Question for the Oscilloscope Experts

[audioguru]

OK the st part is not available in Mexico.

Why can't you order one from USA?

I have now the following circuit as a possible ¿ Buffer As you explained ¿?
but now I found out that the probe of my oscilloscope is doing something strange. If I just connect the probe on Pin 1 I get an offset like the following one

You do not understand that current in a resistor causes a voltage across it.
The input current of an LM358 is the base current of its PNP input transistor. The datasheet specs a current of 250nA (0.25uA) maximum.
Then the voltage across R1 is 0.25uA x 4.7M= 1.175V maximum. But since R3 also has input bias current in it then the output offset voltage is reduced to 0.25V maximum plus any input offset voltage.

If I put a 220pF capacitor between Pin one and the probe i get a nice zeroed reading like the one below

Of course. The capacitor passes the AC signal but blocks the DC voltage.

Now I wired the Pin 1 to pin 5 of the Op Amp B unit and set a gain of 100 using pin 6 and 7 as usual.
but again I only get a zeroed reading putting a 220pF capacitor between probe an Pin 7 OUT

Because the capacitor passes the AC signal but blocks the DC voltage.

If I put the 220pf capacitor between pin 1 and 5 I get a high line and it only amplifies down to zero (0)

Then opamp input pin 5 is floating and is at a positive voltage caused by its input current. Then the opamp amplifies this positive voltage as high as it can go.The input MUST have a DC bias voltage, without the capacitor its voltage was the output voltage of the first opamp.

Why use such a low value as 220pF? Then if it drives a 'scope input that is 1M ohms it cuts frequencies below 727Hz. Vibrations usually produce very low frequencies.

 

[rsfoto]

Hi AG,

Thanks for the 2 posts. I did not open my PC starting Thursday 13th until today as we were visiting some friends

Let me digest your answers and then I will try to give you my comments. Again Thanks

 

[rsfoto]

rsfoto said: ↑
OK the st part is not available in Mexico.
Why can't you order one from USA?

I can order from USA but either the shipping costs are prohibitive or I have to wait a month till I get the stuff ...

 

[rsfoto]

Here I am again

I think I can now say that my project is ready to be soldered on a perforated board.

Here is a bit of info why

The sensor on the wall of my studio



**broken link removed**

The sensor on a tripod of a equatorial mount



**broken link removed**

and then on the window

**broken link removed**

Below the schematics of the circuit



Thanks to all for all your help

 

[audioguru]

Your circuit will not work:
1) The opamp is not biased so it will not have a proper AC output. The positive parts of the input will produce positive output pulses. The negative parts will do almost nothing.

2) The maximum input bias current of an LM358 is 250nA which is 0.25uA. Then if the input resistor to ground is 1M the input voltage will be +0.25V but your resistor is 9.7M so the input voltage will be 2.43V when there is no signal if your LM358 has the maximum input bias current. Use an opamp with a Fet input or reduce the value of the 9.7M resistor.

3) Your full wave bridge rectifier is not needed and does not work because it has only one input signal wire. Also the 10uF capacitor C2 is not needed.
If you need full wave rectification then you could use the unused opamp as an inverter to feed the other half of the full wave bridge rectifier.
I fixed it for you.

 

[cowboybob]

... I fixed it for you.

Nice, clean fix, AG.

 

[KeepItSimpleStupid]

Yep, much better!

 

[audioguru]

But the rectifier is not active so it does nothing until the signal peaks are more than about 0.6V.

 

[rsfoto]

Hi AG,

Thanks a lot for the new circuit.

Please do not misinterprete what I am writing next ...

I do not want to start a debate about this as all of you are by far much more experienced then me, but my findings with the oscilloscope makes me ask myself ...

¿ Am I lying to myself ? when I see differences when putting the piezo sensor at differnt locations and recorded them as you can all see in the screen captures and so I am totally cluless why does it work down to XXXmV (see video of the recording when the piezo sensor is sitting on the wall)after having there a Gain of about 200 times ...

When I hit slightly a wall 12 feet away I can clearly apprecciate a change in the curve at the same rythm I am hitting the wall ...



I am writing from my office and today afternoon I will set up on another breadboard the circuit AG has designed for me and measure with my oscilloscope

Again thanks a lot

 

[rsfoto]

Hi,

I can get the MC3408X here in Mexico for a reasonable price

http://www.hep.uiuc.edu/cleo/trig3/boards/tile/vme021d_data_sheets/mc34084.pdf

 

[rsfoto]

OK, I am home and the first thing I did was to measure the wave in different positions

After C2 but no rectifier

**broken link removed**

and then with no C2 but with rectifier

**broken link removed**

Do you want me to measure on some other point ?



I am waiting for the new opamp MC3408x and some precision resistor in order to build the circuit AG did the favor to design

Thanks and regards

 

[audioguru]

I do not know if the MC3408x opamps in Mexico are the version that must not have a gain less than 2 so they cannot be used for the inverting opamp I drew unless you add an attenuator and change my circuit. Also these opamps will not work in my circuit unless you add an additional negative power supply for them.

