electronics-capsense-simple-qtouch

#!/bin/bash

cat <<'EOT'
Capacitive sensor unit with Active Shield (Driven Shield)
 (Simple OPAMP with QTouch SNSK and SNS lines)

From:
Atmel QTAN0087 Proximity Design Guide
https://ww1.microchip.com/downloads/en/Appnotes/doc10760.pdf

Figure 6-3. QTouch-based driven shield

*-----------* SNSK                 Rsns
|           |-------*---------*-----\/\/----------[Electrode]
|           |       |         |
| QT Device |      --- Cs     |            *----[Driven Shield]
| (any)     |      ---        |  .\ |      |
|           | SNS   |         |  | \|      |
|           |-------*         *--|+ \      |
*-----------*                    |   \_____|
                                 |   /     |
                              *--|- /      |
                              |  | /|      | Unity Gain
                              |  '/ |      | Buffer
                              |            |
                              *------------*

"This circuit may also be used for QTouchADC, the only difference in the
circuit would be Cs – it is not required when using QTouchADC.
...
• QTouch requires two processor pins, one optional series resistor (used to
reduce emissions conducted noise) and one capacitor per sensor
• QTouchADC requires one processor pin and one optional resistor (used to
reduce emissions and conducted noise) per sensor

QTouch proximity sensors are touch keys deliberately made over-sensitive. This can be
accomplished using a combination of the following:
• Make the sensing electrode larger
• Increase the value of the sampling capacitor Cs
• Decrease the touch detection threshold
• Reduce ground loading effects
"

Higher Rsns results in higher sensitivity sensor *I think*.
For long leads (ex. 1.2m), coax cable can be used, with shielding attached to
driven shield, with electrode lead on center conductor 


EOT