documentation.txt

Touch Sensor library for Arduino

Version 1.0

by Scott Daniels

//TBD - See the online documentation here: http://provideyourown.com/

This library uses two almost identical methods for detecting touch. Both methods measure an RC constant and use only one digital pin.

Method 1 - Pin charge

This method discharges tho self-capacitance of the sensor by setting the digital pin to output and low. It then sets the pin to input. Either an external resistor to Vcc
can be used or the internal pull-up resistor can be set when going to input. In either case, the charge time is measured. The charge time is more
reliably longer with a 1 MegOhm pullup resistor than the smaller internal one. In either case, the charge time baseline seems to be the same whether earth grounded or not.

Method 2 - Pin Discharge

This method charges the self-capacitance of the sensor by setting the digital pin high. It then sets the pin to input and turns off the pullup resistor.
When the pin reads low due to the cap discharge, the time taken is measured. A touch will take less time to discharge. The time varies whether the uC
is earth grounded, but a substantial percentage change occurs in both cases - just the baseline reading varies. Subsequent testing has revealed that this method is dependent
on taking repeated samples close together. When they are spread out over time (say a delay of 5 ms between samples) changes become undetectable. Therefore, the real
mechanism of this method is unknown, and therefore not reliable.


Other Documentation
* Excellent article providing overview on how pin charge touch detection works with and without earth ground - http://elm-chan.org/works/capsens/report_e.html




Reccomendation
Here is the latest recommendation for touch sensing:
Use charge method with external resistor of 1 meg-ohm.
For sensor, the larger the better or else bare contact. No ground plane needed. Without earth ground, readings are highly variable. In this application,
either take a large sample (200) and trigger on ave change > 30%, or take 10 samples and trigger if any go over 50%. Non-touched readings stay pretty stable around baseline.

To add tact switch in parallel with charge method, the switch should contact to Vcc, and the timing value goes to zero (the opposite of a touch).
Since the magnitude of change is similar to a touch, but different in direction, a distinction between the two types can be detected if desired.