4 mA TDCS
I recently got interested in TDCS again. I got my old Ebay 2 mA device out and it seemed to work the same way I remembered. It gave me a stimulating feeling somewhat similar to caffeine when the positive electrode was above my left eye (or on my left temple) and the negative was on the opposite arm. The problem was the electrodes that came with the device were not the sponge type so I had to add my own sponges and the wires are real thin so they broke and needed to be repaired. The snaps on the electrodes also rusted.
I built another homemade device with a full 2 mA which worked like my old Ebay one but I still had the poor homemade sponge electrodes.
I finally broke down and got the APEX type A TDCS that has an analog meter and is adjustable up to 4 mA. The design of this device is very nice and the sponge electrodes are the best I have seen. The sponge case is a soft silicone and it has some kind of conductive rubber behind the sponges instead of any kind of metal. The adjustable current combined with the meter makes it easy to ramp up or down the power to keep the stinging sensation to a minimum and the 18 volts (2 nine volt batteries) allows the electrodes to be a lot further away from each other and still work. Instead of placing the negative on my upper arm, I can place it on the back of my hand if I want.
The biggest problem with the APEX device (and other commercial devices) is the high cost.
I redesigned my old schematic to add in a potentiometer and meter and increased the maximum power to about 4 mA. I wouldn’t recommend skipping the potentiometer if going over 2 mA. The meter could be eliminated but it helps to know that the electrodes are making good contact. The LED power indicator is nice but I left it out of the test device I built and I also skipped the switch just because I didn’t have one and I just unhook the power to shut it off. Without the LED light, just removing the electrodes should shut it off but I like to either have a switch or remove the battery to make sure it doesn’t waste the battery when not being used. I couldn't really tell much difference between 9 volts and 18 volts besides possibly allowing the electrodes to be further apart but I calculated the changes to use different voltages without blowing out the LED. I don't know if lower than 9 volts would work.
I know that 2 mA was (and might still be) considered the maximum “safe” current but I like having the extra power especially if trying to place the electrodes in positions with hair. The potentiometer allows lower current settings so it doesn’t need to be used at full power. Use your own judgement if you build this circuit or try higher than 2 mA current. The sponge electrodes and the saltwater electrolyte are an important part of this device. If anyone is interested, I can describe what I have used on homemade devices.