how to control current in stimulating electrode

Discussion in 'Embedded Systems and Microcontrollers' started by surina, Nov 25, 2015.

  1. surina

    Thread Starter New Member

    Nov 11, 2015
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    Hi, I am going to design some kind of stimulating electrode circuit with some limitation parameters. I am using arduino pro mini as my microcontroller and using potentiometer (MCP4131) to control the voltage. I am having a problem on controlling the current produce since what its safe condition is 2mA. I have a rough ideas on this circuit which I will use op-amp (LM358) and transistor (BD135) connecting to the stimulating electrodes but I dont know if it will produce current as low as 2mA. So, my question are :

    1) how can i control the current to be as low as 2mA?
    2) do i need to place more resistors on the circuit and how much it is to be supposed to?

    Any help would be very appreciated !! Thanks.
     
  2. DickCappels

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    Aug 21, 2008
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    Is this for tDCS?
     
  3. surina

    Thread Starter New Member

    Nov 11, 2015
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    Nope. The stimulating electrode will not be apply for neuro stimulation.
     
  4. DickCappels

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    Aug 21, 2008
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    Then what is its purpose?

    A lot of tDCS experimenters use the LM334 or similar programmable current sources. Would that be suitable for your application?
     
  5. surina

    Thread Starter New Member

    Nov 11, 2015
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    The future works for this circuit is to put it on some parts of nose. But in meantime, I want to develop the circuit for stimulating electrode and trying to learn and practice on controlling its current and voltage.
     
  6. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Post your circuit, then we can assist you more,.
     
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A BD135 is overkill if you want to draw only 2mA. The opamp alone could provide that.
     
  8. surina

    Thread Starter New Member

    Nov 11, 2015
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    here is my circuit and it is just a rough idea. but it seems like i have to get rid of BD135? sample.png
     
  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    If you limit the electrode current to 2mA, what is the open circuit voltage allowed between the electrodes (i.e. compliance of the current source)?

    Doesn't the Arduino need to control the magnitude of the electrode current, i.e. 0 to 2mA?

    Isn't a stimulus that has a DC-component a really, really bad idea (Electrolysis of the electrode)?
     
  10. Alec_t

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    Sep 17, 2013
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    Your opamp is configured incorrectly. As shown, its ouput will be permanently low and the BD135 will be off.
    I suspect you will need quite a high voltage to get 2mA to pass through nose tissue, if the electrodes simply contact the skin rather than penetrate it.
     
  11. DickCappels

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    @Alec_t what do you think the best arrangement for the opamp would be for surina's purpose?
     
  12. Alec_t

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    Sep 17, 2013
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    If it were me I'd probably try something like this ...
    2mA-source.PNG
    I don't say it's the best way, since I have doubts on the efficacy of a DC stimulus :).
     
    DickCappels likes this.
  13. surina

    Thread Starter New Member

    Nov 11, 2015
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    Oh great, thanks. but the circuit still can draw high current if the voltage is increase. I just want to make the circuit can have a maximum current as high as 2mA.
     
  14. DickCappels

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    To make @Alec_t 's circuit produce a constant 2 milliamps, regardless of the voltage at + supply 2, when the input voltage is 5 volts (plus some tolerance) As long as the input voltage from your digipot doesn't exceed 5 volts, the current from the collector of Q1 will not exceed 2 ma.
     
  15. Alec_t

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    Dick is right. I assumed the digipot maximum output is 5V. R1 and R2 divide this down to 200mV max. If the electrode current tries to rise above 2mA the voltage developed across R2 will try to rise above 200mV and the comparator (LM358) output will switch low to turn off Q1 and so hold the current at 2mA max.
     
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