The Cognitive Kit, Stealth tDCS

tDCS Takes Off

It’s been amazing to watch the growth of interest in transcranial direct current stimulation (tDCS) over the last couple of years.  tDCS has moved from the obscurity of research center studies to a much wider audience via articles in the likes of Scientific American, the New York Times, Wired Magazine, assorted radio and TV bits, and of course the internet.

Many more people are at least peripherally aware that tDCS can help with depression, chronic pain, memory, learning, and more. Affordable tDCS devices have also become more available meaning fewer do-it-yourself types have to build their own.

At the lowest price are bare-bones 2 mA tDCS devices from for less than $40 and a current-selectable unit from for less than $100.  Of course there are many tDCS devices at the opposite end of the spectrum that cost many hundreds to thousands of dollars.  But all accomplish the same task – providing a very small, regulated current to electrodes that are placed on the scalp in order to achieve a desired tDCS effect.

The Cognitive Kit

(The Cognitive Kit with my “test head”.  The test head allows me to simulate various montages, take measurements, scope wave-forms, etc.)

A new entry to the tDCS market has arrived and I have been fortunate to be able to review an early sample.  Human Bioelectric ( ) has released a new model, The Cognitive Kit, meant to target the higher end of the entry level market.  The Cognitive Kit is a current selectable tDCS device that comes with Amrex electrodes (the most popular choice of professional and advanced DIY tDCS users), a headband and use instructions. The unit is well built and likely to attract attention due to it’s stealth approach to packaging (no labels or anything that identifies what it is).  You can bet the TSA folks will want check it and have you to turn it on if you fly somewhere with it!

Human Bioelectric does not provide montage or medical advice. As with most DIY tDCS device suppliers, they leave it up to you to do your homework to decide which tDCS montage is right for you.  I presume all do this to keep themselves off of the FDA radar screen.


The Cognitive Kit is a slim package somewhat larger than a typical smartphone. There is an on-off slide switch on the side of the case and a current selector slide-switch and session start-stop push-button on top.  There is also an LED indicator for power and session in progress LED. The Cognitive Kit does not include a session timer (typical of devices in this price class).

(What is it? What are all the switches and lights for? The Cognitive Kit, Stealth tDCS)

Inside the box is a very nicely laid-out PC board containing all the major components.  The Cognitive Kit uses an LM234 (functionally identical to the LM334) for current regulation, coupled to a group of resistors and a selector switch that program the LM234 to limit current to 1, 1.5, or 2 mA depending on switch position. This is a very tried and tested design that should prove to be highly reliable and safe for many years of use.

(Neat and solid construction.  Should last for years.)

The Cognitive Kit uses two 9 volt batteries in series to supply 18 volts to the regulator board.  Why 18 volts instead of the 9 volts that some use?  One of the chief difficulties of a tDCS device is overcoming the highly variable resistance encountered depending on the position of the electrodes on the scalp, how wet they are, whether salt has been used, or if some other type of electrode is being use that has higher resistance (stick-on type for example).  In some cases 9 volt powered tDCS devices have a difficult time achieving 2 mA, or even 1 or 1.5 mA in a tDCS session because the lower voltage cannot overcome resistance in the “head circuit”.  The higher source voltage of  the Cognitive Kit helps overcome electrode, hair, skin, etc. resistance so it can more reliably deliver the selected current level. This is exactly the idea behind the “boost” circuits used in more expensive tDCS devices (and professional models) that boost the regulated voltage to 60 or 80 volts – and why some users of higher-end tDCS devices have received a frightening shock when they inadvertently pulled off electrodes during a tDCS session.

Using a lower voltage (9, 12, or 18), makes the tDCS device circuit much simpler to build, probably more reliable in the long-run, and eliminates the possibility of a high voltage shock.

Interestingly, Human Bioelectric claims that the Cognitive Kit has a built-in failure-mode current limit of 6 mA (for 2 k-Ohms of head resistance). In a device failure, 6 mA would be VERY uncomfortable, causing any user to immediately switch off the device or rip off the electrodes. According to what I’ve read, that current level would not do any damage in the brief time a user could tolerate it!

In Use

I set up the Cognitive Kit for several different common montages used in tDCS and found it to reliably deliver the selected current level and to be easy to use.


  • Wet electrodes (slightly saline water). Squeeze out excess water.
  • Place the electrodes in desired locations using a headband to hold them in place.
  • Turn on the Cognitive Kit (slide switch on side)
  • Select desired current level (slide switch on top)
  • Press button (on top) to Start tDCS session
  • Manually time 20 minute session
  • Press button (on top) to Stop tDCS session, switch off the Cognitive Kit
  • Remove electrodes from head (clean sponges as needed)

The montage for “savant learning”, for example, involves placing the anode (+) on the right temple and the cathode (-) on the left temple.  Doing that I found (about 4 k-Ohms of head resistance):

  • Switch set for 1 mA, I measured a current of 0.99 mA
  • Switch set for 1.5 mA, I measured a current of 1.53 mA
  • Switch set for 2.0 mA, I measured a current of 2.01 mA

These numbers show excellent regulation and were reliable with different montages.

One feature that all of the less expensive tDCS devices lack is a slow ramp-up, ramp-down circuit.  When you start a tDCS session, the Cognitive Kit current very quickly goes from zero to your selected level – delayed slightly by how long it takes water to saturate your skin and provide a good conductive path.  Because of this, you may see a weak phosphene (flash of light) as a tDCS session starts or ends. This can seem strange, but all the literature I’ve read indicates that phosphenes are harmless.

What I Like

  • Simple device, very easy to use
  • Low voltage so no possibility of a 60 or 80 volt shock
  • Rugged PC board design – should last many years
  • Amrex 3×3 sponges and headband included
  • Current limited in failure

What Could Use Improvement

  • Labeling the case would be useful and improve appearance (though some may prefer the stealth look)
  • LED brightness is very high (nit picking)
  • You must remove four screws on the back to replace batteries
  • I still love having a mA meter on a tDCS device – it’s an assurance

Final Comments

The Cognitive Kit sells for about $200 and comes complete with Amrex sponge electrodes, headband, connecting wires, and instructions.  Some may question the price, and yes, the Cognitive Kit could be sold for less if it were less solidly built and did not include Amrex 3×3 electrodes. A solid build is worth something and if you are serious about tDCS, no matter what tDCS device you buy, sooner or later you’ll be buying Amrex 3×3’s. They are not cheap! The Cognitive Kit and Amrex 3×3 combination should last you for years if taken care of. As the saying goes, “You get what you pay for”.

Operationally, the Cognitive Kit does what Human Bioelectric claims: it delivers well regulated current at 1, 1.5, or 2 mA.  If you are shopping for a tDCS device, the Cognitive Kit deserves consideration.



3 comments on “The Cognitive Kit, Stealth tDCS

  1. I am the secretary of a small association called PSILAB from Barcelona, Spain. It is registered in the University of Barcelona UB, Spain, the association is made up of psychologists and psychology students, and our goal is make some experiments of psychology.
    I need to buy a TDCS device so that it can be used in blind/sham modes and some electrodes and sponges of the size 5×7= 35cm2, for replay the experiment that made Richard P. Chi and Allan W. Snyder on creativity: Facilitate Insight by Non-Invasive Brain Stimulation, this is the email address, where you can watch the experiment from Allan Snyder:

    The problem is that for this year the university only can give me 1000€ for this research.
    The only tDCS companies with blind/sham modes that have a cheap price and that I have found are:
    – ttp://
    Could you advise me which of these tDCS companies deserves the utmost confidence for my research?
    Kind regards
    Jaume Esteve

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