First, I do understand theory of 2 plate capacitors and enough electrostatics to go a bit more.
The simplest capsense I know charges a bit of metal and takes longer to charge when touch or very close. Most everything else I've done from instruction or tried on my own is some version of that.
The simplest has one "plate" that the user's field interacts with. When I rolled foil in paper as my sensor to see if that would couple with my field more strongly, some of my results seemed to say that the more closely the foil was rolled the more capacitance it had and here I have questions. I could have affected my sensor and caused a difference I dinno.
Would a metal hinge in a wood door and frame have more capacitance closed than open?
Would a metal spring have more capacitance when compressed?
Just looked at my karma. 60 more points and the horsemen ride!
I was hoping one object, same charge folded or squeezed in on itself, smaller, would have more capacity.
It'd be easier to wire.
I wouldn't mind not having to sense a person as part of non-contact buttons that don't have parts that wear/burn out.
So far that's mainly Hall sensors and magnets for me. Capacitive solutions promise to be cheaper, I can't roll my own
Hall sensors from scrap can I? Conductive surfaces are easy.
The spring with a plate under it might do even pushed by a long stick as more spring got closer to the plate.
I was hoping one object, same charge folded or squeezed in on itself, smaller, would have more capacity.
Arh, no in effect if it was an object like a hinge and it was closed then in effect you half the capacitance between that object and a ground plane because the area is in effect halved.
I can't roll my own Hall sensors from scrap can I?
I did do an experiment as an undergraduate where I measured the hall effect on a lump of silicon so I guess it can be done but it would not make a very good sensor.
What I would do to measure people through a door is use three single turn coils wound in the door frame. Excite the center one with about 400KHz and wire the two outer ones so the signals cancel. Adjust one of the coils for maximum cancellation. Then a differential amplifier will give a pulse when a person transits the door, you even get an indication of the direction.
This is based on an industrial metal detector I worked on in the 60s for the pharmaceutical industry. It was very sensitive to the moisture content of what was passing through it.
You have me at the frequency application, Mike. I can only cookbook AC electric!
I don't think that the Arduino capsense I have done depends on frequency, there's nothing about it in the software.
What I do get the idea of is that changing field can transfer energy to maybe make the subject easier to detect at range but I don't think that the ways I've checked so far make much use of that.
Here are some guidelines for resistors but be sure to experiment for a desired response.
Use a 1 megohm resistor (or less maybe) for absolute touch to activate.
With a 10 megohm resistor the sensor will start to respond 4-6 inches away.
With a 40 megohm resistor the sensor will start to respond 12-24 inches away (dependent on the foil size). Common resistor sizes usually end at 10 megohm so you may have to solder four 10 megohm resistors end to end.
One tradeoff with larger resistors is that the sensor's increased sensitivity means that it is slower. Also if the sensor is exposed metal, it is possible that the send pin will never be able to force a change in the receive (sensor) pin, and the sensor will timeout.
Also experiment with small capacitors (100 pF - .01 uF) to ground, on the sense pin. They improve stability of the sensor.
Note that the hardware can be set up with one sPin and several resistors and rPin's for calls to various capacitive sensors. See the example sketch.
I've seen at least two other sensor types that use capacitance differently, one is cap touch screens.
That is not a capacitor sensing system it is actually an inductive sensing system.
I don't think that the Arduino capsense I have done depends on frequency,
No it doesn't. Any capacitave sensing soloution eventually ends up having to measure capacitance. What the Arduino libiary does is to charge up a capacitor, then delay and then measure how much voltage is on the capacitor. This will be less than 5V depending on the value of capacitor, the leakage of the circuit and the length of the delay. It is the dependence on leakage discharge that makes the system so hit and miss.
Other capacitor measuring techniques involve applying an AC voltage and measuring the current to find the capacitave reactance. This depends only on the capacatance and frequency and so is a more stable way of measuring it, although it involves smaller currents that are tricky to measure in a digital environment. Have a look at some of the specific touch sensor chips that are currently out for a better performance than the Arduino libiary.
Grumpy_Mike:
That is not a capacitor sensing system it is actually an inductive sensing system.
No it doesn't. Any capacitave sensing soloution eventually ends up having to measure capacitance. What the Arduino libiary does is to charge up a capacitor, then delay and then measure how much voltage is on the capacitor. This will be less than 5V depending on the value of capacitor, the leakage of the circuit and the length of the delay. It is the dependence on leakage discharge that makes the system so hit and miss.
Other capacitor measuring techniques involve applying an AC voltage and measuring the current to find the capacitave reactance. This depends only on the capacatance and frequency and so is a more stable way of measuring it, although it involves smaller currents that are tricky to measure in a digital environment. Have a look at some of the specific touch sensor chips that are currently out for a better performance than the Arduino libiary.
A third method that I've seen in some sensor datasheets is to use the sensing capacitance in an RC oscillator and measure the output frequency.
The simplest Arduino capsense I've seen, tried and make work measures time/reads to fill as little as a piece of paperclip stuck in a pinhole being filled by INPUT_PULLUP current. It takes 1 or 2usec when not touched and 5+ when touched but is not much good at sensing close without some surface area added. It is 1 pin touch.
My last see-if-it-works put a cap pin in the Uno pinhole and added a paperclip with foil 'plate' to the other leg sticking up. Surprise for me was it didn't up the read times incredibly. That was a couple of years ago.
"Have a look at some of the specific touch sensor chips that are currently out for a better performance than the Arduino libiary."
I dunno about AVR's but I have seen some controllers with pins that have capsense function in hardware. That must be some very low current AC, intuition tells me.