Sensing Capacitive Touch - MPR121 + Arduino
Every now and then you get sick of the typical push buttons and you want something cooler. And what is cooler than touch sensitive things? Remember that old lamp in your Grandma's that changed brightness just by touching the base? Yeah, that's right... We are talking THAT cool! The MPR121 capitative touch sensor gives you 12 of such inputs that we can use with our Arduino, and you can get it in several forms from SparkFun - Im using the basic breakout board.
How? Well... Capacitive sensing is pretty simple in concept. Every object has capacitance, (in laymen's terms capacitance is just an objects ability to hold a charge), and when you come in contact with something you change its capacitance. Well the MPR121 just looks for these changes, and tells us when it happens.
Hooking It Up
Hooking it up to your Arduino is pretty simple, the MPR121 is an I2C device. I2C is a 2-wire serial connection, so you just need to connect the SDA (Data) and SCL (Clock) lines to your Arduino for communication. On your Arduino (everything but the mega) SDA is on analog pin 4, and SCL is on analog pin 5. On an Arduino Mega, SDA is digital 20, and SCL is digital 21.
When the MPR121 senses a change, it pulls an interrupt pin LOW. Now if we were ninja's we would use this oppertunity to create an interrupt on our Arduino and deal with changed immediately. However, to simplify things for us non-ninjas, we are just going to check that pin to see if it is LOW during our loop. To do this, this sensor also needs access to another digital pin, and in this case we are using digital 2.
Electrodes
This is a touch sensor, so we need something to touch right? We call these electrodes. Typically, electrodes can just be some piece of metal, or a wire, or my favorite, metal tape (just make sure you have at least 1/2in of bare wire in contact with the sticky part of the tape for good contact). But some times depending on the length of our wire, or the material the electrode is on, it can make triggering the sensor difficult. For this reason, the MPR121 allows you to configure what is needed to trigger and untrigger an electrode. So if you are getting false or no positives, you may need to change that.
Proximity Sensing
There is a small change in capacitance even as you approach an electrode, just through the air. So if you set the touch threshold, or release threshold too low you may notice it trigger even a few inches away. If you want it to trigger from a decent distance away, the MPR121 supports aggregating all of the electrodes together to use it as a single large proximity sensor - But you will need to read the doc sheet to figure that one out, sorry.
Code
You may have noticed that SparkFun has example code for this sensor already - Actually, this code is slightly based off of theirs. But this setup and code is not limited to sensing one electrode at a time like their's is. Also this is using the Wire Library to simplify the code to only 2 files. And... We can also sense when one of the electrodes is let go as well.
I know this code looks complicated, but most of it is just configuring the sensor (part based off of sparkFun's code). All it does is output in the serial terminal when one of the electrodes is touched or untouched. You can touch all of them at once if you want, and it will show that, then just let go of one, and only that one will show as let go.
Feel free to change the setup values in section C to adjust the sensitivity of the electrodes. You will see constants used in the code such as TOU_THRESH, these values are defined in mpr121.h incase you need to change them.
License.txt
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Copyright (c) 2010 bildr community
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.README.md
MPR121
Arduino code for the MPR121 Capacitive Touch Sensor https://www.sparkfun.com/products/retired/9695
For complete hookup and tutorial see: http://adam-meyer.com/arduino/mpr121
For all my arduino articles: http://adam-meyer.com/arduino/
The code is provided under the MIT license please use, edit, change, and share.
mpr121.h
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/*
MPR121.h
April 8, 2010
by: Jim Lindblom
*/
// MPR121 Register Defines
#define MHD_R 0x2B
#define NHD_R 0x2C
#define NCL_R 0x2D
#define FDL_R 0x2E
#define MHD_F 0x2F
#define NHD_F 0x30
#define NCL_F 0x31
#define FDL_F 0x32
#define ELE0_T 0x41
#define ELE0_R 0x42
#define ELE1_T 0x43
#define ELE1_R 0x44
#define ELE2_T 0x45
#define ELE2_R 0x46
#define ELE3_T 0x47
#define ELE3_R 0x48
#define ELE4_T 0x49
#define ELE4_R 0x4A
#define ELE5_T 0x4B
#define ELE5_R 0x4C
#define ELE6_T 0x4D
#define ELE6_R 0x4E
#define ELE7_T 0x4F
#define ELE7_R 0x50
#define ELE8_T 0x51
#define ELE8_R 0x52
#define ELE9_T 0x53
#define ELE9_R 0x54
#define ELE10_T 0x55
#define ELE10_R 0x56
#define ELE11_T 0x57
#define ELE11_R 0x58
#define FIL_CFG 0x5D
#define ELE_CFG 0x5E
#define GPIO_CTRL0 0x73
#define GPIO_CTRL1 0x74
#define GPIO_DATA 0x75
#define GPIO_DIR 0x76
#define GPIO_EN 0x77
#define GPIO_SET 0x78
#define GPIO_CLEAR 0x79
#define GPIO_TOGGLE 0x7A
#define ATO_CFG0 0x7B
#define ATO_CFGU 0x7D
#define ATO_CFGL 0x7E
#define ATO_CFGT 0x7F
// Global Constants
#define TOU_THRESH 0x06
#define REL_THRESH 0x0Ampr121.ino
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#include "mpr121.h"
#include <Wire.h>
int irqpin = 2; // Digital 2
boolean touchStates[12]; //to keep track of the previous touch states
void setup(){
pinMode(irqpin, INPUT);
digitalWrite(irqpin, HIGH); //enable pullup resistor
Serial.begin(9600);
Wire.begin();
mpr121_setup();
}
void loop(){
readTouchInputs();
}
void readTouchInputs(){
if(!checkInterrupt()){
//read the touch state from the MPR121
Wire.requestFrom(0x5A,2);
byte LSB = Wire.read();
byte MSB = Wire.read();
uint16_t touched = ((MSB << 8) | LSB); //16bits that make up the touch states
for (int i=0; i < 12; i++){ // Check what electrodes were pressed
if(touched & (1<<i)){
if(touchStates[i] == 0){
//pin i was just touched
Serial.print("pin ");
Serial.print(i);
Serial.println(" was just touched");
}else if(touchStates[i] == 1){
//pin i is still being touched
}
touchStates[i] = 1;
}else{
if(touchStates[i] == 1){
Serial.print("pin ");
Serial.print(i);
Serial.println(" is no longer being touched");
//pin i is no longer being touched
}
touchStates[i] = 0;
}
}
}
}
void mpr121_setup(void){
set_register(0x5A, ELE_CFG, 0x00);
// Section A - Controls filtering when data is > baseline.
