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Sound card oscilloscope probe

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JeffWired

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Hi All,

I want to use the PC sound card for some low frequency probing (audio equipment). I don't own an oscilloscope. I found this discussion around a PC sound card based oscilloscope probe. https://www.edaboard.com/threads/259543/ Out of the many designs scattered around the interwebs, this one seems to make the most sense. It provides some degree of sound card protection and allows for testing a variety of input voltages without putting a load on the circuit being tested.

Quick question:

- My sound card has a microphone full scale input of 1V RMS. https://www.conexant.com/wp-content/uploads/2014/06/pb_CX20672.pdf This circuit says it is designed to output 12V max. Rather than constantly have the resistance on the variable resistor cranked up (to bring the output voltage down), how could I bring that down a bit? Or if I just use his switch to go to 1X (instead of 10X) should I be ok? Most voltage I will be measuring will be in the 5-12V range, although could be as high as 67V, as low as audio line out voltages.

I anticipate I will always check the voltage with a multimeter prior to plugging it into my sound card, just for double safety. Is there a simple way to wire in an LED that will light up if the output voltage is over 1V RMS?

Thanks for any input.
Jeff
 

andre_teprom

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Is there a simple way to wire in an LED that will light up if the output voltage is over 1V RMS?

The LED with visible color that has the lowest voltage drop is the RED, whose value is ~ 1,63Vdc which is a little above the value of 1Vrms
 

JeffWired

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Thank you Andre, I could buy and test a few of those LED and see if I can find one that lights up a little lower.

On my other question, is there a way to adjust that circuit so that the maximum output voltage is about 1.5V instead of 12V? This way I could avoid the light test (or checking with my multimeter for that matter).

Thanks,
Jeff
 

andre_teprom

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Using a resistive divider in the operational amplifier output is enough for that.
 

JeffWired

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Thanks again Andre,

My understanding is that the circuit will currently perform as follows (assuming R6 is set to 0):

- between +150 and -150 V input the circuit will follow linearly in output between +12 and -12V
- over 150 V (-150) the circuit will shunt and the output will be max at 12V (-12)
- for small voltage inputs, for example 1V, the circuit has the ability to increase output x10, so for 1V input the output will show 1*(12/150)*10 = 12/15 V = .8V (when using the x10 switch)

I would like the same behavior, except where currently you see 12 in the above behavior, I want 1.5. Seems there should be a better way than dividing the output?

Thanks again and please be patient as my circuits course was 35 years ago.
 

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The datasheet for the TL082 shows that its output does not go to the supply voltages. With a plus and minus 12V supply and a 10k ohms load then its maximum output will be about plus and minus 10.5V. The datasheet has a graph that shows the maximum output voltage is less when the load resistance is less.

The TL082 is a dual opamp. If the second opamp is not used then it must be properly disabled. Use a TL081 single opamp if you want.
 

JeffWired

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Thank you again for the inputs.

So looking at that datasheet for the TL081 https://www.ti.com/lit/ds/snosbw6a/snosbw6a.pdf

- according to the Voltage Swing chart, if I put an input voltage on it of +-5V, then the output voltage swing would be about 8V with a 2k load
- going to the next chart to the right (Output Voltage Swing), it shows that for a Vs of +-15V reducing the output load from 2k to .2k drops the swing from 28 to 7, or a factor of 4.
- assuming the curve remains the same for Vs of +-5V, then applying that same factor of 4 it would reduce the swing from 8V to 2V for a +-5V Vs
- this would be right about where I want it, leaving me a little room in my roughly +-3V sound card input (1V RMS)

In the circuit design then, I change the Vs from +-12 to +-5. I want to leave the x10 switch in there to boost up low voltages. What would be my values for R4 and R5 to accomplish the above? I don't know how to figure this out. Or please suggest how I can figure this out.

Assuming the above is correct, then in theory I should be able to take the variable pot out, since my output voltage will never be above the input limit of my sound card. (exception would be if I happen to leave the switch in the x10 position and feed in a high voltage, then it could blow the sound card). Would need to always test with x1 first, then switch to x10 if the output is low enough. Could put a little dummy light on there to protect against that.
 

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Hi folks,

Can anyone help with my last post? At least tell me if I am on the right track?

Would like to actually build this little probe.

Thanks,
Jeff
 

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Your INPUT voltage is not +-5V, instead your SUPPLY voltage is +-5V.
You are looking at the maximum output swing. With a 200 ohm load then you are almost shorting the output so of course its maximum output swing is less. Do not overload the output to reduce the maximum output swing. instead use a voltage divider or a pot.

If you use a +-5V supply then the maximum output swing will be about +3.5V to -3.5V but will be lower if you overload it with a load resistance of less than about 2k ohms if the supply is +-15V or a load of 470 ohms if the supply is +-5V (7.5mA peak).

The voltage gain of a non-inverting opamp is 1+ (R5/R4). When the switch disconnects R4 then the voltage gain is 1. When R5 is 27k and R4 is 3k then the voltage gain is 10.
 

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Your INPUT voltage is not +-5V, instead your SUPPLY voltage is +-5V.
You are looking at the maximum output swing. With a 200 ohm load then you are almost shorting the output so of course its maximum output swing is less. Do not overload the output to reduce the maximum output swing. instead use a voltage divider or a pot.

Sorry for the terminology issues, I'm trying. Yes, definitely looking at the supply voltage and maximum output swing, I don't want to blow up my sound card.

If you use a +-5V supply then the maximum output swing will be about +3.5V to -3.5V but will be lower if you overload it with a load resistance of less than about 2k ohms if the supply is +-15V or a load of 470 ohms if the supply is +-5V (7.5mA peak).

But above you say don't overload it correct? this would be the wrong way to go about it.

The voltage gain of a non-inverting opamp is 1+ (R5/R4). When the switch disconnects R4 then the voltage gain is 1. When R5 is 27k and R4 is 3k then the voltage gain is 10.

So in summary, you are saying don't screw with the circuit? I can drop the supply voltage to +-5V but then use the pot to adjust the output voltage as necessary.

One last question, which is kind of where I started. How can I insure that the output voltage is never enough to blow up my sound card? Or is the answer to always turn the pot all the way down, hook up my test points, then gradually increase the pot until I get a measurable signal?

Thanks much for your assistance.
 

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