ADCTouch is a library that allows users to create a capacitive sensor without ANY external hardware.
Most capacitive touch libraries require two pins and a large resistor to acquire precise readings. This library makes use of the AVRs internal wiring to get decent resolution with just a single pin.
int val = ADCTouch.read(byte analogChannel, int samples = 100);
- analogChannel: the pin to use, you can just use A0 - A5
- samples: (optional) the amount of samples to take, defaults to 64. ...range 1 - 32767, powers of two save around 50 bytes of flash and run faster. ...(value does not change with different amounts of samples)
To Acquire a reading, the library does the following:
- charge the test pin to 5V via pullup resistor (not directly to prevent short circuits)
- discharge the internal ~14pF capacitor
- set the pin to tristate
- connect the 14pF capacitor with the pin so that charge distributes evenly
- measure the voltage of the internal cap with analogRead() .if the pin has a low capacitance, the stored charge will be small as will the resulting voltage, if the external capacitance is equal to 14pF, the volatage should be around 2.5V. Even higher capacitances will result in volatges > 2.5V. The chip and arduino board already have stray capacitances that will produce an output of ~390 and just a single external wire can boost that up to 500, so you really need offset compensation. The accuracy is really good, most often even the LSB/smalles digit can still yield usable data and just vary by a single unit between readings (at only 100 samples, more will mean even less variation). The sensitivity is phenomenal, with a large enough surface, it can sense a person in more than 2 feet distance.
A more accurate explanation can be found here
Depending on the geometry of the sensing element and ambient electronic noise, it is likely that the touching individual acting as a larger antenna to pick up more noise from the room is also a significant factor. One piece of evidence in support of this is the observation that with no change in whether it is being touched - or even anyone being nearby, the value returned oscillates. The swings are much wider when it is being touched, and this can still be used as method to do touch sensing, however, rather than simply returnign the average reading, it would likely be more effective if it were to return a more processed value.
It is expected to work with anything based on an ATmega328p or other part which has ADMUX = 0b11111 read the internal ground reference, as well as the ATtiny85/45/25/13 on any core.
It has been generalized to work with tinyAVR 0/1/2-series (megaTinyCore) and Dx-series parts (DxCore), and any other AVR devices where the core provides an ADC_GROUND channel that works with analogRead() - that allows all direct register manipulation code to be removed entirely. The stock Arduino cores do not support this.