Question for the Oscilloscope Experts

KeepItSimpleStupid said:

Just TRYING REALLY HARD to try to explain Tony's second integral of g (m/s^2) is displacement (m) WITHOUT actually doing the math.

The similarity that you SHOULD be able to relate to is the car example.

G (acceleration due to gravity) is generally a constant for most problems, but in your case the change in g (now a small letter) is related to vibration (displacement).

Click to expand...

Hi KISS,

AH now I got. Why did you just not put at first sentence.

" Let me try to explain wht Tony wanted to express about displacement "

And then your first post and you have not needed to make the second long one.

Thanks

Tony Stewart said:

Unless your telescope has some slack in the moving parts which can cause high frequency buzz motion from low frequency vibration or a resonance, the only frequencies of interest you should be looking at are the seismic frequencies I posted. These of course are not sounds but vibrations you can feel and see.

10% or 1/10 of the signal in log scale is 20 dB

This requires a two stage low pass filter so that frequencies above 10 Hz are attenuated , so anything at 100Hz will be 1% of input or 40 dB down and at 1kHz 0.01% of input.

Since DC offsets with high gain saturate of clip the signal, DC must be balanced or blocked thus a high pass filter above 0.1Hz

So your range of interest is 0.1Hz to 10Hz. This 2nd order LPF slope effect is the same as converting acceleration, g to motion, x

everything else you might see is just interference or noise.

the initial assumption is critical however... no slack in moving parts, like the focus parts or scope mounts, bearings etc.

Click to expand...

Thank you Tony.

In about 2-3 weeks I will be back at my Observatory and I will do some tests with the circuit I have to see if anything happens.

I also have an DE-ACCM3D accelerometer there and I will connect it to the Oscilloscope to see is anything happens by knocking on my piers or any other thing I stick into the ground.

https://www.dimensionengineering.com/datasheets/DE-ACCM3D.pdf

Also I just got another accelerometer like the following one https://www.robodacta.mx/index.php?dispatch=products.view&product_id=85

So I can make some tests with them too.

You are putting at the moment too much emphasis on the telescope set up. That is the next stage I want to analyze.

I know you are en expert in this matter but I am not so it takes somewhat longer for me to get into this matter until I get it, so please bear with me and have a bit of patience until I start to understand it.

Thanks for the tips about buying the seismograph but as said where is the fun in just buying something. One that soon I will retire from my day job I have to find things to do in order that my retirement life does not get boring

Muchos Saludos

The fun is understanding what makes a good motion sensor, how to specify it and how to get the lowest frequency response for the low cost using moving coils or MEMs.

Hi,

I am back from my trip to the north after holding a presentation on how to build observatories for Amateur Astronomers.

Will now continue with my experiment and in the meantime it looks like I found an interesting commercial alternative which is similar to my first thinking about using operational amplifiers with the difference that there is at the output stage a comparator ¿?

Here is the info

**broken link removed**

and there is even a PDF of the schematic circuit

**broken link removed**

Taking a look of the datasheet of the used components looks like my LM358 as well as the LM393 I have here are 100% compatible

Will setup a breadboarded version and see what comes out.

I know I will have antennas, breadboarding it, but in the meantime looking at the Oscilloscopes curves and analyzing with FFT I can get an approximate idea if it works better then the first trial.

If it does not work I will buy the finished solution which is in the range of US $ 10.00 finished and then I can add the LM3914 for having an optical feedback.

Again these will not be an accurate indicator of ground movement as they are force (g) sensors (F=ma) and not position sensors ( 2nd integral) nor have a very good low frequency response.

The quality of a geophone is measured by its low frequency response and accurate of position w.r.t. freq.

The Seeedstudio schematic has a 10k bias resistor R1 for an electret microphone at the input that will seriously load down the output level from a piezo sensor.
Without that bias resistor the first opamp is simply a buffer with no gain that has a lowpass filter on its output. The second opamp has high gain at high frequencies and reduces its gain below 1600Hz so will have very low gain at the frequencies you want.
The comparator shorts an LED to turn it off then allows the 3k output resistor to very dimly light the LED when the comparator turns off.
Where will you connect an LM3914? The output is simply high or low.

Hi,

I am here again and using a very simple Op Amp LM358 circuit and got great results in regard to sensitivity. Also I am using a dual voltage supply with a LiPo delivering me -4.14V, 0V and +4.15V (a difference of 0.01V) I do not know how much this voltage difference is or is not affecting the result.

Look at the circuit with a Gain of ~100 except that at the moment of writing this I am feeding -4.14V into Pin 4 and +4.15V into Pin 8. BTW the pins 5,6 and 7 are connected to ground.



and here on the Oscilloscope the behavior of the LM358 using a double rail voltage supply



I thought that using a dual voltage supply the middle line of the wave would be along the 0V line but it happens to be that in the amplified signal = Yellow Line the center is on ~1.7V.

If I use a capacitor between R2 and GND my sensitivity does suffer a lot.

In the configuration I have now if let fall down a ball pen from 15cm and 3 feet away I get a nice visible signal on the amplified wave. I know I have again the 60Hz noise, but at the moment that is less of my concern as I can eventually live with it when connecting the output to the LM314 and just set the minimum limit on 1.85V for example and everything over that will be signalizing vibration.

Any comments about this behavior so I can understand what I am doing wrong ?

Thanks in Advance

PS I was watching yesterday something about a Inverting Amplifier, ¿ would I have some sort of advantage or is it the same ?

The offset of the OP amp and the transducer get's amplified by 100. 10 mV * 100 is 100 mV as an example.

So do what the suggested circuit does. One stage gain use some gain or a gain of 1. Then use a 2nd stage and capacitively couple the output from the first stage. The input Z of the second stage can be tailored to what you want.

