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==Spec== The basic circuit for TDCS is ridiculously simple. We are creating a 2ma DC current and running it through 2 inch square or round electrodes that are then connected to the human body. There are various configurations that can be used to place the pads, but the most common is placing the active electrode over the area of the skull corresponding to the brain region to be effected and the other electrode, called the return electrode, is placed on the opposite shoulder. Using the anode, or positive electrode, over the brain will stimulate the target region. The cathode, or negative electrode, will retard the brain region. A standard simulation time in most research is 20 minute sessions. The effect, depending on the type of stimulation and region seems to last up to an hour after the stimulation is removed. Again, depending on the type and location of the stimulation, repeated exposure daily for up to 9 sessions can cause the effects to persist for 30 days or longer in depression studies. The simplest working circuit for this is a pair of 9v batteries in series, an LM334 current limiter and a fixed resistor across the LM334 reference and output to set the 2ma limit. A better circuit involves replacing the fixed resistor with a 200 ohm pot and adding an ammeter anywhere in series. One of the better designs of this nature is located [http://imgur.com/a/CrzTG here]. Although the fuse in the circuit is of unlikely utility in reality. A fairly simple, slightly unsafe and highly voltage limited arduino project that is somewhat clever is located [http://sourceforge.net/p/openbrainstim/wiki/Hardware/ here]. The cadillac of designs here would probably look something like an arduino controlling a SPI variable resistor connected to an LM334 for current control, another LM334 with a fixed resistor to provide failure protection (a fuse is going to blow too slowly and fuses that small are expensive and difficult to source), a shunt ammeter connected back to the Arduino and a display/input to allow the user to set different times, current levels, sham stimulation and so on. Also we would want to put a boost transformer on the battery supply in order to boost the output voltage being used. Discussion of why we want to up the voltage here will happen in the section on electrodes. (need to add a long discussion about electrodes here, since there is more complexity in the electrodes than the circuit itself)
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