There are a lot of good circuits online for motivated people to try to build their own tDCS circuit. For these schematics, the direct current output levels are controlled either with resistors in series (perhaps varied with a potentiometer), or current regulated mostly using a three pin regulator like the LM334 and feedback resistor to set the current level. The discussion whether regulated current or line resistor is better is not the topic here, personally I think regulated current is miles better, safer, more rugged. I would like to discuss and present a simpler, safe, tDCS circuit that is even easier to build than the three pin LM334 with resistor.
Current regulating diodes (CRD) have been discussed previously on reddit/tDCS and other tDCS websites, but the schematic has not been described. CRDs (or they are also called Current Limiting Diodes) basically are the current equivalent to the zener diode. With a zener diode, once you reach its rated voltage, it turns on. For the CRD, once the bias voltage is applied (and bias current allowed to flow), that device will continue to regulate (maintain) that current regardless of changes in input voltage or output load. This is perfect for tDCS. As the battery voltage drops, the current remains the same (to a certain point of course), also if the head electrodes shift, or dry a little and the head resistance increases, the current remains the same.
Above is a schematic for a simple tDCS circuit that will supply 1mA. The CRD (E-102) maintains a 1mA regulated current to the head (between the Anode and Cathode). The E-102 can be purchased at www.mouser.com. Two 9V batteries are better than one, particularly if you have a less than perfect electrode-head interface, and it will last much longer without the need to change the battery; also the CRD has a 1V-2V drop so the full battery voltage is not present at the Anode. In using the CRD, you have a two pin regulator instead of a three pin regulator and a resistor for the LM334, which means simpler construction.
The 2k ohm resistor in series is for current limiting. If there is a fault condition (although I feel the CRD is more rugged than the IC LM334), and the 18V is at the anode, the absolute maximum current that the circuit can provide is 9mA. This current limiting resistor is used instead of a fuse. A 5mA fuse will blow after a surge of dangerous current, the resistor will not have that surge; the fuse is $20 and the resistor is $0.20. Also for most simple regulator circuits, if you accidently short the Anode and Cathode (if you let the electrodes drop on the table and they come together), the fuse blow and have to be replaced. This will not happen with the resistor. Yet 9mA is high, but not immediately dangerous, and if it happens when the electrodes are on the head, it will be felt and taken off (yanked off) immediately.
This circuit is not good if you want numerous current levels at say 0.1 mA increments. The LM334 with adjustable feedback resistor and microcontroller control is best for that. If you want to have multiple levels then there are other value CRDs available (1.5mA, 2.0mA) and these can be switched in or out.
The above explanation is for informational purposes only, and I take no responsibility for the use or intended use of this information. tDCS is currently not FDA regulated, and there is no intention here in providing a tDCS device. Also I suggest you read, read, and read more about tDCS, particularly any safety considerations before undertaking any project like DIY tDCS.