This is a general electronics question, not directly PIC related:

I have a circuit that looks like this:


               +------R470K------+
               |                 |
               |          |\     |  |\ 74HC14 (2 gates)
               |   /      | \    |  | \
          +----+--/ ---+--|  >O--+--|  >O----+----  OUT
          |  Btn(N.O.) |  | /       | /      |
          |            |  |/        |/       |
          |            |                     |
         _+_           +-------R10K----------+
         ___ 0.1uF
          |
         GND

(Some of you may remember the "Button toggle" thread a while back, this
was the result.) Every time the button (normally open) is pressed, the
output toggles: if it was low, it goes high, and vice versa. A PIC reads
the output. It works great. But...

Yesterday I went for a "pre-test" for EMC and ESD certification for the
product, and discovered that a static zap (indirect, applied to a metal
plate near the unit) will often toggle the output state of this circuit.

It was amusing, actually, and we had a grand time zapping it and watching
it change, but now I'd like to fix it. I can repeat the problem here at
home using the piezo spark unit from a barbecue lighter (instead of the
very expensive gun the test lab used), so I'm in a good position to know
when it's fixed :-)

My only thought so far is to tie some point to a rail, +5V or gnd,
perhaps between the two 74HC14 gates, but rather than just blindly start
chewing away at the board, I thought I'd throw it out there for some
advice. Anybody got a suggestion where to start?

Thanks in advance,

Dave Johnson