microcontroller - How can I use a generic doorknob as a capacitive touch sensor?

I would like to use a generic doorknob that's installed in a typical wooden exterior door as a capacitive touch sensor. I have found various references for the circuit, but I have some questions about the sensor itself.

First, the doorknob in question seems to be non-ferrous (as a magnet doesn't stick to it), but it's still conductive. The "back-plate" that the doorknob rotates in is ferrous (and conductive also).

The image below shows the two options I am considering for fashioning the sensor foil. The option that I am most confident will work without interference from the doorknob itself is a copper ring that surrounds the backplate of the doorknob, but doesn't touch it anywhere. The obvious drawback is this will require some visible modifications to the surface of the door.

The cosmetically preferable option would be to conceal the copper sensor foil beneath / behind the backplate of the doorknob, as in the illustration of the inner copper ring below. But will the sensor function there, with a plate of ferrous metal between it and the hand it is supposed to sense?

Answer

You could try and use the entire doorknob as a single-node capacitive sensor:

The electrode for such uses need not be ferrous or cuprous, as long as it is conductive.

The mechanism involves sensing capacitance changes between the electrode (doorknob) and any earthed conductive body (the human). No second electrode is required; Instead the other pole of the capacitive sense circuit is connected to ground.

For bonding a wire to the metal of the doorknob back plate, a simple method favored by DIYers is the MG Chemicals Silver Conductive Epoxy. It comes in convenient little syringes:

• Clean the back-plate carefully first with alcohol or acetone, then with a fine emery paper.


• Also scrape any oxide off the wire.


• Epoxy the wire and a bit of its insulator onto the back-plate: The glued-in insulator works as a strain buffer so the wire won't break off too easily.



• Try and get as much wire in contact with the doorknob as possible, since the epoxy isn't quite as conductive as a good solder junction.


• Avoid getting any of the stuff onto your fingers.


• Use a hair-dryer set to minimum airflow, to warm up the epoxy - this cures it rapidly and maximizes its conductivity as well.

That should sort it out.