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How can I generate 0.1v output?

Discussion in 'General Electronics Discussion' started by vtdave, Aug 18, 2014.

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  1. vtdave

    vtdave

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    Aug 18, 2014
    Hello,
    My colleague and I would like to conduct some simple experiments on electroreception in small sharks (in a lab at a marine bio facility in Woods Hole), and need to generate approx 0.1v across two silver wire electrodes in a salt water tank.

    Keeping in mind that I know very little about electronics, is there a simple and inexpensive way to generate the 0.1v, ideally from a dc source such as a couple of aa batteries or a single 9v battery?

    Thanks in advance for your help!!
     
    chothuexeotosaigon likes this.
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    0.1V is lower than most voltage regulators can produce (most are limited to 1.25V (or thereabouts) as a minimum.

    However there are several ways to do this, so don't despair!

    Firstly, can you answer the following:
    1. Are you familiar with three terminal regulators?
    2. Are you familiar with op-amps?
    3. Can you solder?
    4. Do you have access to a multimeter and a soldering iron?
     
  3. vtdave

    vtdave

    7
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    Aug 18, 2014
    Hello, thanks for the response.
    Per your questions:
    I am not familiar with either 3 terminal regulators nor op-amps.
    I do know how to solder and have access to both a multimeter and a soldering iron.
     
    Last edited: Aug 18, 2014
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    What current do you need?
    Which frequency has the signal (I'd guess it is almost DC)?
    Would it be feasible to manually adjust the voltage, or di you require an automatic regulation of the voltage?

    Manual adjustment could be done with a series resistor and a potentiometer, provided you need only a small current. For automativ adjustment of higher current you'd need an active voltage regulator. An operational amplifier with rail-to-rail output can be used for this purpose. If you use two batteries, almost any operatinal amplifier can be used plus the polarity of the signal can easily be inverted.
    A very basic circuit looks like this:
    [​IMG]
    It requires 2*9V batteries, 2*1kΩ resistors and a 100Ω potentiometer. The potentiometer is used for adjusting the output voltage on electrode_1. Almost any operational amplifier can be used, e.g. a µA741.
    Note that the output voltage depends also from the stability of the power supply (batteries). You will have to re-adjust the output voltage as the batteries get drained. Alternatively you can use voltage regulators to stabilize the supply voltage. Use 78L05 for the positive rail and 79L05 for the negative rail. You will need additional capacitors with the regulators for stable operation, see the datasheets (links above).
     

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  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I was thinking of using a pair of adjustable three three terminal regulators. One is adjusted to have a voltage 0.1 volts above the other. The lower voltage one is loaded with a resistor so that the maximum required current is drawn. The output is taken across both regs. Because these regs can't sink current, the max available current is the static load on the lower voltage regulator.
     
  6. vtdave

    vtdave

    7
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    Aug 18, 2014
    Thanks!
    A few questions (please excuse my ignorance about basic electronics!):
    1. In the circuit you provided, can I assume that U1 is the operational amplifier?
    2. If so, is an operational amplifier such as a µA741 likely to be available at a local radio shack or will I have to order something like that online? If something simmilar to a a µA741 is likely to be available at radio shack, what exactly should I ask for at the store? The folks that work there tend to be teenagers who know as little as I do about electronics!
    3. A quick look online shows a huge range of op-amps... what specifications do I need?
    4. The op-amps I saw all seem to have 8 pins. Which pins should I use?
     
    Last edited: Aug 18, 2014
  7. vtdave

    vtdave

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    Aug 18, 2014
    Thanks Steve.
    I'd be grateful for more specific instructions (sorry, I'm pretty clueless with this stuff)...
    For example, what type of 3-pin regulators would I look for?
    Am I likely to find them at a local radio shack?
    How do I adjust them?
    Could you please provide or describe the circuit I'd need?
    Thanks again!
    So grateful for the kind folks who post here.
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    1. yes
    2. rs should have them
    3. nothing very demanding. The 741 should be ok
    4. you read the datasheet.
    also you need to answer the question about the current you require.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'm not using a device where I can draw you a circuit right now. I'll draw you something in 8 hours of so if nobody beats me to it.

    knowing the required current is very important
     
  10. vtdave

    vtdave

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    Aug 18, 2014
    Hi, it looks like we need a very low current, something in the order of 6.0 μA (but this number is not set in stone).
    Not sure if this matters, but the electrodes will be in salt water, about 1 cm apart. I believe the conductivity of salt water is such that there will be negligible resistance, but I could certainly be mistaken. The goal is to create a weak electrical field to simulate prey.
     
  11. Laplace

    Laplace

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    Apr 4, 2010
    There may be a simpler way. Where I live there is no access to seawater so I put 3 teaspoons of salt in a cup of water. I don't have any silver wire so I took two pieces of 18 AWG brass wire each one-inch long and clipped on VOM leads. Then I inserted these into the saltwater and measured the resistance, initially at around 2K and rising to 3K after about 15 seconds.

