MPR121 module - how to know when an electrode is touched?

...did dig up the code:

`
// resonance.js
// resonance experiment
// PWM signal generator with variable frequency
// agitates LC with variable L and C
// observed to find resoncances

// *** Exciter, resonance, detector circuit
//
// <--sigPin ----------------------------------------­-. 
//   resonance                 TP3  TP4           TP5 | 
//   signal s      multi-tap  /    /     D       /    | 
//   detected  MW(AM) HF Air .----.-----|>|-----.-----: 
//   0..290mV  Coil 100 wind |    |  Ge E110LF  |     | 
//               0.5mm Cu on (    |  Si 1N4148  |     | 
//               41mm/1-5/8" (    |             |     | 
//         _                 (    |             |+    | 
//    TP1 | |_| 3.3VPP   TP2 (   ===           ===    / 
//   /                  /    (    | Cx      C1 ---    \ 
// >.-outPin--/\/\/\-->.     (    | 1nF    3.3u |-    / 
//   PWM      R1 330R  )     (    | 2.2n*       |  R2 \ 
//   variable          )  8x (    | 3.2n        | 56K / 
//   frequency f    20 )  10 (    | 4.7n        |     | 
//   32K..262KHz...  w |   w |    | 5.7n        |     | 
//   for receiver      |     |    | 9.4n        |     | 
//   antenna          ___   ___  ___           ___   ___
//   simulation        -     -    -             -     - 
//                     .     .    .             .     . 

// *** UI / Control / Feedback circuitry to vary frequency f
//
// <-slidePin------------------------------­-------------------. 
//   0..3.3V                                                  | 
//            3.3V      3.3V      3.3V      3.3V  3.3V        | 
//              ^         ^         ^         ^     ^         | 
//              |         |         |   extra |     |         | 
//              |         |         | read of |     |         | 
//              \  f down |    f up |  signal |     /         | 
//           R3 /  button |  button |  button |     \         | 
//    100..300R \     red |   green |  yellow |  R4 /    R5   | 
//              /   SW1 |      SW2 |     SW3 |  10K \<-/\/\/\-: 
//              |         |         |         | Sli /    1K   | 
//             .-./|      |         |         | der \         | 
//     speaker | | |      |         |         |     /         | 
//             '-'\|      |         |         |     \         | 
//    _        +|         |         |         |     |         | 
//   | |_|     ===        |         |         |     |         | 
//   3.3VPP    --- C2     |         |         |     |         |+
//   PWM       -| 33u     |         |         |     |        ===
// >-audioPin-->'         |         |         |     |     C3 ---
//   wave 7 octaves lower |         |         |     |    3.3u |-
//                        |         |         |     |         | 
// <-btnPins[1]-----------'         |         |     |         | 
// <-btnPins[0]---------------------'         |    ___       ___
// <-sRBtnPin------------------------------­---'     -         - 
// internal input_pulldown (30k..40K)               .         . 

var go        = true   // keep going on self invocation / 'interval'
  , f         = -1     // frequency put out
  , fMin      = 32768  // lowest used freequency
  , fMax      = 262144 // highst, 65536 * 4 = 18 bits unsigned ()
  , fUp       = true   // frequency hopping up (wobble)
  , outPin    = B7     // wave exciting antenna coil
  , duty      = 0.5    // 0..1
  , audioPin  = B10    // speaker
  , audioDiv  = 128    // 7 octavae lower into audible space
  , s         = 0.0       // signal (resonance, rectified) - discrete
  , smV       = 0.0       // signal in [mV], continuously
  , sigPin    = A0        // signal pin (analog read 0..1=0..3.3V)
  , sChngSens = 0.7       // signal change sensitivity relative
  , sCalib    = 303.334   // signal calibration to show mV
  , sRBtnPin  = B13       // singal extra read button pin
  , ctrlMode  = 0         // 0=buttons, 1=slider
  , ctrlModeI = 1000 // [ms] delay after ctrlMode chnage
  , btnPins   = [B15,B14] // down|up button, both = ctrlMode change
  , debounced = false     // on button press do custom debounce
  , debounceI = 10   // [ms] debounce interval
  , btnHopInt = 10   // [ms] button hop interval min, slider modified
  , btnHopMul = 49        // button hop interval extension multiplier
  , slidePin  = B1        // analog to read voltage divider from slider
  , slideVal  = 0.0       // last read different slider value
  , slideSens = 0.003     // slider sensitivity to detect different val
  , slideAdjM = -0.005    // adjust slider min to cover [0..1]
  , slideAdjR = 1.05      // adjust slider range to cover [0..1]
  , slideHopI = 50   // [ms] slider check/hop interval
  ;
var hTFact = 1.059463094359; // half tone factor (2**1/12)**12 = 2 oktav
var qTFact = 1.029302236643495; // quarter tone  (2**1/24)**24 = 2 oktav
var eTFact = 1.014545334937521; // eight tone f  (2**1/48)**48 = 2 oktav