Your latest scope video with the LM358 shows that its output is at a positive voltage determined by its input bias current in R1. But R1 connects to ground so the input and output should be referenced to 0V. Since you have the (+) input of the opamp biased at 0V with R1 then the output is also supposed to be referenced to 0V then the signal would be half-wave rectified by the opamp but it isn't due to the input bias current in R1 raising it.

You use 4 diodes in the bridge but only D3 is doing anything and it acts like a half-wave rectifier.

The MC34084 is obsolete and is not made anymore. It is not even listed on ON Semi's website so I could not find out its minimum supply voltage.

 

[rsfoto]

But the rectifier is not active so it does nothing until the signal peaks are more than about 0.6V.

Hi AG,

So this means that also in your circuit nothing happens if the output signal from Unit1B is lower then 0.6V ¿ did I understand that correct ?

I took a look at the upper part and if I understand correctly you are feeding additionally 6V into pin 3 from Unit1A as well as into pin 5 from Unit1B.

What is the purpose of that ?

If I interpret the wiring of the Unit1B correctly is has a gain of 1 and it just takes the negative part of my Sinus wave at Pin 1 and sends it to the full bridde rectifier and so I get both portions of the sinus wave from Pin 1 at my outputs marked Out + and Out -

 

[rsfoto]

I do not know if the MC3408x opamps in Mexico are the version that must not have a gain less than 2 so they cannot be used for the inverting opamp I drew unless you add an attenuator and change my circuit. Also these opamps will not work in my circuit unless you add an additional negative power supply for them.

Your latest scope video with the LM358 shows that its output is at a positive voltage determined by its input bias current in R1. But R1 connects to ground so the input and output should be referenced to 0V. Since you have the (+) input of the opamp biased at 0V with R1 then the output is also supposed to be referenced to 0V then the signal would be half-wave rectified by the opamp but it isn't due to the input bias current in R1 raising it.

You use 4 diodes in the bridge but only D3 is doing anything and it acts like a half-wave rectifier.

The MC34084 is obsolete and is not made anymore. It is not even listed on ON Semi's website so I could not find out its minimum supply voltage.

Hi,

Here is the spec of the MC3408X

http://www.hep.uiuc.edu/cleo/trig3/boards/tile/vme021d_data_sheets/mc34084.pdf

 

[audioguru]

I guess your rectifier's load is the 1M or 10M input resistance of your 'scope so when D3 conducts slightly it is enough to show some half-wave rectified output.
My circuit biases the (+) input of the opamps at half the supply voltage so that their outputs can swing equally up and down so that the full-wave rectifier works properly.

 

[rsfoto]

I guess your rectifier's load is the 1M or 10M input resistance of your 'scope so when D3 conducts slightly it is enough to show some half-wave rectified output.
My circuit biases the (+) input of the opamps at half the supply voltage so that their outputs can swing equally up and down so that the full-wave rectifier works properly.

Thanks AG. I understand

 

[rsfoto]

Hi,

It is me again and as you see I am quite stubborn with this project.

Maybe not the best circuit but what comes out, I like it



and here a 1 min video. The sensor is on my desk and the vibration comes from tapping with the fingers.

**broken link removed**

Thanks for reading and watching.

regards Rainer

 

[cowboybob]

That's remarkable, rsfoto. Bravo!

You might consider adding a linear potentiometer (like this:

) so that you can adjust the gain, if need be.

 

[rsfoto]

That's remarkable, rsfoto. Bravo!

You might consider adding a linear potentiometer (like this: View attachment 96085 ) so that you can adjust the gain, if need be.

Thanks cowboybob for nice comment.

I still do not really understand what all the Capacitors and Resistors do there but I have been playing around with different values and so I found the depicted values in the circuit. Have read a lot, compared many circuits, etc. perhaps one day I will understand

.. and yes about the potentiometer, I think that will be necessary in order to adjust the gain when the sensor is on my pier and so I can find out where I put the limits and then compare with other parts of the building and the house so I gain experience in judging what and how moves everything ...

Again thanks a lot for your help.

 

[audioguru]

Your circuit has many parts that are not needed.

Capacitor C2 passes AC but not DC so the second opamp amplifies vibrations with a gain of 23 times but does not amplify its DC input offset voltage.
The first opamp has a gain of only 2 so it is not needed and the gain of the second opamp can be doubled instead.
You are using a polarized capacitor for C2 which has AC across it that is wrong. If the values of R4 and R5 are increased then capacitor C2 can be a small film (non-polarized) capacitor.
C3 is SHORTING the output of the second opamp so it should be removed. If you want to cut high frequencies then add an RC filter at the output of the opamp, my R6 and C3 cut frequencies above 97Hz.
You do not need two rectifying opamps.
My added diode prevents the rectifier opamp from saturating as low as it can go.
EDIT: I deleted my circuit because its halfwave rectifier was completely WRONG!