    So I would try a simple voltage divider with a Thevenin equivalent resistance about 10% of the seawater resistance. Take one AA battery, one 2700 ohm resistor, and one 220 ohm resistor all connected in series. Attach the electrodes to the 220 ohm resistor which should have about 0.1 volt across it when the electrodes are submerged.

    Note the distance between the electrodes in the seawater did not seem to have much affect on the resistance but the length of the brass wires in contact with the water did. Depending on the length (or surface area) of the silver electrodes the resistor values may need to change.
     
  12. vtdave

    vtdave

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    Aug 18, 2014
    Thanks for all the advice. I'll let you know how it works...
     
  13. dcac

    dcac

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    Jul 10, 2013
    The effect of voltage might be hard to interpret. Let me illustrate for you. The sea water might have keys say 2k resistance at normal conditions. Then creating a constant 0.1v across two points means keeping the resistance at constant and adjusting current. If resistance changes ( waves and temperature of the water) then the current have to be adjusted. So at different times the current is different. The change in the current you also changing the magnetic field of of the wires . Sharks are well known for being sensitive to magnetic fields keep in mind that too.
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    One answer is to use wires that are insulated for all but (say) the last couple of mm. If they are always immersed, waves on the surface will not chage the resistance so much.

    You might also consider using a constant current rather than a constant voltage.
     
  15. BobK

    BobK

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    Jan 5, 2010
    Could you measure the resistance of the probes as they will be used?

    If you think 6uA is required, that would translate to 16K Ohms. I doubt that it is that high.

    And by the way, you cannot specify both current and voltage.

    Bob
     
  16. vtdave

    vtdave

    7
    1
    Aug 18, 2014
    I was able to generate a voltage down to 0.1V by simply using a 9V battery and a 25Ω potentiometer (actually called a rheostat on the package, not sure if there's a difference) along with a device I dug out of an unused lab that was designed specifically to multiply the voltage by 0.1 or 0.01. I suppose I should have looked for that first...
    We performed a few initial experiments which appeared to be successful -- the sharks did indeed strike at the electrodes when the voltage was down around 0.7V (they did NOT like 9V, which we accidentally sent into the tank at first).
    In any case, we now intend to use AC because that will better mimic prey (apparently the current alternates as a prey fish opens and closes its mouth.
    I have located a function generator (wavetek model 190 20MHz function generator) and an oscilloscope (tektronix TDS 1002). I am now doing my best to generate approx 6uA from the generator while simultaneously measuring the signal from the generator on channel 2 of the oscilloscope. The signal from the generator is split, with one lead going to the oscilloscope (ch 2) and the other lead going to the silver electrodes in our shark tank (obviosly not using it with live animals until the system is set up and has been determined to be robust and safe). A second set of electrodes, used for measuring the signal underwater, is placed in the tank approx 10 cm from the generator electrodes, with the lead going to channel 1 of the oscilloscope. The two signals can thereby be compared, and the measurement electrodes can be moved closer or farther from the source to watch the drop in current.
    So far, the biggest problem has been that the measurement electrodes are clearly not measuring the electric field. We have had a constant signal (a bit weaker than, but matching the frequency of the input from the generator on channel 2), but that signal does not get much weaker when moved away form the source electrodes, even when placed 4-5 meters away, on the opposite end of the tank! I believe the signal we are picking up is somehow coming from the 2nd lead out of the generator and into channel 2 of the oscilloscope, because when channel 2 is unplugged, the signal on channel 1 is reduced to background noise despite the fact that there should still be current going from the generator to the input electrodes (only the split signal from the generator to the oscilloscope was removed). I have checked and the math function is not on (adding channels 1 and 2). I'm baffled. Going to sit and read through both manuals today. any thoughts would be most appreciated.
    Thanks again for the continued help!
     
    Last edited: Aug 20, 2014
  17. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    It is possible that you get a coupling between the different channels of generator and oscilloscope. However, from your verbal description the setup is somewhat hard to decipher. Can you post a schematic diagram of your setup?
    Similar to my post #4. Maybe someone at your facility can help you sketch the diagram with meaningful symbols.
     
  18. Laplace

    Laplace

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    Apr 4, 2010
    How fast does a prey fish open and close its mouth? Or what is the frequency of the AC signal? Consider using an isolation transformer to couple the signal into the submerged silver electrodes. You don't want any common mode voltage inserted from the power system. If the tank is metallic, consider grounding the scope (and the isolation transformer shield) directly to to the tank. For measuring AC fields in the water consider using capacitive coupling to the scope probes and use the A-B display to show the potential difference between the pickup electrodes. It may be useful to externally horizontal sync the scope directly to the signal generator.
     
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