You have to "juggle" stuff so it works. The high Z of the transducer messes certain things up. The buffer essentially does an impedance transformation where the filters (capacitive coupling) will now work.

Select C = 1/(2*PI*R*C); PI is that silly number 3.14...., C will be in Farads, R is the input Z or external added input load of the next stage. At f, the response will be down -3 db or the sqrt(2) or about 70.7%

The second stage will only amplify the input offset voltage of the LM358 and not the transducer.

The LM741 https://www.ti.com/lit/ds/symlink/lm741.pdf (DO NOT USE THIS OP AMP) has what's called an offset/null pin. This can be used to null the amplifier offset
So, you either need an OP Amp with a low offset or an offset null pin or offset null can be implemented using a summer.

KeepItSimpleStupid said:

The offset of the OP amp and the transducer get's amplified by 100. 10 mV * 100 is 100 mV as an example.

So do what the suggested circuit does. One stage gain use some gain or a gain of 1. Then use a 2nd stage and capacitively couple the output from the first stage. The input Z of the second stage can be tailored to what you want.

You have to "juggle" stuff so it works. The high Z of the transducer messes certain things up. The buffer essentially does an impedance transformation where the filters (capacitive coupling) will now work.

Select C = 1/(2*PI*R*C); PI is that silly number 3.14...., C will be in Farads, R is the input Z or external added input load of the next stage. At f, the response will be down -3 db or the sqrt(2) or about 70.7%

The second stage will only amplify the input offset voltage of the LM358 and not the transducer.

The LM741 https://www.ti.com/lit/ds/symlink/lm741.pdf (DO NOT USE THIS OP AMP) has what's called an offset/null pin. This can be used to null the amplifier offset
So, you either need an OP Amp with a low offset or an offset null pin or offset null can be implemented using a summer.

Click to expand...

Hi KISS,

Like the following one https://www.st.com/st-web-ui/static/active/en/resource/technical/document/datasheet/DM00062815.pdf

or https://www.ti.com/lit/ds/symlink/opa2134.pdf available in Mexico

OK. BTW I forgot to write in my previous post that using a single voltage I get the same behavior as using positive and negative voltage. Just as a side note.

Thanks and I am looking for a similar circuit you just described.

Hi KISS,

This is a bit of confusing

C = 1/(2*PI*R*C)

The st part doesn't look horrible and it has input protection. With a GBW of 400 kHz, a gain of 100 reduces the bandwidth to 40 kHz.

GBW = gain * bandwidth and is usually a constant for an amplifier. So, when you increase the gain, you reduce the BW. GBW is known as the gain Band Width product.
"product" comes from the multiplication

rsfoto said:

BTW the pins 5,6 and 7 are connected to ground.

Click to expand...

Pin 7 of the LM358 is the OUTPUT of the opamp you are not using. It might be trying to amplify its input offset voltage 100,000 times and go up in smoke because you should never short an output to ground.

Instead you should connect its (+) input to ground and connect its (-) input to its output so its gain is only 1 and its output will be close to 0V.

Hi,

OK the st part is not available in Mexico.

Now your formulas and explanations are getting more and more confusing.

OK. 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



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



I am clueless as usual

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. Look below



but again I only get a zeroed reading putting a 220pF capacitor between probe an Pin 7 OUT Look below



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

R2 out, R1 =0 C1 = out
You say your using a dual supply, yet your schematic is single.
So, are we single or dual?

This should be about what you had.

Now...

Put a 1 uf cap between pin #1 of OPA and pin #5 of OPB
Put a 1000 ohms to ground from pin #1 on OPA

This would reduce the bandwidth to 160 Hz or so.

KeepItSimpleStupid said:

R2 out, R1 =0 C1 = out
You say your using a dual supply, yet your schematic is single.
So, are we single or dual?

This should be about what you had.

Now...

Put a 1 uf cap between pin #1 of OPA and pin #5 of OPB
Put a 1000 ohms to ground from pin #1 on OPA

This would reduce the bandwidth to 160 Hz or so.

Click to expand...

Hi KISS,

Sorry, I am now working single supply from a 11.1V LiPo

rsfoto said:

Hi KISS,

This is a bit of confusing

C = 1/(2*PI*R*C)

Click to expand...

Yep would be. Sorry.

R = 1/(2*PI*f*C) or C = 1/(2*PI*f*R); or f = 1/(2*PI*R*C) is frequency. the 2*PI converts to radians

My head can't process the single/dual differences tonight.

Hi KISS,

Well, here you can see the latest circuit



and here the output directly read from the bridge rectifier with the Oscilloscope probe connected to + and the Oscilloscope ground to ground. The result looks quite good and the sensitivity is also great but still have to test it somewhere to see more.

Your tip of using a buffer at first stage made the trick.



Now I will add the LM3914 as I have seen that the slightest movement already gives me a good signal. Will glue it onto the window or wall and observe what happens.

Even when I walk at about 10 feet away I already get a vibration signal

Thanks a lot

audioguru said:

Pin 7 of the LM358 is the OUTPUT of the opamp you are not using. It might be trying to amplify its input offset voltage 100,000 times and go up in smoke because you should never short an output to ground.

Instead you should connect its (+) input to ground and connect its (-) input to its output so its gain is only 1 and its output will be close to 0V.

Click to expand...

Hi AG,

Thanks. LM358 is still alive and on the previous post you can see how it is working now

Again Thanks. Will remember that for future circuits.

Since your opamps do not have a negative supply in your latest schematic and their (+) inputs are also at 0V then they rectify the signal so your diodes are not necessary. The outputs of the opamps cannot go negative without having a negative supply.
Why use two opamps?
Your capacitor value is much too small so it cuts most vibration frequencies.
Look at this: