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Transcranial Direct Current Stimulator

Why

Transcranial direct-current stimulation is somewhat effective in suppression of certain neuro/psychological symptoms and somewhat helpful in maintaining concentration. After an experiment with a jury-rigged unit, a de-luxe unit was assembled.

Construction

Current source

The basic design was taken from the BRMlab's presentation. The LM317 chip in the original circuit was replaced with LM334, which, while substantially similar, is more suitable for such low currents.

The output current is regulated between 140-1900 microamperes using a potentiometer. The value commonly used in tests is 1000 microamperes.

Indicators

A red LED was inserted into the circuit as the indicator of operation. The LED was chosen to give significant brightness at submilliamp currents. The red color was picked for the low forward voltage of red LEDs.

A miliammeter was improvised from a cheap VU meter, to which a resistor was connected in parallel. Its role is indication of the current through the electrodes.

A voltmeter was improvised from the same type of VU meter, this time with a series resistor and a trimpot for maximum-range adjustment. Its role is indication of the voltage between the electrodes, which correlates to the connection resistance (the lower the better) and eventual electrolysis effects on the electrode-skin interface and corresponding irritation.

Due to the low cost of the instruments, the current flowing through the voltmeter and the voltage over the sensing resistor are both fairly substantial, and the meters are influencing each other. As the current value is more important than the voltage (which is used only as a rough indicator of the quality of electrode contacts), the voltmeter connection was chosen to not influence the ammeter, for the price of some inaccuracy dependent on the electrode current.

The scales were hand-calibrated, computer-generated and laser-printed.

Outputs

A pair of header pin female connectors was used as an expedient cheap way to attach the electrodes. For alternative connection of electrode sets, a 3.5mm audio jack was employed as well, with ring as negative and tip as positive electrode.

Power supply

The first version of the device relied on a 9 or 12V battery with a barrel jack. It turned out to be somewhat cumbersome.

There is no power switch in this version.

Batteries are much preferred over a wall-wart or other mains-connected power source, due to the (low but existing) risk of voltage spikes breaking through from mains to the user and causing injury or death. With a wall wart, you are putting yourself in the mercy of the power company and the power supply manufacturer, both trying to supply the lowest quality of their products that the consumers let them get away with.

For next version, it was decided to use USB as power supply (whether from a battery-powered netbook or notebook, or a portable charger (e.g. Power Brick). A step-up converter was therefore built around the MC34063 chip, and a microUSB connector was used as the power source.

This circuit also allows future use of a built-in Li-ion battery.

Electrodes

The electrodes were cut from polyethylene foam and wrapped in silver-coated conductive fabric. Thin wires with header pins on their ends were used as connectors.

For satisfying electrical contact, a sheet of tissue paper wetted with water (optionally salt water), or a little of electrode gel, has to be placed between the electrode and the skin.

Power supply

TODO

Images


Inside view

Inside view

Inside view

Inside view, stabilizer side

Inside view, stabilizer side

Inside view, stabilizer side

Inside view, stabilizer side

Outside view

Outside view, power supply side

Outside view, output side

Outside view, output side

Outside view, meters

Outside view, meters

Electrodes

Electrodes

Step-up board with microUSB connector

Step-up board, top

Step-up board, bottom

Step-up board with microUSB connector

Step-up board and connector in box

Step-up board and connector in box

Step-up board and connector in box

Step-up board and connector in box

Step-up board and connector in box

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