Cap Recharge

[Jim Patchell]

    No, not capacitive.  You are gettting an inductive "kick" from the
relay.  Connect a reversed biased diode across the relay terminals, this
should give a path for the current to flow when you lift the cup.
(reversed biased means connect the cathode towards the end of the relay
where the positive voltage is applied).

    When I was in grade schools I would make booby traps like this.

    -Jim

Plinio Barraza wrote:

> 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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