First question is with regard to the capacitor 330P and resistor 100k marked in red A.
What do these do with regard to the signal from the piezo sensor?
Next I found the smaller schematic "C" on line.
Using that as a reference, I assume that the circuit flips the ac in from the piezo wave into full wave rectified. is that correct?
Where I marked D, isn't this the same as a single 28k?
Where I marked B, what is the purpose of the 2 volt zener diode?
The resistance values in the circuit are higher then the "C" circuit. In general what to these resistor values do?
My view on the circuit. Indeed a full-wave rectifier.
Second opamp is a 20x DC amp (28k:560k) that changes into a 10x amp (2x560k parallel) above the zener voltage.
Sort of gearbox/anti-clipping circuit.
Input impedance of the circuit is "100k to ground +56k to virtual ground (opamp#1) + 56k to virtual ground (opamp#2)" = ~22k (21k875).
That input impedance creates a high-pass filter with the capacitance of the piezo.
The 100k resistor also keeps DC off the (capacitive) sensor.
I think the 330p is just some high-cut (RF stopper).
2x56k parallel. Sometimes it's cheaper to use one resistor value. And save a reel in the pick and place machine.
Second circuit has less gain, no "gearbox", and a lower input impedance (6k66).
Leo..
That circuit will have a cutt-off frequency of ~2Khz with the average 1" piezo (3-4nF).
Forget about 100 - 400Hz with those circuits.
Any general oscilloscope will do. They have a 1Megohm input impedance that can be changed to 10Megohm by setting the probe to 1:10.
A load of >=1Megohm is needed to measure below 100Hz (depends on piezo capacitance).
Why not connect the piezo directly to Arduino's analogue pin.
The "knock sensor" example shows you how.
If you want to measure full-wave vibrations, use a (e.g. 2x2.2Megohm) voltage divider connected between +5volt and ground, and connect the piezo and analogue pin to the mid-point.
Sample many times. Code can extract positive and negative vibrations, amplitude, and frequency.
Leo..
I don't think there's a cut-off frequency, the system acts as a differentiator - piezo's act as charge sources, the
resistor produces a voltage proportional to rate-of-change of charge. There's just a constant 3dB/octave slope
in voltage response. The resistor also sets the output voltage magnitude to something convenient.
Wawa:
..... Why not connect the piezo directly to Arduino's analogue pin.
The "knock sensor" example shows you how....
Yes I have seen this. I guess I started to explore amplification as an option.
Certainly I will try cheap and simple if I can get it to work. While I am waiting for my disks to come in, I thought it would be good to learn a little about amplification, should it be needed. I am guessing that it will. The vibration I am attempting to detect will be small. Won't really know until I try.
... If you want to measure full-wave vibrations, use a (e.g. 2x2.2Megohm) voltage divider connected between +5volt and ground, and connect the piezo and analogue pin to the mid-point.
Sample many times. Code can extract positive and negative vibrations, amplitude, and frequency.
Leo..
Is there any concern with biasing the piezo at 2.5 volts with regard to +/- voltage on the analog pins?
I understand the chip says internal impedance > 100Megohm. would there be any need to clip the potential voltage?
MarkT:
I don't think there's a cut-off frequency, the system acts as a differentiator - piezo's act as charge sources, the
resistor produces a voltage proportional to rate-of-change of charge. There's just a constant 3dB/octave slope
in voltage response. The resistor also sets the output voltage magnitude to something convenient.
Hi Mark, thanks for the response.. you may be a little over my ability on this. I thought a piezo was sort of a oscillatory voltage source when responding to periodic vibration, with the voltage proportional to the stress on the element. When not vibrating it is like a open in this circuit. Is that not the right way to think about a piezo?
Qsilverrdc:
Is there any concern with biasing the piezo at 2.5 volts with regard to +/- voltage on the analog pins?
I understand the chip says internal impedance > 100Megohm. would there be any need to clip the potential voltage?
The knock sensor example biases the piezo at ground potential.
Worse than mid-biasing, because ~0.5volt negative pulses are already at the input voltage limit of the chip.
Common piezos can't deliver the current to damage the internal pin protection diodes.
The input protection diodes clip the piezo voltage at "ground - 0.5volt" and "VCC + 0.5volt".
If you frequently see A/D clipping (0 and 1023), consider using a voltage divider between piezo and analogue input.
Leo..
Wawa:
The input protection diodes clip the piezo voltage at "ground - 0.5volt" and "VCC + 0.5volt".
I see.
With the op-amp circuit, if the voltage output is > VCC could this be a problem for the input?
