You've got the magic touch

Wed, 11/19/2008 - 11:07 — Sofian

Mimosa Pudica (touch-sensitive plant) at ComPeung

Since Jean-Marc showed me these plants, lots of ideas have started flowing through my mind. The mimosa pudica grow on the ComPeung ground and they are very impressive. Watch the video to get the idea... As thus, I've started looking at touch sensing electronic devices.

I've put my attention at very simple circuits at first. Here are a few links I found, I put them here for later reference:

How does a touch lamp works? provides basic info on the topic
Touch sensitive switch
Transistor Circuits (look for "Darlington pair")
Very simple detector (you can do the same by pluging a small antenna to an analog input on a Arduino)
Simple touch sensor shows how to build simple sensors using an ATiny1213 and insulated copperplates. This page uses approximately the same idea but the code is easier
Interesting discussion on the Arduino forum, many solutions are proposed. Read also this post

Fri, 08/06/2010 - 10:52 — Sofian

CapSense Arduino Library

There's now an Arduino library available for such applications, I haven't tested it yet but it's worth trying.

Thu, 11/27/2008 - 05:45 — Sofian

Interesting link: EMLI

This looks like an interesting project wrt plants reactions. The idea is to send a small current to a plant and amplifies the signal.
http://www.vslvx.org/EMLI/II/

It also has links to other interesting projects (scroll down the page). In particular this page contains a long explanation of the process as well as a schematics.

Wed, 11/26/2008 - 01:10 — Sofian

TouchSensor.h

Here is a small class that I implemented which works GREAT for detecting touches. It also explains the circuit that needs to be built.

/*
 * TouchSensor.h
 * 
 * (c) 2008 Sofian Audry (info __AT__ sofianaudry __DOT__ com)
 *
 * This code was adapted from a technique by Pontus Frössander described at
 * http://www.e.kth.se/~pontusf/touchsensor.html
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program.  If not, see .
 */
#ifndef TOUCH_SENSOR_INC
#define TOUCH_SENSOR_INC

/*

Circuit schematics
------------------


                ~0.5 M 
  PIN_OUT -----/\/\/\----*
                         |
  PIN_IN  ---------------*------------- SENSOR
                         |
                         = 100 pF
                         |
                        GND

The sensor is a simple copper plate, with or without insulation.
The 100pF capacitor can be dropped if the circuit is grounded, it is
especially recommended for battery-operated systems that are not plugged
to ground.

Example Arduino code
------------------------------
#include "TouchSensor.h"

#define SIGNAL_OUT 5
#define SIGNAL_IN 6

#define N_SAMPLES 8
#define THRESHOLD 0.2f

TouchSensor sensor(SIGNAL_OUT, SIGNAL_IN, N_SAMPLES);

void setup() {
  sensor.init();
  Serial.begin(9600);
}

void loop() {
  float s = sensor.sense();
  if (s >= THRESHOLD) {
    Serial.print("Touch: "); Serial.print(round(s*100)); Serial.println("%");
  }
}
  
*/
class TouchSensor {
  byte _sendPin;
  byte _receivePin;
  int _nSamples;
  
public:
  TouchSensor(byte sendPin, byte receivePin, int nSamples = 8)
    : _sendPin(sendPin), _receivePin(receivePin), _nSamples(nSamples) {}
    
  void init() {
    pinMode(_sendPin, OUTPUT);
    pinMode(_receivePin, INPUT);
  }
  
  boolean detect(float threshold = 0.5f) {
    return (sense() >= threshold);
  }
  
  boolean senseOne() {
    digitalWrite(_sendPin, LOW);
    delay(2);
    digitalWrite(_sendPin, HIGH);
    delayMicroseconds(5);
    return (digitalRead(_receivePin) != HIGH);
  }
  
  float sense() {
    float nTouches = 0;
    for (int i=0; i<_nSamples; i++) {
      if (senseOne())
         nTouches++;
    }
    return nTouches / (float)_nSamples;
  }
  
};

#endif

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