Improving Capacitive Sensor Speed

[Dblackb99]

Trying a little project using the Capacitive Sensor Library. The sensors are actually copper tubing caps connected via 1M ohm resistors to digital pins on a UNO. The setup seems to work okay as I get increased readings when I touch the sensors, so it should suit my needs, but the response to touching the sensors is slow, with a 1-2 second lag. I looked into the library code to see if there was any way to accelerate the measurement process, but got lost pretty quickly (still a newbie). Any suggestions?

Here's my code (demo sketch):

#include <CapacitiveSensor.h>

/*
 * CapitiveSense Library Demo Sketch
 * Paul Badger 2008
 * Uses a high value resistor e.g. 10M between send pin and receive pin
 * Resistor effects sensitivity, experiment with values, 50K - 50M. Larger resistor values yield larger sensor values.
 * Receive pin is the sensor pin - try different amounts of foil/metal on this pin
 */


CapacitiveSensor   cs_4_2 = CapacitiveSensor(4,2);        // 10M resistor between pins 4 & 2, pin 2 is sensor pin, add a wire and or foil if desired
CapacitiveSensor   cs_4_6 = CapacitiveSensor(4,6);        // 10M resistor between pins 4 & 6, pin 6 is sensor pin, add a wire and or foil
CapacitiveSensor   cs_4_8 = CapacitiveSensor(4,8);        // 10M resistor between pins 4 & 8, pin 8 is sensor pin, add a wire and or foil
long start;
void setup()                    
{
   cs_4_2.set_CS_AutocaL_Millis(0xFFFFFFFF);     // turn off autocalibrate on channel 1 - just as an example
   Serial.begin(9600);
   start = millis();
}

void loop()                    
{
    
    long total1 =  cs_4_2.capacitiveSensor(30);
    long total2 =  cs_4_6.capacitiveSensor(30);
    long total3 =  cs_4_8.capacitiveSensor(30);

    Serial.print(millis() - start);        // check on performance in milliseconds
    Serial.print("\t");                    // tab character for debug window spacing

    Serial.print(total1);                  // print sensor output 1
    Serial.print("\t");
    Serial.print(total2);                  // print sensor output 2
    Serial.print("\t");
    Serial.println(total3);                // print sensor output 3

    
}

[Paul_KD7HB]

OF course it's slow! You are charging a capacitor through a 1 meg resistor with no ground connection.
Try connecting the resister between the Arduino pin and the Arduino ground, not in series with the pin, but parallel.

[Wawa]

@Dblackb99

...when I touch the sensors

The whole point of capacitive sensing is that it should work without touching the sensor.

The metal part should be behind an electric insulator.
Sensors I made still worked through a sheet of perspex or 6mm of glass.
Must have some ground plane near though.

Maybe you should try PPT223 sensor boards.
Leo..

[Dblackb99]

What I'm trying to do is to make a quick and dirty touch point (actually, a few of them) to control some servos. They will likely end up behind some sort of barrier, but that doesn't solve the issue of slow responsiveness.

[Dblackb99]

@Paul_KD7HB

OF course it's slow! You are charging a capacitor through a 1 meg resistor with no ground connection.
Try connecting the resister between the Arduino pin and the Arduino ground, not in series with the pin, but parallel.

I was following the Arduino playground example for this: [Arduino Playground - CapacitiveSensor].

Here's the schematic for my current setup.

.

Are you suggesting something like this?

[polymorph]

Incorrect. The resistor charges through the parasitic capacitance of the sensing surface. What you suggest will do nothing.