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I would appreciate any input

Discussion in 'General Electronics Discussion' started by Gadgetastic, Jun 27, 2013.

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  1. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    The intent of the attached circuit is to create a portable device that outputs a steady 2mA to electrodes that will be applied to the skin. I have an LED to indicate the battery is working and an ammeter to tell me what the exact current is. The ammeter runs off a separate power source and I've used a 7805 to bring it down to 5volts.

    I'm looking for any comments, advice, tips, basically anything that can make it better or correct mistakes I've made. I'm very new at this stuff, so I am fully open to being schooled.

    Thanks.
     

    Attached Files:

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,282
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    Nov 17, 2011
    Sorry to say, but this circuit has several flaws:

    1) The ammeter shorts out the electrodes. Any current will flow through the ammeter, the current through the electrodes will be minimal. You need to connect the ammeter in series with the electrodes, not in parallel.

    2) While 9V/4.5kOhm=2mA, this is not the current that will flow through the electrodes, because there will be a measurable resistance of the skin. This resistance adds to the 4.5kOhm. The skin's resistance depends on many factors (humidity, fat, dirt etc.) and the distance between the electrodes. It can range from a few 100Ohms to Megaohms (On the back of my hand I measured 5Meg using an Ohmmeter and a pair of rather sharp probes approximately 1cm apart).

    Look into current sources if you need to control the current. Watch out that you will need to limit the voltage and power delivered to the subject under test.

    IN NO CASE USE THIS ON PEOPLE WHO WEAR ANY KIND OF ELECTRONIC LIFE SUPPORT INSTRUMENT!!!!
     
  3. Gadgetastic

    Gadgetastic

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    Nov 22, 2011
    Please don't be sorry. This is exactly the sort of input I need. Good stuff. Keep it coming. Thanks.
     
  4. BobK

    BobK

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    Jan 5, 2010
    You are going to need a LOT more than 5V to get 2ma through skin.

    I just tried this on myself. At 10V the most I could get is 20uA by squeezing the probes very hard, one in each hand. Placing the two probes nearby on my leg, I could not get more than 1uA. So, 500K to 10Meg for my skin resistance.

    Bob
     
    Last edited: Jun 27, 2013
  5. Gadgetastic

    Gadgetastic

    27
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    Nov 22, 2011
    With tDCS there is the expectation that a large part of the circuit output will be lost before it gets to it's destination. The uA range is about right for what is supposed to get through.
     
  6. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    Sorry, but if the only connection between the two probes is the skin, and you have 2m
    A going through the probes, you also have 2 mA going through the skin.

    Bob
     
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    There have been several threads on these forums about trans-cranial direct stimulation (TCDS). Search for TCDS, and TDCS (people often seem to misspell it).
     
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