Or will the internal clamp take care of?
Would I need an external zener to ground to clip? Say a 5volt?
Qsilverrdc:
With the op-amp circuit, if the voltage output is > VCC could this be a problem for the input?
Or will the internal clamp take care of?
Would I need an external zener to ground to clip? Say a 5volt?
No. Common piezos haven't got the power to damage.
There is 56k between piezo and opamp anyway.
I don't see the advantage over the opamp circuit vs. direct A/D.
Unless you want to measure very small vibrations.
Leo..
This is something I am thinking of trying for AD input for vibration detection. The frequency is on the order of 100 to 400hz.
Vibration is typically measured with accelerometers. This has been the case for as long as I can remember. Before Arduinos came along, an accelerometer would have been expensive. Mass production to meet the demand of arduino hobbyists has lowered the price of an accelerometer to pocket change. It really makes no sense to use such an inefficient circuit , (no matter how exotic it might seem) to measure vibration when accelerometers and arduino code to use them is so cheap and available. I don't see the application of A/D when accelerometers are available (unless of course it is an analog output accelerometer, like THIS ONE)
A basic example sketch that reads from analog three-pin three-axis accelerometers, such as the common ADXL330 chip found at Seeedstudio SparkFun online vendors.
raschemmel:
Vibration is typically measured with accelerometers. This has been the case for as long as I can remember. ......
So use an accelerometer to detect relatively low frequency vibration.
I like the idea with the following exceptions for my need.
The sensing package has to fit in a space about the size of a nickle.
Not sure I could fit the sensor and board in the space.
The sensors are on a hand held component.
I'm not sure that I would be able to easily distinguish between, inadvertent motion and vibration.
This is primarily the reason I am "exploring" this option. I expect that I will need a great deal of amplification. I don't know if simple motion of the device may also create a trigger event with the piezo's either.
Keep in mind that the op amp circuits shown in the first post won't work properly with a single supply voltage, because the precision rectifier (the first op amp) has to have a negative-going output.
I'm not sure that I would be able to easily distinguish between, inadvertent motion and vibration.
No matter what sensor you decide to use, you will need to use signal processing to distinguish the desired events from noise and other disturbances.
jremington:
Keep in mind that the op amp circuits shown in the first post won't work properly with a single supply voltage, because the precision rectifier (the first op amp) has to have a negative-going output.
So the VCC pin 8 would be at +5vdc and the V- pin4 would need to be like -5vdc?
So the VCC pin 8 would be at +5vdc and the V- pin4 would need to be like -5vdc?
Yes, or higher (+/- 15V is common). In these circuits, GND is 0V.
There are alternative circuits that allow unipolar power sources. In such cases, the + inputs of the op amps are typically biased to Vcc/2, which is also "signal ground".
Not that it applies to your application but just for the record , an op amp could run from GND on pin-8 and -5 on pin-4 ( if you needed a negative output). The only criteria is that pin-8 be more positive than pin-4 and +Vcc to -Vee does not exceed the Vcc to Vee maximum . I've used a combination of single ended positive op amp and single ended negative op amp before when the + Vcc to -Vee for a bipolar split rail was > the maximum for the chip, so I had to split it into two single ended circuits.
jremington:
Yes, or higher (+/- 15V is common). In these circuits, GND is 0V.
There are alternative circuits that allow unipolar power sources. In such cases, the + inputs of the op amps are typically biased to Vcc/2, which is also "signal ground".
So a voltage divider 5v to 0v with 2.5v as the data ground. So the output would centered about 2.5 volts.
So you are all waiting for the "Dumb" question. (I am sure I know the answer, but I'm not sure.)
:-[ Can the output voltage of an op amp ever exceed the V- to Vcc potential?
So this circuit were powered +5v -5v would at maximum produce a +5v output, like a clipped wave?
Can the output voltage of an op amp ever exceed the V- to Vcc potential?
No, and in most cases the maximum output will be significantly less (in magnitude) than either extreme.
The op amp data sheet will specify "output swing".
If you want to experiment without having to build actual circuits, I recommend the free simulator LTSpice. There is of course a learning curve, but there is lots of on-line help and an active user forum. For understanding circuits like this it is really useful.
jremington:
If you want to experiment without having to build actual circuits, I recommend the free simulator LTSpice. There is of course a learning curve, but there is lots of on-line help and an active user forum. For understanding circuits like this it is really useful.
That is the answer I thought. I have played a little with some simulators, but there is nothing like others real life experience and knowledge.
Thanks to all who have shared and contributed to this thread. I have learned a great deal.
