According to Instructables/reddit user quicksilv3rflash…

This device can be used for any kind of human electrical stimulation, such as tDCS, tACS, tRNS, or tVNS. Its hardware, when properly constructed and tested, limits output current such that -2.1mA < output_current < +2.1mA , based on the 2mA safety limit recommended in these published guidelines.

Transcranial Electrical Stimulator, Arduino Compatible
Discussion on Reddit Clear instructions to build a precision computer-controlled tACS/tDCS/tRNS device, <$100
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