Pulse Necklace 15July2009
Pulse Choker Hack Notes for July 15th, 2009
Silk Screening Conductive Paint
We tried silk screening silver conductive paint (MGchemicals 842-20G) onto two different types of fabric, a water resistant one and a water proof "leather like" one. Both showed very high resistance (higher than the meter can read) even for very short thick traces. It looks really cool though - hopefully we can figure out what we're doing wrong, and make this work. We also tried measuring the resistance of a huge dollop of paint on the spoon: silver = infinite resistance (!), copper = 200kOhms. It didn't matter if the paint was dried or wet. Note that we shook the bottle prior to use to ensure that the silver particles were dispersed evenly.
Our new Axial LEDs look great (small, short wires), but they are dim, perhaps 1/4 the brightness of our other surface mount LEDs for the same current. We tried putting more power through it, but it remained dim. This product was the 5mcd LED P419-ND from digikey.
We wired up our favorite LED/transistor sensor (CNB2301-ND) with 3K on the resistor and 4.7K on the transistor. With room lights on we are able to observe considerable signal from the transistor when we darkened it (by placing something in front of it), however with room lights off the device doesn't work. This is because there is no current flow through the LED (as measured by 0V across the LED resistor). So we swapped in a 500 ohm resistor for the LED instead, but still no current flowed. Did we damage the LED somehow?
We decided after measuring the LED that the stat sheet is wrong, and the LED needs to be wired the other way. So we switched power and ground for it, and now the device seems to work even in the dark. For skin, the transistor seems to be (a) open for no object in front of it, (b) totally closed even with the finger pressed in tight, indicating that we are saturating the available current flow through the transistor. Inspecting the stat sheet further, its Rload should be much smaller. We swap in a 750ohm resistor. This indeed fixed it so that the transistor isn't totally open with finger pressed to it. With the o-scope on AC triggering, 5mV per division and my finger through the window, we were able to observe a 2mV "heart beat" at ~1Hz over top of the 50mV 60Hz noise. Clearly we need some filtering. Equally clear is the fact that the o-scope is going to be hard for prototyping this stuff without an amplifier.
So Eric's homework: figure out a suitable circuit for use with this sensor and an arduino. Chung-Hay's homework: figure out why the silver conductive paint isn't conductive.