ok, I need help

Discussion in 'General Electronics Chat' started by jimbarstow, Dec 10, 2011.

  1. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I'm trying to build a water tank depth sensor that logs measurements over time. The first thing I need to do is get the pressure sensor circuit working. I'll then hook it up to an arduino and do the rest. I've got 30 years of experience in software but I can't get this circuit to work. I'm using the simplified circuit here:

    http://cache.freescale.com/files/sen...f?fsrch=1&sr=1

    I really have no idea on what is wrong. I've built it on a breadboard and gone over it 100 times. Perhaps I haven't translated from schematic to physical circuit correctly. Perhaps I destroyed some component with static (I didn't use a wrist strap). I really don't know.

    With software, I have a huge arsenal of tools to use to track down problems but I haven't a clue how to start when debugging hardware.

    What I'd really like is to find someone with a lot of circuit building experience who can show me how to figure out what is wrong. The next time this happens, I want to be able to figure it out myself.

    So, where do I find someone like this? I'm in the SF Bay Area and there is a place in SF called "Tech Shop" but they want $95/hour to provide help. I'm willing to pay but that is a little steep.
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
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    The link doesn't work.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
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    The link doesn't seem to work for me.

    hgmjr
     
  4. praondevou

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    Jul 9, 2011
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    Can u please repost the link. It doesn't work.

    Please also post a picture of your breadboard in good resolution. png or jpg files are preferred.
     
  5. tracecom

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    Post deleted.
     
    Last edited: Dec 10, 2011
  6. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    sorry about that, should have checked the copied link:

    http://cache.freescale.com/files/sensors/doc/app_note/AN1324.pdf?fsrch=1&sr=1

    The simplification is to eliminate the trim pot and 1 resistor. I'll just calibrate it in software.

    The behavior is that I get a constant 3.85V on the output and there is no change in the voltage when I change the pressure on the transducer. Basically, I've built a complicated circuit that converts the output from a 9V battery to a constant 3.85V; not very useful.
     
  7. tracecom

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    The first problem is that the regulator needs at least 2.0 V higher input than its output. If you are using a 9 V battery, the output won't reliably be 8 V. What does it measure under load?

    I don't see a trim pot. Maybe I am looking at the wrong schematic? Is it Figure 2. Sensor Mini Block?
     
    Last edited: Dec 10, 2011
  8. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    on page 3, r7 is the trim pot. If you're willing to calibrate in software (I am), you can eliminate R7 and R6. They provide alternative values for R3 and R5 on page 4 if you do this.

    Before I decided to start on this project, I read everything I could on interfacing depth sensors to microcontrollers. The design in the linked document comes from the supplier of the pressure transducer so I thought it might work better. (It probably does but not when I build it...)

    Any advice on finding a local resource who could look at my circuit and basically teach me how to figure out what I did wrong?
     
  9. tracecom

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    Apr 16, 2010
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    OK, I see R7, but it's not drawn as a pot, just as a fixed resistor.

    My only idea is to look for an amateur radio club; many "hams" still like to build and are only too willing to help others.

    I really think there are some quite knowledgeable people on this forum who can and will help if you are willing to answer some questions and post some good photo's of what you have done.

    I am a relative beginner to electronics design and construction, so I tend to look for the simple errors first, i.e., is the power properly regulated and connected? That is the reason for my question above. The datasheet for the voltage regulator you are using clearly states that the input voltage must be at least 2 V higher than the output. That isn't the case with even a brand new 9 V battery, and those small batteries drop in voltage very fast. Have you measured the voltage output from the regulator, and have you measured the voltage on the power pin of the op-amp? If not, that's the place to start.

    ETA: I have read enough of page 4 to realize that you have likely made some significant changes to the circuit. Thus, the first thing that is needed is a schematic drawing of exactly how you have built (or think you have built) the circuit. That will be required for anyone trying to help, whether on this forum or local to you.
     
    Last edited: Dec 10, 2011
  10. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I measured the output from the voltage regulator and it was 8 volts, as expected. I'll try with a higher input voltage thought to see if that makes a difference.

    The document states:
    I haven't made any changes to the circuit other than the suggested simplification on page 4. The changes are really minor; 2 resistors (R6 and the one labeled "R7 trim") are eliminated and 2 others use different values.
     
  11. thatoneguy

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    Feb 19, 2009
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    Post a high resolution image of your breadboard and we can look it over.

    Do you have a decent DMM and a scope? Scope isn't really necessary for this, but just in case, I'll ask.

    I attached the PDF to this post so it doesn't vanish again.
     
  12. jimbarstow

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    Nov 8, 2011
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    I don't think a picture of the breadboard layout would help since it's pretty dense. I'll pick up a bigger breadboard at Radio Shack tomorrow and re-do my assembly so it will be easier to see. I'll post that. Thanks for the offer of help.
     
  13. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I got a larger breadboard and rebuilt the circuit. I had the unrealistic hope that it would magically work but I got the same results as before: a constant 3.85V on pin 7 when it should be close to zero. (Or at least pretty low.)

    Image attachments are limited in size so I zipped them up.

    If anything else would help, please let me know.

    Probably pointless but I'm also going to call UC Berkeley to see if there is some EE undergrad who'd like to pick up some beer money.
     
  14. thatoneguy

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    Download irfanview, it will save your images to be smaller than 200k (the limit) without loss of quality. It is free software to boot!

    Most here are extremely wary of .ZIP files, due to viral loads being common.
     
  15. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I resized the pictures down to 1024x768 and they are still pretty good. They are attached (not in a zip this time).

    thanks
     
  16. tracecom

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    It's a little hard to tell from the photo, but it looks to me like the regulator in and out connections are reversed. As you look at the flat side of the regulator, the input should be on the right and the output on the left. Is that the way you have it?

    ETA: It may be there, but I also don't see where you have the ground side of both busses connected together.
     
    Last edited: Dec 12, 2011
  17. thatoneguy

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    Try to get it one big picture, the wires and all, at 3200x1500, then save it to your computer, open it in irfanview, pic "Save for Web" and "Specify Size" of 200kB.

    It will make those pixels fit into 3200x1500, slight artifacts, but at that resolution, they aren't noticeable when one zooms in to see detail.
     
  18. jrm

    Member

    Oct 11, 2011
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    Freescale makes pressure sensors with a 5v supply that will simplify using with an Arduino.
     
  19. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    Yes, the ground busses are connected. (You can just make it out in the 954 image.

    I use one input power bus for power from the battery. This feeds to the voltage regulator only. The output from the voltage regulator is connected to the power bus on the opposite side of the breadboard. Pin 1 of the regulator should be power out and that is the one I connected to the alternate bus.
     
  20. jimbarstow

    Thread Starter New Member

    Nov 8, 2011
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    I'll do this tonight.

    thanks
     
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