A Very Simple Current Regulated tDCS Device

Simple tDCS

The Holy Grail of tDCS seems to be availability of a very simple, current regulated device that is easy to build and use.  Keith Spaulding of http://dcstim.blogspot.com got us about as close as we are likely to get with this design:

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I decided to use Keith’s design, with some minor variations, and build a tDCS device.  Keith’s tDCS design is based on using a current regulating diode (CRD).  CRD’s are available in many variations including three values that are of particular interest to DIY tDCS.  They are 1 mA, 1.5 mA, and 2 mA.  You pick the value you desire, build Keith’s circuit and away you go – current limited tDCS.  Here are some plus and minus items to consider:

PLUS

1. Very simple to build
2. Current limited by resistor if CRD shorts in failure

MINUS

1. Current is fixed – you can’t ramp it up or down.
2. You may see a phosphene at start or end of tDCS session
3. Keith’s design does not include a meter to confirm current level

I decided to make the following changes to Keith’s design:

1. Use a single type 23A 12 volt battery instead of two 9 volt batteries. This limits CRD failure current to 12 / 2200 = 5.5  mA instead of 8.2 mA in Keith’s design. Neither is dangerous according to studies published on the web. Both would be very irritating and immediately noticable to a user! I do use a 9 volt battery to power the display (below). You can expect 3-6 months of battery life from the type 25a tDCS battery and a year or more from the 9 volt display battery given regular use.
2. Added a digital mA panel meter so that actual current delivered can be monitored
3. Added a DPST switch to act as an on-off switch for the tDCS circuit and on-off for a separate battery and resistor to power the meter.  The digital meter I purchased needs 5 volts to operate. I use a 9 volt battery with a 1 k Ohm resistor in series to achieve the desired operating voltage.

Before I built a “permanent” CRD tDCS device, I built a test unit using a breadboard.

The type 23A battery is at the left side of the pic, the leads that go to the sponge electrodes are at the right.  Follow the red lead from the battery and you will see the CRD. It’s tiny! In the breadboard example I used two 1 k Ohm resistors (what I had handy), one in series with the plus lead of the battery and one in series with the negative lead (instead of a single 2 k Ohm resistor in series with the plus lead.  Either method is fine.)  I also threw in a 100 uF capacitor (the blue cylinder) across the leads to the sponges to ease start-up and shut-down current.  I did away with it in the final build – it didn’t seem that helpful.

Close-up of the CRD.  Note the black band is away from the plus of the battery. I decided to use a 1.5 mA CRD.  Why?  Studies posted on the web show that 2 mA is better than 1 mA for treatment effectiveness.  However, it is my experience that 2 mA irritates the skin of many individuals causing them to cease using tDCS.  As odd as it seems, backing the current off to 1.5 mA reduces reports of irritation to almost zero.

For the final build, I used a RS project box I had on hand.  You could build a CRD based tDCS device into something much smaller and more attractive.

In the bottom left is the digital mA meter circuit. To its right is a DPST switch. The left portion of the switch is used as on-off for the meter.  A 1 k Ohm resistor is in series with a 9 volt battery and the switch to provide the 5 volts needed by the meter.  The right portion of the switch is the on-off connecting the 12 volt type 23a battery to the series connected 2.2 k Ohm resistor, the CRD and the electrodes. You can see the CRD at top-center of the pic, just before I placed heat-shrink tubing over it to protect it.

Wires dressed, ready to close up the box.

Completed unit with electrodes attached shows it’s regulated current level, 1.5 mA.

So Where Do the Parts Come From?

The case, switch, resistor, battery clips, heat-shrink tubing, and leads all come from Radio Shack. The CRD came from www.mouser.com. I used PN 954-E-152, a 1.5 mA CRD with axial leads.  Be careful not to buy a surface-mount device unless you are well prepared to deal with one. The “3 digit Mini Blue LED DC 100mA meter” came from “Coldfusionx” via EBay.  They are in California – not China – so delivery was quick. I use Amrex electrodes which are available via Amazon and other suppliers.

By the way, I found the meter, as delivered, to be slightly out of calibration (it read too high).  I was able to check and recalibrate  it (adjustment screw) against a couple of DVMs.

Total cost for this project was about $50.

Wrap-Up

Keith’s design is as simple as it gets.  If you want more flexible current control, you could add a selector switch and CRDs of different values – or you could use one of the many LM device designs on the web and build a tDCS device with fully variable control.  Either way, tDCS is an amazing and wonderful thing.  Please proceed with caution and read all that you can before attempting to build your own device – especially read safety related articles and papers!  Best of all would be for you to seek out a medical professional like www.transcranialbrainstimulation.com .

Disclaimer

The information presented in this article represents an accumulation sourced from articles, papers, popular press, and other sources easily accessed on the internet. While evidence so far indicates that tDCS is very safe, your use of information in this article is completely at your risk. You are  advised to seek out a trained medical professional for assistance with tDCS.

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