Static meter incorperating op amp

Discussion in 'The Projects Forum' started by Richtea, Mar 24, 2010.

  1. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    I'm trying to build a static meter by using an op amp (see attached circuit diagram). When I created a similar circuit using a single power supply (the negative end defined as ground) the circuit worked fine, but would only sense static charges of one polarity, to try to solve this I changed to the circuit shown in the diagram using 2 batteries and in between the 2 defined (arbitrarily) as ground.

    This circuit gives a reading on the moving coil meter even when no static is at the aerial, even when the inverting and non inverting inputs are wired together in fact.

    If anyone could give me a steer in the right direction I’d very much appreciate it
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I believe you should use a center 0 meter that can show signal variation from both poalrity
     
  3. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    You are definately right that i will eventually need a moving coil meter that can read +ve and -ve, but that isn't currently my main problem, because that meter would also show a positive value when theres no voltage at the aerial. (and the moving coil meter i'm using will go slightly negative so i could tell the difference between zero output and negative output)

    P.s. i should mention this is a hand held device which is why ground is hard to put into the circuit

    I have also attached the circuit diagram for the static probe that will only sense 1 polarity of static (i presume negative voltage static)
     
  4. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Post ur resistor values too
    It is induced voltage you are seeing
     
  5. Richtea

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    Mar 18, 2010
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    Resistors are variable to adjust gain, but we're talking of the order of 200 ohms in the resistor connected to the aerial and 5 Meg Ohms in the other (feedback) resistor.

    I don't think i'm seeing induced voltage, there are no magnetic fields near what i'm measuring, only electric fields. Although the possibility i might be measuring change in electric field not actual electric field has occured to me (and worried me)
     
  6. Wendy

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    Mar 24, 2008
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    I have a major problem with your design. You should have as high an input impedance as you can get, instead the input resistance is the resistance you have on the aerial (look up virtual ground in context with op amps). Use a non inverting op amp design, and put a 10MΩ (or bigger) resistor on the plus input to ground. This will give the high impedance you need.
     
  7. Richtea

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    Mar 18, 2010
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    I have built what I think you describe, and the circuit diagram is attached below, however it gives no output at all now.
     
  8. Wendy

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    Actually you didn't. You used a 100Ω resistor for the arial, so the input impedance is 100Ω. You need a high resistance, say the 10MΩ you are throwing around.

    Did you try to look up the concept of virtual ground? It is key to where you are going wrong.

    When I get a chance I'll sketch up something.
     
  9. Wendy

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    Mar 24, 2008
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    [​IMG]

    R5 can be upped to 100KΩ or 1MΩ to increase the gain X10 or X100 respectively.

    What kind of op amps are you using?

    Do you want to go over virtual grounds in op amps?
     
    Last edited: Mar 26, 2010
  10. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    I did lookup virtual ground and my understanding was that the first op amp (with the output connected to the non inverting input) generates the virtual ground at the output, is this not correct?
     
  11. Richtea

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    Mar 18, 2010
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    P.s. And if I am correct (a dangerous assumption) what does the ground in your diagram mean?
     
  12. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    I think i'm there! I couldn't get your circuit to work (probably because i'm still building it wrong) but ive got a design of my own (modified version of something in the All about circuits book if im 100% honest) to work. I think it includes the high impedance between the antennas and ground you wanted (there was a slight offset before i put those in). They're 2MOhm resistors because thats the highest resistor i have available (and long series chains of resistors get in the way a bit). There are 2 antennas now so one can be either put a long way from a source of static or prefereably on some grounded bare metal and the circuit will compare the 2
     
  13. Wendy

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    Mar 24, 2008
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    Nope. Virtual ground is on the other input, which matches the grounded input. It is "virtually" a ground.

    You built a instrumentation amplifier, which is OK if overkill.

    Like I said, if you want to go into it I would be happy to diagram the concept. It is fundamental to op amps.

    The ground symbol in this case is nothing more than the negative side of the power supply (battery). Ground in electronics can have several meanings, usually it is a common connection point.
     
  14. Richtea

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    Mar 18, 2010
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    If you're happy to go through it i'd be very happy to hear about virtual ground, it'd be a useful concept to understand.

    Assuming my circuit is ok (if ludicously complicated) i'll stick with it for the time being though, under the basis that if it works it works
     
  15. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    Hm, slight oddity, theres still a slight offset that reads even when the two anntenas are in identical electric enviroments (right next to each other but not touching), actually touching them removes the offset. The offset (unsurprisingly) gets bigger as the gain is increased, decreasing the resistances of the resistors between the antennas and the output of the first op amp (which is at ground but not connected to ground i think) decreases the offset but (again unsuprisingly) makes the static meter less sensitive and generally a bit rubbish
     
  16. Richtea

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    Mar 18, 2010
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    p.s. i've made the two antennas as identical as i can make them
     
  17. retched

    AAC Fanatic!

    Dec 5, 2009
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    Take a picture of your setup, if you could. I would like to know the size of your antenna (shut up) and what fields you are measuring. Are you using a ground rod or the ground pin on your home power? and are you using a diode to dull reverse measurements coming from ground?
     
  18. Richtea

    Thread Starter New Member

    Mar 18, 2010
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    I can take a picture when i get home tonight but for the moment, both antennas are 0.5 meters long of normal circuit building wire, with perhaps 2cm exposed at the end. I am using no ground of any kind as its a portable devive (except the leftmost op amp that i'm hoping is acting as the common reference point for 0V and what i'm refering too when i say ground. And i'm not using a diode in my circuit
     
  19. Richtea

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    Mar 18, 2010
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    heres the photo
     
  20. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    You just need to apply some offset compensation. Check with the application notes of your particular op-amp on how best to do this.

    Eric
     
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