var lon = true; // log on
function log() { console.log.apply(console,arguments); }

pinMode(outPin,"output");
pinMode(sigPin,"analog");
pinMode(sRBtnPin,"input_pulldown");
btnPins.forEach(function(b){ pinMode(b,"input_pulldown"); });
pinMode(slidePin,"analog");

var cnt = 0;
function upAndDown(fInp,fCdPre) {
  var fChgd = (fInp != f); // f changed
  if (fChgd) { // ----- put freq out if changed
    f = fInp; fChngedT = getTime();
    analogWrite(outPin,duty,{freq:f});
    analogWrite(audioPin,duty,{freq:f/audioD­iv});
  }
  var btnVals = digitalRead(btnPins), nextF = f, hopI, val;
  // if (lon) log(++cnt, btnVals, ctrlMode);
  if (btnVals && ( ! debounced)) { debounced = true;
    setTimeout(upAndDown,debounceI,fInp,fChg­d); // ...w/ same f
  } else {
    debounced = false;
    if (btnVals === 3) { // --- ctrlMode change
      ctrlMode = (ctrlMode === 0) ? 1 : 0;
      if (lon) log("New ctrlMode =",ctrlMode);
      setTimeout(upAndDown,ctrlModeI,fInp,fals­e); // ...w/ same f
    } else {
      if (ctrlMode === 0) { // --- calc next freq on button presses
        slideVal = -1.0; // forces slide value on ctrlMode change
        if        (btnVals === 1) { // --- up
          nextF = (f<fMax) ? f*tFact : fMax;
        } else if (btnVals === 2) { // --- down
          nextF = (f>fMin) ? f/tFact : fMin;
        } hopI = Math.floor((1+analogRead(slidePin)*btnHo­pMul)*btnHopInt);
      } else { // --- calc next freq from slider pos / voltage divider
        val = (slideAdjM + analogRead(slidePin)) * slideAdjR;
        if (Math.abs(val - slideVal) > slideSens) {
          slideVal = val;
          nextF = Math.floor(fMin + (fMax - fMin) * slideVal);
          nextF = (nextF<fMin) ? fMin : (nextF>fMax) ? fMax : nextF;
        } hopI = slideHopI;
      }
      val = analogRead(sigPin);
      if (lon) {
        if (    digitalRead(sRBtnPin)
             || (fChgd || fCdPre)
             || (    (Math.abs(s - val) > s*sChngSens)
                  && (f !== fMin) && (f !== fMax) ) ) {
          s = val;
          smV = val * sCalib;
          log(++cnt,"f =",Math.round(f),Math.floor(smV));
      } }
      if (go) setTimeout(upAndDown,hopI,nextF,false);
    }
  }
}

function h() { go = false; } // halt

function onInit() {
  go = true;
  tFact = eTFact;
  ctrlMode = 0;
  fUp=true;
  upAndDown(fMin);
}

setTimeout(onInit,500); // while dev'ing
`