Building a good tDCS device – part 1

After reading all up about tDCS, and deciding it might be a good idea to take a crack at it, I was faced with a dilemma. You see, professional tDCS devices cost a lot, and are medical devices, ie. you can’t just buy them off Amazon.

Of course, there were a few options beside that. One of the awesome things about tDCS is that the technology is so easy it’s almost ridiculous. To have a fully functional tDCS device that does exactly what the professional ones do, all you need is a current regulator set for 2mA, some wire, and two saline sponge electrodes.

Since it caught the fancy of many geeks and makers, there was an awful lot of resources out there. I first came across an initiative to create a kit, called GoFlow. Their webpage is still up and dandy, though the project itself went under years ago. It was intended to be a simple device with a standard bifrontal electrode layout (more on that stuff later, do some googling if you’re interested).

Now even if it was available, it has its shortcomings. For example, they have this big ass, 470uF electrolytic capacitor across the ‘trodes. I fail to see how that would EVER be a good idea in a current regulated loop. I do love the concept pic tho’, all those Apple-esque colors…

The next candidate was the foc.us. If you haven’t seen it already, it’s the big game in town. I’m not entirely convinced what they are doing is legal, but they are popular and the device looks cool. That said, it’s expensive for what it is, requires an iPhone 4S to access all functionality (a regular iPhone 4 just won’t cut it), and seems to have been designed by a herd of insane marketing managers high on MDMA.

So, I decided against it. Since then, it turns out I was even more right than I thought I was. The foc.us is a dog with fleas. I’m particularly struck by the fact that it goes to the length of having 64 Volts as its maximum driving voltage. How irresponsible is that?

That’s when I read this article on the Speakwisdom blog. It’s a beautifully simple design, and as far as I know at least two commercially sold kits (the kind of pricey Biocurrent kit, and the humbly named – and priced – tdcs kit) are based on it.

Simple tDCS design

The design is based on a so-called current regulating diode, or CRD, which is a simple and robust device. It’s a FET wired in a current limiting configuration, and factory-tuned for a specific current level. They are quite stable, very low maintenance.

So I got some, plugged them up in a breadboard, wired up a pair of Amrex electrodes, and well… It worked. However, I felt quite shocked when I turned the thing on, and had lights flash in front of my eyes when I disconnected it. Of course these side effects are well known and harmless (the flashing lights are called phosphenes and are due to some neurons firing in the retina or the optical nerve), but I just couldn’t shake the thought that it could be made safer and nicer.

Of course I wouldn’t think of connecting a big ass “soft start” capacitor across the terminals, like the crazy GoFlow guys, especially not that my design is not a nice, single package like theirs, ensuring constant electrode contact. Things can get disconnected, a wire can break, and then the “soft start capacitor” gets a new job as a “very hard start capacitor”, or more aptly the “help a donkey kicked me in the head capacitor”.

Adding a potentiometer for manual starts and stops seemed like a good idea, but it wasn’t trivial either, due to the nonlinear nature of the entire system. The trick is to get a setup where the intensity is at least somewhat linearly distributed across the movement range of the pot. I didn’t want the current to come up in a jolt at either end – I wanted a smooth, nice ramp.

While I was thinking about solving that, I also gathered a few further requirements pertaining to the device:

  • Should be cheap,
  • Should be simple enough to build on a veroboard,
  • Should be as safe as I can make it,
  • I should be able to build it in a day or two once I have the components delivered…

I think I came pretty close.

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