Good idea on testing with a 555 oscillator. It would give a good rough estimate of the capacitance. It might be a little more difficult to determine a realistic lower bound though.
Hmm, empirical evidence and the ESD model put it around 100-150pF then. I guess I'll sand bag the minimum to around 30-50pF and see if I can design to that.
Thanks for the insight. I'll share the results when it gets built.
Hello DP! I'm currently in the planning phase for a new project, and would find it useful in development to have a model for the human body. I know of the ESD testing model, but don't think it is appropriate for the behavior I need to predict. Does anyone know of any models used in the biomedical engineering field?
A little background: I'd like to build a touch interface for my project where the object being touched for input is an exposed piece of metal. I'm familiar with the standard touch solutions where there is a dielectric between the finger and sense pad. In those cases there is likely a cap change in the range of 3-15pF. Due to the operating environment of my project, it would not be practical to coat the metal surface with an insulator (it would quickly get scraped off). If a human finger were to directly contact a metal sense pad, how much cap would you suspect this to add? The project will have on the order of 200 contact points, so I'd like to tie 5, 10, or maybe 20 of the pads to one line (it's okay that the micro can't tell which is touched). Having this many pads together will of course reduce the ratio of finger cap to total cap, reducing sense margin. I'll most likely be using a PIC24F with the CTMU module.
I'm away from home for a couple months, so I don't have any access to tools (LCR meter), to test the setup myself.
I'd love to hear any thoughts you guys have. Without the dielectric, will the finger have an additional resistor to ground? I will be paying attention to ESD issues, and the "body discharging" that I suspect will be necessary.
Also, does anyone see an issue with soldering a copper wire to Zinc nuts/bolts?
