2014-10-11 (Sa) Adjustable tDCS

In this blog post I was tempted to rearrange some pictures and words to make it seem like I properly breadboarded a circuit before soldering it in place with untested parts. But that's not what I did. In truth I started with a soldered circuit on the hearsay of internet forums for component sizes. It bit me and I wasted a lot of time. None of my components let their smoke out although some of them were too hot to touch. I hope you learned your lesson.

Enough background
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A set of electrode wires with a proprietary end was altered to use a 1/8" (3.5mm) jack so it would be compatible with the tDCS units and milliammeter already built.

Electrode with new 3.5mm end

The LM334Z order was picked up from a shipping center. One unit was soldered in place and the circuit was switched on. The milliammeter could not register the high current no matter the setting of the 50 Ohm potentiometer. A multimeter replaced the milliammeter test box and showed 18mA which was six times higher than the range of the milliammeter testing box and nine times higher tun the intended maximum current. The lowest the potentiometer could bring the reading was 13mA.

 LM334Z soldered into circuit

Dangerously high amperage reading

The circuit was rebuilt on a breadboard with a different LM334Z. A 1K Ohm potentiometer replaced the resister-potentiometer series assembly but even at full resistance the current could not be sufficiently lowered. A 10K potentiometer was used and a 2mA reading was obtained. Lower values could also be obtained so a chart was made with a list of desirable values and corresponding resistances. Resistance readings were taken by pulling the potentiometer from the circuit and testing with the multimeter. The 10K Ohm resistor could not bring the current below 1mA. The trend was graphed and showed a clear curve indicating that exponentially higher resistances would be necessary to achieve smaller amperage limits

 Breadboarded circuit adjusted to 2mA

Graph showing amperage and resistance

A 20K Ohm potentiometer was selected from stocked items to act as the sole means of adjusting current on this revision of the Adjustable tDCS. A 19/64" (7.5mm) hole was drilled in the side of the tin to fit the 20K Ohm potentiometer. The anti-rotation tab was removed from the potentiometer since the minimal clearance made it difficult to drill a second small hole. The LM334Z and potentiometer were soldered in place and installed in the tin. The milliammeter was hooked up and set to read short circuit current. Current as low as 0.4mA was obtained which is lower than the smallest desirable value.

 Potentiometer hole in tin

Anti-rotation tab removed from potentiometer on right

Circuit with 20K Ohm potentiometer installed

LM334Z installed

Range of current adjustments

To do:

  • Update schematic
  • Update spreadsheet
    • Resistor
    • Potentiometer
    • Potentiometer knob
  • Buy:
    • 4K5 Ohm resistor
    • 10K Ohm potentiometer
    • 20K Ohm potentiometer
  • Install new resistors
  • Test + Verify safety

Journal Page 1

Journal Page 2


The rest of the posts for this project have been arranged by date.

A list showing of all the final posts of COMPLETED projects.




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Comments

  1. Very great efforts. Nicely done!Properly bread boarded a circuit before soldering it in place with untested parts and the system.Very much informative. Thanks for the post.

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  2. Thanks for sharing this informative post about "properly breadboarded a circuit". Your step by step in details information will help anyone to do this.

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