Transcranial Brain Stimulation (tACS) creating Visual Phosphenes
I did some research on transcranial alternating current stimulation today after hearing about experiments  in which visual Phosphenes (colorful shapes) can be induced with (non invasive, low-current) electrical stimulation of the visual cortex. Phosphenes are stimuli like those perceived when light pressure is applied to closed eyes.
This sounds interesting to me as a potentially useful machine-input interface for private and clearly perceivable information and the experiment itself is a very hands-on exploration of the mind (or the visual sensory apparatus ) and the effects might be recreationally interesting in themselves.
When searching for devices for transcranial electric stimulation, most DIY efforts seem to focus on tDCS (direct current) and seem to my gut feeling to be rather pseudoscientific or at least focus on not very measurable effects, but I might be wrong and would like to hear possible experiences.
At the moment I'm trying to get a technical understanding of how exactly tACS works and is defined in this case.
I would like to propose the following as a first step into building an electronic experimentation toolkit:
- battery powered stimulator
- multiple current-limiting safeguards
- ±2-3mA power use
- no mains or USB connection for safety reasons
- adjustable frequceny in a range of around 5-30Hz (findings are strongest phopshenes in the 10-20Hz range)
- triangle or sine wave oscillation (triangle waves are usually easier produced)
- two sponge-electrodes
Some more general work on tACS and established safety guidelines can be found in  and .
Looking for DIY tACS devices, I have found some schematics with very little documentation and much more in the way of tDCS, where it seems to be common to simply use a LM334 or similar adjustable current source.
There are multiple designs for tACs stimulators from neuroplustech.com and related instructables , .
These designs and the commentary is quite sophisticated and above my level of thorough understanding of discrete electronics at the moment, but the designs are quite convincing.
The first design  is very complex and some flaws are documented. As I mentioned above I would focus on a battery powered and fixed-function implementation. From  it seems like it would be quite easy to use a simple Op-Amp oscillator as the control signal for the voltage-controlled current source instead of the photoresistor and LCD system.
With this design I am mainly confused about what seems to me as unusually high and potentially dangerous voltages on the power supply (±60V ?!). Maybe this is a requirement due to losses in the circuit, a simple estimate of 2mA * 10kOhm (mentioned head-resistance) yiels a 20V draw, so this would be equivalent to about a 10-15% effectiveness plus safeties.
I'd be happy to hear any opinions on the topic, experiences with brain stimulation and especially more qualified evaluations on the schematics than I can currently offer.
: Frequency-Dependent Electrical Stimulation of the Visual Cortex, http://www.cell.com/current-biology/fulltext/S0960-9822(08)01396-1
: Transcranial electrical stimulation over visual cortex evokes phosphenes with a retinal origin, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3545027/#!po=7.60870
: Transcranial alternating current stimulation (tACS), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3695369/
: Establishing safety limits for transcranial direct current stimulation, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2754807/
: DIY USB tACS, http://www.instructables.com/id/DIY-tACS-Hardware-for-machine-human-telepathy-expe/
: LDR-controlled Simple tACS, http://www.instructables.com/id/Simple-tACS-for-analogue-electromancers/
btw, is all rich text gone? Neither BBCode nor Markdown seem to work for me.
EDIT: the lab notes from neuroplustech.com also mention Phosphenes using tDCS. I also just realized there is an updated arduino-controlled instructable as well (http://www.instructables.com/id/Transcranial-Electrical-Stimulator-Arduino-Compati/)
I still think that a simpler version is a necessary starting point at least for my own attempts but it seems that in 2017 the creator of the designs hasn't fried their brain yet.