constant current stimulator/generator

Discussion in 'Embedded Systems and Microcontrollers' started by Daan, Mar 16, 2010.

  1. Daan

    Thread Starter New Member

    Mar 16, 2010
    2
    0
    Hi

    I am new here and tried to find a similar project with the same problem as mine, but I am afraid I can't find anything anywhere (on the internet), because it's probably a pretty newbie and specific question. So I hope I could find some help.

    I have to develop and build a constant current stimulator. The current has to be a squarewave between 0.0 mA and 50.0 mA. It has to start with 0.0mA when a button is pushed and it has to go up in steps of 0.1 mA with 1.0 mA/s. The squarewave has to be 100 Hz with a duty cycle of 2%, one pulse should be 0.2ms. The resistance of the stimulant will be variable between 30kΩ and 150kΩ, it can differ a lot, but will remain relatively stable during the stimulance.

    The goal is to measure with how many mA different people start feeling a stimulance (they have to push the button again or stop pushing the button and the stimulance should stop and a small LCD screen should show how much the max applied mA was.

    (eventually the duty cycle and mA/s should be able to be editted for different protocols, but that is future work, first I have some basic problems because of a lack of knowledge)

    After talking with some people I decided/thought it would be the best idea to do this with a microcontroller. There are a few questions I still have.

    1. Can I get 50mA out of a microcontroller?
    2. How do I do it?

    Next to this I am already searching for the best suitable microcontroller for the job. I have been doing a lot of reading how to chose the best microcontroller for the job and have made some decisions, but also a few I can't really decide upon yet.

    Peripherals:
    USB: (Protocols have to be setup (different duty cycle mA/s and some other stuff) by using a program on the pc
    PWM controller: for setting the duty cycle?
    POT: for measuring the resistance?
    Timer dunno if this one is really neccesary yet

    prefer a (0.1in)DIP package

    CISC or RISC?: RISC is easier I guess


    Architecture: Harvard or Von Neuman or Accumulator based (I don't really mind at this point as long as there is a small amount of ROM for setting up protocols)
    (offcourse in need of EEPROM and maybe RAM)


    clockspeed and busspeed needed I don't know yet?


    8bit 16bit or 32 bit I don't know yet?


    I would prefer to be able to program in something like C or java


    (I was thinking about something from AVR btw)


    In conclusion I still don't know a lot, but my questions are:

    1. Can I get 50mA out of a microcontroller?
    2. How do I do it?
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
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    This is not a easy project to do. First of all you said the skin impedance will be 30kΩ and 150kΩ. That is wrong as the skin conductance will quickly break down. Then the skin conductance will break down will depend on the current density and the frequency http://books.google.com/books?id=v3...re+grimnes&cd=1#v=onepage&q=breakdown&f=false I think you will see a resistance between the stimulator skin electrodes below 2 Kohm. But the latter number will only be true if you use proper medical electrodes. I also think you do not need to go up to 50 mA. I did some test on my self and at about 20mA I could really feel a strong pain. But I have given 50m A to sedated people during general anaesthetic procedure. Another thing you have not considered is Ohms Law. If we say as an example the you have a resistance between the electrodes equal to to 1 Kohm. In order to get 50 mA current you will need a voltage equal to 1000*0.05=50 Volt. If you do not use medical electrodes the numbers will be much higher. To be honest I think you should purchase a commercial apparatus for your experiment. Like this one http://www.life-tech.com/anesthesia/stimulators/ezstim.shtml Then I read your posting I have a feeling that your electronics skills are not quite up to such a project yet.
     
    Last edited: Mar 16, 2010
  3. Markd77

    Senior Member

    Sep 7, 2009
    2,803
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    This is a potentially very dangerous project. Passing these kinds of currents through people could kill them. I'd recommend using professional equipment and in a highly supervised setting.
     
  4. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
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    You are somewhat wrong. Such a apparatus that the OP want to make is used all over the world in operating theatres to check if the patients are properly sedated. The electrodes are placed on the Ulnar nerve and a 50 mA stimuli is given as the OP describe. I have done this many times my self. And the current used during electrosurgery is higher. As long as the apparatus is used correct it is not hazardous.
     
  5. Daan

    Thread Starter New Member

    Mar 16, 2010
    2
    0
    The story is that at this moment a neurostimulator is used, but this is not working properly so a device has to be made that automatically goes from 0 to 50 mA.

    I have used medical skin electrodes of 3M with the device used now and I measured a resistance between 35kΩ (sweaty hands) 30kΩ (with ultrasound gel added) and 70-75kΩ (dry hands), but this was without current. But that is a good book, I already didn't understand what was the deal with the resistance not begin correct when I applied Ohms Law on the measured current and voltage (1 neuro stimulator to use as example).

    You are correct that at 20 mA it gets pretty painfull however this measurements should also be for people with for example less feeling in their hands. (until 50mA it is safe).

    and indeed my electronics knowledge is pretty bad. I have still have lots and lots of time I have to spent reading and learning, but I'm quite stuck on this at the moment.

    Don't be worried this is just a school project (with incompetent teachers).
     
    Last edited: Mar 16, 2010
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is an important issue, especially if the apparatus is not properly designed.

    The safety issue is paramount here. I was unable to come up with specific values, but the literature cited in the article at the link - http://www.analog.com/library/analogDialogue/archives/37-11/ecg.html#ref - will serve as a necessary guide for what the linits are, as well as testing and certification procedures.

    The project is not well suited to a first-timer's skills. That 50 ma current level is enough to cause lethal shock if misapplied. A commercial unit would be the better choice.
     
  7. Markd77

    Senior Member

    Sep 7, 2009
    2,803
    594
    If you do decide to go ahead with it the important thing to consider is that the main danger occurs where current can pass through the heart or brain. This can happen if current passes up one arm and down the other. It is important that the electrodes can only be touched with 1 hand and also that there is no danger of current going from 1 electrode through the body and to an earth (any electrical equipment with a metal case, PCs, water pipes, etc). You could do this by making the device battery powered.
     
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