RE: Sensor switch - update

["Paul Gordon" <paul_gordon@xxxxxxx>]

Right, Just had a conversation with David at sensor.co.uk, (very helpful
chap BTW), and am happy to report that I now have the switch working
perfectly at a distance of about 30mm (following some Blue-Peter-esque
fiddling with sticky-tape, paper, and tin-foil!!

Basically, the sensor works by detecting varying capacitance, so there are
a
couple of Gotchya's that need to be understood...

The sensor will not work if it is fixed face-down onto a conductive surface
(say a chrome faceplate), because where the 5V connector wires are soldered
onto the PCB, the solder tags go right through the board and are exposed on
the sensing grid side. (and thus are shorted out if the grid is placed onto
a metal surface).

So rule 1 is: insulate the sensing grid side of the PCB from anything
conductive.

Next, the range can be increased by enlarging the sensing area with a
bigger
capacitor. (this is where the paper & tin-foil come in). Having
insulated
the sensor grid, place a layer of tin-foil (about 2-3cm square) over that.
Then, if using a metal faceplace, place another insulating layer between
the
foil & the plate (I used paper), effectively constructing a
capacitor...

So my sandwich is now:
PCB -- Insulating tape  --  Tin foil  --  Paper  --  Chrome faceplate

There are some additional precautions that need to be observed when fixing
into the wall as well... If a metal backbox is used, that should be
earthed,
however, if a metal faceplate is also then used, the screws connecting the
faceplate to the backbox will also earth the faceplate, and kill the sensor
stone-dead.

Rule 2 is therefore: Do not use both a metal faceplate _and_ a metal
backbox. If at all possible use a plastic backbox.

Finally, when using a metal faceplate, it should also be remembered that
the
wall itself will provide some earthing, and will reduce (or eliminate) the
range of the sensor.

So Rule 3 must be: If using a metal faceplate, insulate it from the
surrounding wall with some kind of gasket bwetween them, - one of plastic
fingerplates will do nicely.

Of course, if you use a plastic faceplate, most of the abovementioned
gotchya's are eliminated.

Paul G.

(Now I just have to hope SWMBO is impressed when I show it to her this
evening...)  ;-)



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