Re: gEDA-user: CR2032 with supercap, was Re: [SOT] suggestion for a SMT switch

[andrew whyte <ajwhyte@xxxxxxxxx>]

  Sorry, I've read the thread properly now.

  You have a bit of a problem, most supercaps will be larger than a
second coin cell.  You could charge a capacitor through a p-channel
mosfet and resistor, turn this off from the pic, and release the power
in the capacitor onto your vibration motor from an n-channel mosfet.
For a small system like this you should be able to use the body diode
of the mosfet to remove back EMFs.

  The challenge will be to charge the Cap, as fast as possible without
browning out the coin cell,  this shouldn't be too hard, it is the
potential divider between the cell's internal resistance and the
series resistor | P-channel 'on-resistance'.  That will give you an
idea of worst case voltage droop.  Perhaps with some fiddling you
could find a resonant frequency of the vibration motor and get the
system to pulse repeatedly... anyone?

  I've seen something like this done in the Beam Photopopper [1]
(circuit at[2]).  By Philosophy, they aren't worried about brown outs,
but they are running a pager motor off of a 4700uF capacitor at 0.5V,
this makes me think that you *might* get away with something like
this...

  Do some checking/calculations before you start laying out a board
though.  Maybe just charging a cap up to 2.7V, and shorting the
terminals with the motor would tell you how big the cap has to be to
get the motor moving experimentally.

[1] http://www.solarbotics.com/products/k_pp (circuit diagram in the
'resources' tab)
[2] http://www.solarbotics.com/assets/documentation/solarbotics_photopopper_kit_may032007.pdf


N.B. All of this info is supplied without warranty, please remember
all the usual safety stuff.  I am assuming that you are a skilled
professional capable of doing your own risk assessment.

-- 
╒═════════════════════════════════════════════╕
     Andrew Whyte MEng CEng
               paramita ltd
╘═════════════════════════════════════════════╛


_______________________________________________
geda-user mailing list
geda-user@xxxxxxxxxxxxxx
http://www.seul.org/cgi-bin/mailman/listinfo/geda-user