Hello,

Just want an opinion on whether the idea is ludicrous or not. So I'd like to take a femto farad capacitance meter which there are circuits already available online and use it as a proximity that can detect change from up to 6 inches away. A square inch copper plate will be the sensing capacitor.

Thanks

 

You can calculate the capacitance between the two objects with these pages.

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/pplate.html

http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/diel.html

After you know the capacitance that will be created, you will know if it is more than a femtofarad.

 

In my opinion, not ludicrous. A while back I was working with a very high impedance circuit that could sense hand movement nearby. It was not the intended purpose in my case, but it showed that it was possible. I think it might take some interesting filtering and logic to deal with the signal size and other noise sources.

 

look up circuit for theramin, a musical instrument that produces tones determined by distance of a hand to a probe. an oscilator has one side of the capacitance brought out to a probe, and another oscilator is hetrodyned with it to produce varying tones determined by the distance of the hand.
cliff

 

I guess its not completely ludicrous but definitely a challenge. I'll give it my best shot.

Thanks

 

I'm completely serious about doing the math to find the expected capacitance change. I don't see how you can proceed without knowing how much you are trying to measure. I expect the answer is in the picofarad range.

 

Just want an opinion on whether the idea is ludicrous or not. So I'd like to take a femto farad capacitance meter which there are circuits already available online and use it as a proximity that can detect change from up to 6 inches away. A square inch copper plate will be the sensing capacitor.

No, not ludicrous at all... but perhaps overkill.

*ANY* circuit that depends on some capacitor value to operate can act as the basis of a capacitive sensor. One such circuit is the ubiquitous '555 timer. I used one of those back when I was working on a projective capacitance sensor.

My sensor only had to see a half inch ahead to sense water. Here is a post of some of my early findings. I have the test sensor around here somewhere... think it's in the shed, and with a foot of snow coming in tonight it may be months before I make it back out there again to confirm what I remember.

It occurs to me to maximize the effect of the fringe field over the direct (directly between the plates) one may simple remove the direct plate like so:

What the drawback may be is in the original version the back plate also acts as a shield: it did not seem to matter a bit what went behind the back plate, just what went in front of the small plate.