Pulse Necklace 22July2009
 Pulse Choker Hack Notes for July 22th, 2009
 Silk Screening Conductive Paint Part 2
Last week we tried silk screening silver conductive paint (MGchemicals 842-20G) onto two different types of fabric, but had no luck. Neither fabric showed low resistance, despite our shaking the paint. How could conductive paint possibly not conduct?!
Well, it turns out that the key was to 'stir' the paint vigorously. The metal particles had settled at the bottom and formed its own "layer" with a consistency of a swamp-like mud. After stirring, we silk screened on the two fabrics and got successful electrical connection. We were able to thread conductive thread through with a needle and light up an LED through a circuit that went from the power supply through conductive thread and conductive paint to the LED, then to ground.
We tested out different paint techniques (silk screen vs paint on by brush) on our two different fabrics (water-resistant polyester vs leather-like fabric) for durability:
- Silk screening on the "leather"-like fabric didn't survive our wear and tear testing. The paint cracked when we wrinkled the fabric.
- Silk screening on the polyester survived wear and tear, though the silver particles started dusting off when I (Chung-Hay) scratched it with my nails. We'd say this is not cat-proof.
- Painting on the "leather"-like fabric definitely didn't survive wear and tear. The paint cracked when we wrinkled the fabric.
- Painting on the polyester survived wear and tear with no noticeable differences compared to silk screening.
We then turned to testing the fabric we used for the actual screen in silk screening. That porous fabric turned out to be better than either fabrics. The conductive paint soaked into the holes between the weaves. We successfully threaded conductive thread to the painted wires and were able to light up and LED.
Addition note: silk screening required more paint than just painting with a brush over a stencil.
From our experimentation, we have concluded that the best way to draw our circuit will be to paint by brush over a stencil on porous fabric.
 Soldering on Conductive Paint
Soldering on conductive paint was remarkably easy. The solder tins onto the paint nicely! Surface mount LEDs were easy to solder directly onto the painted fabric. The conduction was very solid, better than the soldering connection on regular PCB. The fabric literally becomes integrated with the paint and solder.
We term our circuit-making creation a painted circuit.
 Stencil Idea
Looking at the circuit in the choker that we have, you can just layout traces and have nice contact pads. We envision a "fake tie" armature, like this:
The traces at the bottom could be soldered to a header if they were spaced right. Hopefully making holes through the fabric will be easy.
 Heart Rate Sensor Parts
We're exploring methods of filtering and amplifying the signal. There is a interesting part which does some filtering right inside the device: digikey EE-SY310.
Eric's homework: options for stencils Chung-Hay's homework: options for thinning the silver conductive paint to keep costs down