lack of sensitivity was not a problem, but in this application they were connected to paddles that are designed to be tapped with your finger, not detect a small insect landing on them! :)
 

darren adcock

Sep 26, 2016
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That circuit is similar to what I've recommended a local musician use to interface a large number of these devices to an Arduino.

You can get by with something significantly simpler than that if you only want to hold a peak voltage for a short amount of time before reading it.

In my case, we used the Arduino to first read the voltage from the cap before using the same pin to reset the voltage to zero. A lack of sensitivity was not a problem, but in this application they were connected to paddles that are designed to be tapped with your finger, not detect a small insect landing on them! :)
Which museum do you work for Steve? It's been a dream of mine to intern at a museum helping out with tech display needs
 

Audioguru

Sep 24, 2016
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When you amplify the signal from a piezo sensor then I think the input of the amplifier (transistor, jFet or opamp) should have diodes protection from the high voltage produced when a piezo is hit hard or is dropped. The high voltage can zap the input device.

A piezo sensor produces a voltage with hardly any current so use Ohm's Law to see the voltage become reduced when its load resistance is low. The linked circuit shows a 1M input resistor on an opamp. But some opamps cannot use an input resistor as high as 1M .
 

darren adcock

Sep 26, 2016
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Hey. Gonna use this circuit as part of a sound installation At Manchester's science fair.

How do I credit the circuit?

I would like to cite electronics point, as support and also to help direct people here, Is there anyway you would prefer this worded? It's an art thing so all the room for creativity humour etc....I'll only reject pretention!

Can I print the circuit it and have it as part of the display relating to the installation? Is their copywrite infrangment or permission I need to ask?

I would also like to have a some examples of this circuits use for other applications aswell, especially musical, but not fussed really. Just it's a science fair so wanna indulge the geeks and my own geekness.

The feedback resistor on first amp stage (100k) I am going to change that to a pot, would it be accurate to call this pot simply gain?

Thanks in advance.

Darren
 

Audioguru

Sep 24, 2016
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You can label your circuit anything you want, it is your circuit.
Yes, if the 100k feedback resistor is changed to a 100k pot then it controls the gain from 1 to 11 times.
If the piezo is hit hard enough to produce a voltage higher than the supply voltage then the opamp might be destroyed without adding protection diodes. A nearby firecracker or hand clap might do it.
 

darren adcock

Sep 26, 2016
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Hey AG.

On testing it, It seems it won't be pushed much past 15v. We will be using it to pick up vibrations from drawing. Where would the protection diodes go please? I tried to see on the tl072 where I might find it's input voltage tolerences but couldn't work it out if you don't mind telling me what I need to be searching for on the datasheet please?
 

(*steve*)

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It may be worth placing RS/2 on either side of the junction of the diodes. It's not strictly necessary, but it does provide significantly more protection.

Don't make RS too high or you'll end up with a low pass filter formed by RS and the junction capacitance of D1 and D2.
 

darren adcock

Sep 26, 2016
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Thanks Steve.

This is where it becomes obvious i've not been doing my maths homework :( how do I work out what the value of RS should be. The circuit is powered by -12v/+12v.... at it's peak the sensor tends to reach around 15v.

I'll follow your advice on RS/2 and placing either side of the D1 D2 junction.
 

darren adcock

Sep 26, 2016
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Am i reading this correctly that VI is either the inverting or non inverting inputs?

If so it means I have to make sure I don't have any voltage that exceeds this? In terms of in circuit, i'm going to need to follow the op amp section to see if it get's amplified anymore and see where that voltage reaches a cap and stops?

VI Input voltage (2) VCC– – 0.3 VCC– + 36 V
 

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