Part of the project is to detect finguring at the instruments. So, we have to sense the analog aperture of seven holes. For this we will use the microcontroller Arduino and the Capacitive Sensing Library by Paul Badger.
The capacitiveSensor library turns two or more Arduino pins into a capacitive sensor, which can sense the electrical capacitance of the human body. All the sensor setup requires is a medium to high value resistor and a piece of wire and a small (to large) piece of aluminum foil on the end.
The physical setup includes a medium to high value (100 kilohm - 50 megohm) resistor between the send pin and the receive (sensor) pin. The receive pin is the sensor terminal. A wire connected to this pin with a piece of foil at the end makes a good sensor. For many applications, a more useful range of values is obtained if the sensor is covered with paper, plastic, or another insulating material, so that users do not actually touch the metal foil. Research has shown that a small capacitor (100 pF) or so from sensor pin to ground improves stability and repeatability.
When the send pin changes state, it will eventually change the state of the receive pin. The delay between the send pin changing and the receive pin changing is determined by an RC time constant, defined by R * C, where R is the value of the resistor and C is the capacitance at the receive pin, plus any other capacitance (e.g. human body interaction) present at the sensor (receive) pin. Adding small capacitor (20 - 400 pF) in parallel with the body capacitance, is highly desirable too, as it stabilizes the sensed readings.
The methods that this libray is using and the class interafce are the following:
// library interface description
class CapacitiveSensor
{
// user-accessible "public" interface
public:
// methods
CapacitiveSensor(uint8_t sendPin, uint8_t receivePin);
long capacitiveSensorRaw(uint8_t samples);
long capacitiveSensor(uint8_t samples);
void set_CS_Timeout_Millis(unsigned long timeout_millis);
void reset_CS_AutoCal();
void set_CS_AutocaL_Millis(unsigned long autoCal_millis);
Resistor Choice
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.
Grounding and other known issues
The grounding of the Arduino board is very important in capacitive sensing. The board needs to have some connection to ground, even if this is not a low-impedance path such as a wire attached to a water pipe.
Capacitive sensing has some quirks with laptops unconnected to mains power. The laptop itself tends to become sensitive and bringing a hand near the laptop will change the returned values.
Connecting the charging cord to the laptop will usually be enough to get things working correctly. Connecting the Arduino ground to an earth ground (for example, a water pipe) could be another solution.
Another solution that seems to have worked well on at least one installation, is to run a foil ground plane under the sensor foil (insulated by plastic, paper, etc.), and connected by a wire to ground. This worked really well to stabilize sensor values and also seemed to dramatically increase sensor sensitivity.