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 Posted: Jun 24, 2012 - 04:30 PM |
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Joined: Mar 24, 2012
Posts: 17
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I bought a poScope sometime back and have not been able to use it effectively. This is my first attempt to use an oscilloscope of any type, so to be honest, I don't know where my ignorance stops and the limits of the USB scope start. I was thinking of buying another scope but not sure if I will do any better. Anyone got any ideas that might help me either learn to use this scope better or decide I need a different one? Are there standard projects out there somewhere that will help develop oscilloscope techniques, or at least get enough smarts to decide if it's time to cut bait and try another one?
Any imput would be greatly appriciated.
Example of what doesn't seem to work for me... I have a LED, a 5v source, a regular resister and a photoresister. Setup a simple circuit that will dim the LED when I place my finger over the photoresister. Tryed to observe this on the scope (ground to positive side of LED), but could not even though I could see the LED dim and brighten. |
Last edited by CRChisholm on Jun 24, 2012 - 04:50 PM; edited 1 time in total
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Posted: Jun 24, 2012 - 04:44 PM |
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Joined: Nov 22, 2002
Posts: 12198
Location: Tangent, OR, USA
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Each input has an AC/DC switch. For your LED measurement, the switch has to be on "DC". I think you want triggering off so that it will free run.
Connect the ground for the scope input to the ground side of the LED and connect the probe to the other side of the LED.
Hope this helps
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jun 24, 2012 - 07:26 PM |
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Joined: Mar 24, 2012
Posts: 17
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Quote:
Each input has an AC/DC switch. For your LED measurement, the switch has to be on "DC". I think you want triggering off so that it will free run.
The only switch I see is on the probe and it is labeled X1 and X10. Tried both no diffence. The circut is as follows
GND--[180k]--[photo resister]--[- LED +]---5v
i attached the probe ground to GND and the probe to LED + Shading the photoresister dims the LED but there is no percievable dip in the scope (unless I unplug the 5V compeletey.
Thanks for the response. |
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Posted: Jun 24, 2012 - 07:58 PM |
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Joined: Mar 29, 2006
Posts: 568
Location: Western New York, USA
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Quote:
The only switch I see is on the probe ...
I just downloaded the user manual for the PoScope Basic. Picture 1 on page 2 shows a DC/AC switch next to each input, right where Jim said they should be.
Don |
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Posted: Jun 24, 2012 - 09:22 PM |
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Joined: Nov 22, 2002
Posts: 12198
Location: Tangent, OR, USA
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Thats the picture I looked at.
Maybe you got something that is always AC coupled. If this is the case, then you won't see any change in a DC circuit, such as what you have.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jun 25, 2012 - 12:29 AM |
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Joined: Nov 01, 2008
Posts: 192
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CRChisholm wrote:
GND--[180k]--[photo resister]--[- LED +]---5v
Try probe ground to GND and probe tip to the 180k-photo resister junction. |
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Letting the smoke out since 1978
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Posted: Jun 25, 2012 - 01:31 AM |
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Joined: Jul 02, 2005
Posts: 6039
Location: Melbourne, Australia
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CRChisholm wrote:
The circut is as follows
GND--[180k]--[photo resister]--[- LED +]---5v
i attached the probe ground to GND and the probe to LED +
OK, I see two problems  by connecting to Gnd and the LED+ you are only looking at your power supply which is probably constant.
If you swap the connections so that your LED is in place of the resistor you will see the voltage across the LED more easily.
GND --[- LED +]--[180k]--[photo resister]---5v
^................^
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CRO probes
The second problem is the value of your current setting resistor. With 180Kohms, you will probably not get any visible light. If the photo resistor is fully illuminated, its value will be low (high in darkness); lets say zero and that the Vf of your diode is say 2 volts. That leaves 3 volts across your 180K resistor producing about 16 uA which is not enough to produce light. If you used a 180ohm resistor you could have closer to 16 mA and definitely see light. However in reality, photo resistors have a fully illuminated resistance of closer to 500 or 1000 ohms ... not zero. So your current would be closer to 4 or 5 mA {3/(180+500)}. This would still be visible.
Cheers,
Ross |
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Ross McKenzie
ValuSoft
Melbourne Australia
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Posted: Jun 28, 2012 - 05:03 PM |
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Joined: Mar 24, 2012
Posts: 17
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| I'm very sorry. The 180k IS in fact 180 ohm. I will try your suggestion and see if I can scope the change. There is a side of me that wants this poScope to be useful even if I didn't spend a lot on it, but another side that say, "Yah, but another scope would be cool". Adult time I guess. |
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Posted: Jun 28, 2012 - 07:25 PM |
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Joined: Nov 22, 2002
Posts: 12198
Location: Tangent, OR, USA
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If there is no AC/DC switch, it must ALWAYS be in DC mode or in AC mode.
How do you tell? Use the scope probe to look at the power supply voltage. Ground probe lead to the negative side of the supply. If the trace shifts when you first turn the power supply on, then drifts back to zero, it is permanently AC coupled. If it shifts and stays shifted, then it is permanently DC coupled.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jun 28, 2012 - 08:34 PM |
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Joined: Mar 24, 2012
Posts: 17
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| Thanks Jim, but this thing is really bare bones apparently. There are no switches anywhere on it except for the x1 and x10 I told you about on the probe. All other setting are through the software and pretty much limited to voltage, sample size, analog, digital and trigger setting and that's about it. Stricktly a USB scope. |
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Posted: Jun 28, 2012 - 08:55 PM |
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Joined: Nov 22, 2002
Posts: 12198
Location: Tangent, OR, USA
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Sure, but you still need to know whether or not it is AC coupled. I would hope not, but it is important to find out.
And, making that determination will give you some important learning about using it.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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Posted: Jun 28, 2012 - 09:10 PM |
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Joined: Apr 25, 2004
Posts: 3817
Location: Denmark
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Could "Analog" be AC & Digital DC ?
/Bingo |
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Posted: Jun 28, 2012 - 10:10 PM |
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Joined: Nov 22, 2002
Posts: 12198
Location: Tangent, OR, USA
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Bingo has a good suggestion there.
Try my coupling check in each of those settings.
Jim |
_________________
Jim Wagner
Oregon Research Electronics, Consulting Div.
Tangent, OR, USA
"The only thing standing between us and victory is defeat" P.G.Wodhouse in Wooster & Jeeves series
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