Transcranial Direct Current Stimulation (tDCS) is a method of external neural modulation that uses a small current run through the brain in order to alter cortical excitability. The details of the mechanism of action and exact enhancements possible are beyond the scope of this article, but start with the wikipedia entry, examine commercially available products, and look at safety data and ethical reviews before deciding if this is something you would like to pursue. Some google scholar searches will turn up interesting things too.
The photo on this page is from this article.
Step 1: Circuit Principle of Operation
The circuit shown is a regulated current sink. You may find it a useful building block in your future projects. It regulates the current through R[L], preventing it from exceeding a set value. This circuit doesn't have active drive capacity, though, and so V[DRIVE] must be large enough to drive the desired current through R[L].
The current through R[L] is equal to I[C]. I[C] is roughly equal to ( V[REF] - (V[BE] of T1) ) / R[LIM] .
To see where this equation originates, begin by noting that the sum of the voltages around the loop formed by V[REF], the base-emitter junction of T1, and R[LIM] must be zero (by Kirchhoff's voltage law):
V[REF] - V[BE] - V[RLIM] = 0
V[RLIM] = V[REF] - V[BE] .
The current through R[LIM] (also known as I[E]) is defined by Ohm's law, and we can substitute using the previous equation:
I[E] = V[RLIM] / R[LIM] = (V[REF] - V[BE] ) / R[LIM] .
Ignoring the base current,
I[C] = I[E] ,
so the current through the load resistor is approximately defined by
I[LOAD] = I[C] = (V[REF] - V[BE] ) / R[LIM] .
If you wish to include the effects of the base current of the transistor, you must also factor in the current gain of the transistor, h[FE].
Viewing the transistor as a node, by Kirchhoff's current law,
0 = I[C] + I[B] - I[E]
I[B] = I[E] - I[C] .
We know that h[FE] is the factor we can multiply by I[B] to find our I[C]. Thus,
I[B] * h[FE] = I[C] .
Substituting for I[B] from a previous equation,
(I[E] - I[C]) * h[FE] = I[C] .
Solving for I[C],
I[C] = I[E] - (I[E] /(1 + h[FE] ) ) ,
and since I[E] = (V[REF] - V[BE] ) / R[LIM] ,
the exact equation then becomes:
I[C] = ((V[REF] - V[BE] ) / R[LIM] ) - (((V[REF] - V[BE] ) / R[LIM] ) / (1 + h[FE] ) ) .
Step 2: Practical assembly
This is the schematic of a working 2mA current supply that may be used for tDCS. It is based on the transistor regulator described in the previous step. Parts were added to allow on/off functionality, on state indication, and redundant safety measures.
B1: 4 9V battery clips, series configuration (add 9V batteries to provide power)
S1: SPST switch
D1: indicator LED
D2-D4: 1n400x (I used 1n4003)
T1: TIP31C (or TIP29C)
R1,R2: 12 kohm 250mW
R3,R4: 2.2 kohm 250mW
R5: 560 ohm 250mW
R6:100 ohm 250mW
Wires and gel electrodes are easiest to find sold for TENS devices, but will allow tDCS, though only in areas that are hairless.
W1: electrode leads (such as these TENS leads)
E1,E2: gel electrode pads (also sold for TENS units)
Perfboard is best for assembling this circuit permanently.
Hot-melt glue is useful for gluing wires in place to prevent strain.
Step 3: Testing and quality verification
Once your device is constructed, you should test it before adhering it to your head and torso and activating it. Check the short-circuit output current with an ammeter. The value should be 2 mA +/- 10%.
Have fun. Try to get better.
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