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Nice, I'll be paying attention. I've been planning on messing around with tdcs for a while now but I've never sat down and actually built the (admittedly really simple) device needed. Possibly a dumb question but do you have to use electrodes that require that gel substance for good conductivity, or are dry eeg electrodes sufficient? Or is this irrelevant if the signal comes from a current source?

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Gel is probably the way to go.

One usually specifies the stimulation in terms of current (1 mA, for example). You want to deliver that with as little voltage as possible and because V=IR, that means minimizing the electrode resistance.

Minimizing voltage is good because the power delivered is proportional to V², and the power is what makes stimulation uncomfortable by heating/burning the skin (and I guess, the tissue below if you go really crazy). Most commercial systems have a voltage cut-off so they'll deliver (say) up to 2 mA but no more than 12 volts.

The good news is that gel and saline are both very cheap. I bought $50 worth of SIGNAGEL in 2015 and the lab has hardly made a dent in it. Do shop around though, because some "wellness" places mark it way up.

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