Pulse Necklace 16Sept2009
Pulse Choker Hack Notes, Sept 16th, 2009
Our new stencil is here! It supports 5 independent control lines, with 7 LEDs total. We are still using the silver conductive paint, and the white mesh fabric (although the new mesh "organdy" is stiffer than the old mesh). We used paper as the backing during the painting step, but this turned out to be a mistake - pealing up the stencil was fine, but when we pealed up the mech fabric from the paper, over half of the paint went with the paper rather than staying with the fabric. This created several visible areas of lack of paint, which the multimeter confirmed as non-conductive spots (and the multimeter also confirmed that the paper has fully conductive traces, what a waste). We had to re-touch the paint job by hand.
On the up side, we had only a little bit of bleed, and once we removed the backing no traces were shorting with each other.
In the future, we should use a non-absorbent backing such as plastic to prevent this problem.
After all the touch-ups, we measured the resisitivity of each trace. Placing the stencil in an upside-down 'T' manner ( around neck __||||||__ around neck ), going from left to right, we had the following readings in ohms:
30, 14 or 60, 11, 41 on power line to the left, 34 on power line to the right, 21, 15
If we want the LEDs to be completely isoluminant, we need to account for the fact that different colored LEDs have different current needs and the differences in the above resistances.
Progress on Wiring
We had some serious trouble soldering headers unto the lower end of the display, so we switched plans. Instead, we hot glued a peice of thick paper to the traces (for stability and to prevent twisting, etc), and then soldered stranded wire to each of the paint traces. This worked well in 5/6 cases, the 6th case it seems that the hot glue somehow absorbed into the fabric and was preventing a proper solder connection - we had to move that solder connection up to an area without the glue.
Then we soldered LEDs to the other end of the paint traces, and resistors to the small stranded wires (500 ohm for single LED ones, 270 ohm for outside dual LED ones). A little shrink wrap to protect the resistors, and we're almost done.
Out of Time
But we ran out of time to finish the prototype tonight - we still need to wire it into the electronics, write code to support 5 LEDs, and see if the ECG stuff is still working after a month of down-time. We will meet again next Wednesday, and thence forth we'll meet on Thursdays as that will work out better for Chung-Hay due to wed night assignment deadlines.