Basic tDCS
Link to basic tDCS Instructables page
I began experimenting with tDCS during 2014. The initial devices I designed used an LED as a voltage reference, and I even designed some to allow for placebo stimulation for experimentation. However, use of an LED as a voltage reference causes production to become dependent on the consistency of LED voltage drops which is not well controlled in manufacture, leading to dependency on individual batches. Using a surplus supplier's LEDs as a design-specific requirement was an especially poor choice in retrospect.
These and other difficulties with the first several iterations led to me optimizing the design for simplicity and ease of construction. The instructable shows the design optimized for simplicity and use of only common through-hole parts available from multiple suppliers. This device costs about about $20 (as of 2016) in raw parts.