Cap Recharge

[Plinio Barraza]

Just yesterday I ran into an interesting
problem.  It might seem like something obvious
to some of you, but it really surprised me.

I was helping out a friend who wanted to make a
circuit such that a video signal entering a tv
would switch from one vcr to another when
someone lifted a cup from a table (sounds
wierd?).  

Anyway, I said, "easy".  I made a circuit with
a dual relay that would switch the video and
audio source everytime you closed a circuit
consisting of the relay controll input, and a
9v battery.  We tested the circuit and it
worked great.  THe next step was to make a
table whose top was two isolated metal plates
that would close the connection when a metalic
cup was placed in the center of the table (ie
gapping the two plates. 

Well, the circuit worked great, but someone
noticed that one would get a shock when
touching both plates and lifting or replacing
the cup at the same time.  I thought the
circuit would be safe, (who can get shocked by
a 9v battery), but I was wrong.  

I guessed that the shock was somehow the result
of some form of capacitance between the plates,
or charging of these.  The shock was actually
quite hard.

I then tried some intuitive trics to limit the
current, like putting a small resistor or an
inductor in series in the controll circuit:
-battery, inductor, resistor, table, relay...
but that didn't  work. 

Obviously I have no idea what is going on, nor
how to fix it.  I thought of a couple
alternatives.  Isolating most of the table
except the place where the cup goes, or using a
nonconductive tabletop with thin copper wire
strands accross it  -every other strand
connected to one end of the open circuit in
order to ensure circuit closure wherever the
cup was placed.... But my friend is so happy
with the table top he made, that he would like
to know if there is another solution.  

What do you guys recomend I do ?

Can anyone explain this effect.  THe plates are
side by side, laying flat, with a thin gap
separating them...

 Thanks in advance ..

Plinio

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