[RFI] Undercounter Touch Lights #4

[Jon Ogden]

> 
>> 	I also just got some advice from a ham friend who is also an
>> oscillator
>> 	expert.  He suggested putting an LC parallel resonant 
>> circuit at 14
>> MHz in
>> 	series with the sense line.
> 
> 	That would reduce the currents in the line, and increase the
> impedance.  Could be tricky since you'll need to keep the C 
> low so as not to
> interfere with the sensing lines function (change of C equals 
> change of
> state). 

Actually, I haven't been able to have any capacitor interfere with the
sensing line function.  I've tried all sizes of caps and none affect the
workings of the circuit.  Since both the L and C are in series with the line
(parallel to each other), I think that just might do the trick.

>I think it'll also need to be a high Q circuit 
> (Q>10) with a low DC
> resistance (you mentioned it quits working when a resistor is added).

Yeah, he suggested a Q of about 12 or so.  You don't want the Q too hi as
that will reduce the RF bandwidth.  He said I might want to de-Q it a little
by adding a resistor in parallel also.  That shouldn't affect that sensing
ability since the resistor is in parallel with everything else.

>> 
>> 	My friend says that one reason
>> 	why a choke may not work is that the choke has 
>>    capacitance between
>> the
>> 	windings.  This capacitance then acts as a pickup for the touch
>> sensing line
>>	and actually counteracts the effect of the choke.  

> 	I think it has more to do with a change in capacitance.  The
> distributed C is quite small so anything that changes the C 
> even a tiny bit
> (like your hand being nearby) can have a large delta effect on C.

Well correct.  Perhaps I didn't say it so well!

I am going to try some smaller chokes as well.

73,

Jon
NA9D

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