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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.
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Library Methods
The library contains three main methods and some utility methods:
CapacitiveSensor CapacitiveSensor(byte sendPin, byte receivePin)
CapacitiveSensor creates an instance of the library (please note the capital letters, this is not the same method as below)
long capacitiveSensorRaw(byte samples)
capacitiveSensorRaw requires one parameter, samples, and returns a long integer containing the absolute capacitance, in arbitrary units. The samples parameter can be used to increase the returned resolution, at the expense of slower performance. The returned value is not averaged over the number of samples, and the total value is reported.
capacitiveSensorRaw will return -2 if the capacitance value exceeds the value of CS_Timeout_Millis (in milliseconds). The default value for CS_Timeout_Millis 2000 milliseconds (2 seconds).
long capacitiveSensor(byte samples)
capacitiveSensor requires one parameter, samples, and returns a long containing the added (sensed) capacitance, in arbitrary units. capacitiveSensor keeps track of the lowest baseline (unsensed) capacitance, and subtracts that from the sensed capacitance, so it should report a low value in the unsensed condition.
The baseline is value is re-calibrated at intervals determined by CS_Autocal_Millis. The default value is 200000 milliseconds (20 seconds). This re-calibration may be turned off by setting CS_Autocal_Millis to a high value with the set_CS_AutocaL_Millis() method.
void set_CS_Timeout_Millis(unsigned long timeout_millis);
The set_CS_Timeout_Millis method may be used to set the CS_Timeout_Millis value, which determines how long the method will take to timeout, if the receive (sense) pin fails to toggle in the same direction as the send pin. A timeout is neccessary because a while loop will lock-up a sketch unless a timeout is provided. CS_Timeout_Millis' default value is 2000 milliseconds (2 seconds).
void reset_CS_AutoCal()
reset_CS_AutoCal may be used to force an immediate calibration of capacitiveSensor function.
void set_CS_AutocaL_Millis(unsigned long autoCal_millis)
The method set_CS_AutocaL_Millis(unsigned long autoCal_millis) may be used to set the timeout interval of the capacitiveSensor function. Re-calibration may be turned off by using set_CS_AutocaL_Millis to set CS_AutocaL_Millis to "0xFFFFFFFF".
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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.
