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goldfingerfif
08-10-2006, 06:54 AM
I cannot find any examples or an explanation on how the LM358 works.· I am trying to make this http://www.teamfc3s.org/info/articles/fcd/diy_fcd.htm but instead of a potentiometer I wanted to use a digital pot controlled by the basic stamp.· I made this last night and the only difference is that my supply is 5v.· Even if I turn the potentiometer all the way to 0 so it is being clamped at 0v I still get an output of 1.6v.· If I clamp it at 5v (the input is 5v as well)· then the output is only 3.4v, I cannot understand why I do not get 0v or 5v and anything inbetween depending on what it is clamped at.· I tried looking up how the LM358 but the datasheets tell me nothing.· I do not get the in(-) and in(+) or why the in(-) is connected to the input.· I think that the gound voltage is going through the diode in the begining and that is why I still see 1.6v when it should be 0v.· I am very confused and have been googling about this and how it works with no luck, i even tried searching the forums. Nothing still.· Please help! :-)

Chris Savage
08-10-2006, 07:37 AM
Hello,

·· The LM358 is used in our Basic Analog And Digital manual...That may or may not offer you some help on this.· Here is a link.· http://www.parallax.com/dl/docs/books/edu/baad.pdf

It was also used in the Industrial Control (Replaced by Process Control) at the following links:

http://www.parallax.com/dl/docs/books/edu/ic.pdf

http://www.parallax.com/dl/docs/prod/sic/Web-PC-v1.0.pdf

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Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)

goldfingerfif
08-10-2006, 07:52 AM
Thanks.

Any idea on why my circuit doesn't go 0-5v?


Post Edited (goldfingerfif) : 8/9/2006 11:58:42 PM GMT

Chris Savage
08-10-2006, 07:58 AM
I was thinking maybe instead of focusing on this particular part you should search for a tutorial on Op-Amps...The minus and plus inputs are referred to as the inverting and non-inverting inputs, respectively.· The Op-Amp can also be configured as a comparator.· A tutorial would be the way to go.· I learned many years ago about Op-Amps from a Radio Shack book written by Forest M. Mims III.

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Chris Savage
Parallax Tech Support
csavage@parallax.com (mailto:csavage@parallax.com)

Sawmiller
08-10-2006, 08:24 AM
was reading understanding signals tutorial today, and in it , it says the lm358 is not a rail to rail op amp, it will lose 1.5v off the top and like .02 v off the bottom of your scale so if you want 0 - 5 v you have to have Vcc at + 7.5 v and Vss at - .02 ( this is the voltage feed to the op-amp )

so theres the reason... its not a rail to rail op-amp

dan

goldfingerfif
08-10-2006, 08:49 AM
Sawmiller said...

was reading understanding signals tutorial today, and in it , it says the lm358 is not a rail to rail op amp, it will lose 1.5v off the top and like .02 v off the bottom of your scale so if you want 0 - 5 v you have to have Vcc at + 7.5 v and Vss at - .02 ( this is the voltage feed to the op-amp )

so theres the reason... its not a rail to rail op-amp


Awesome!· Thanks, I was thinking that since they had a 12v supply, I will use the 9v supply off the basic stamp development board.· I also read that you need to put a 4.7k to 10k ohm resistor on the output to get it to drop to 0v when the input is 0v.· I found an awesome tutorial incase someone comes across this thread.
http://homepage.mac.com/tbitson/weather/TI_OpAmp_Tutorial.pdf
Thanks everyone again.· I am starting to think in 1's and 0's from thinking about electronics so much.· I knew nothing only a couple weeks ago and now have been obsesively learning with my basic stamp.