set_register(0x5A, MHD_R, 0x01);
set_register(0x5A, NHD_R, 0x01);
set_register(0x5A, NCL_R, 0x00);
set_register(0x5A, FDL_R, 0x00);
// Section B - Controls filtering when data is < baseline.
set_register(0x5A, MHD_F, 0x01);
set_register(0x5A, NHD_F, 0x01);
set_register(0x5A, NCL_F, 0xFF);
set_register(0x5A, FDL_F, 0x02);
// Section C - Sets touch and release thresholds for each electrode
set_register(0x5A, ELE0_T, TOU_THRESH);
set_register(0x5A, ELE0_R, REL_THRESH);
set_register(0x5A, ELE1_T, TOU_THRESH);
set_register(0x5A, ELE1_R, REL_THRESH);
set_register(0x5A, ELE2_T, TOU_THRESH);
set_register(0x5A, ELE2_R, REL_THRESH);
set_register(0x5A, ELE3_T, TOU_THRESH);
set_register(0x5A, ELE3_R, REL_THRESH);
set_register(0x5A, ELE4_T, TOU_THRESH);
set_register(0x5A, ELE4_R, REL_THRESH);
set_register(0x5A, ELE5_T, TOU_THRESH);
set_register(0x5A, ELE5_R, REL_THRESH);
set_register(0x5A, ELE6_T, TOU_THRESH);
set_register(0x5A, ELE6_R, REL_THRESH);
set_register(0x5A, ELE7_T, TOU_THRESH);
set_register(0x5A, ELE7_R, REL_THRESH);
set_register(0x5A, ELE8_T, TOU_THRESH);
set_register(0x5A, ELE8_R, REL_THRESH);
set_register(0x5A, ELE9_T, TOU_THRESH);
set_register(0x5A, ELE9_R, REL_THRESH);
set_register(0x5A, ELE10_T, TOU_THRESH);
set_register(0x5A, ELE10_R, REL_THRESH);
set_register(0x5A, ELE11_T, TOU_THRESH);
set_register(0x5A, ELE11_R, REL_THRESH);
// Section D
// Set the Filter Configuration
// Set ESI2
set_register(0x5A, FIL_CFG, 0x04);
// Section E
// Electrode Configuration
// Set ELE_CFG to 0x00 to return to standby mode
set_register(0x5A, ELE_CFG, 0x0C); // Enables all 12 Electrodes
// Section F
// Enable Auto Config and auto Reconfig
/*set_register(0x5A, ATO_CFG0, 0x0B);
set_register(0x5A, ATO_CFGU, 0xC9); // USL = (Vdd-0.7)/vdd*256 = 0xC9 @3.3V set_register(0x5A, ATO_CFGL, 0x82); // LSL = 0.65*USL = 0x82 @3.3V
set_register(0x5A, ATO_CFGT, 0xB5);*/ // Target = 0.9*USL = 0xB5 @3.3V
set_register(0x5A, ELE_CFG, 0x0C);
}
boolean checkInterrupt(void){
return digitalRead(irqpin);
}
void set_register(int address, unsigned char r, unsigned char v){
Wire.beginTransmission(address);
Wire.write(r);
Wire.write(v);
Wire.endTransmission();
}If you want to trigger individual functions based on the electrode touched, you could do something like this and just add the call to your function under where it says "//X was touched"
if(touchStates[i] == 0){ //pin i was just touched Serial.print("pin "); Serial.print(i); Serial.println(" was just touched"); switch (i) { case 0: //0 was touched break; case 1: //1 was touched break; case 2: //2 was touched break; case 3: //3 was touched } }
I hope this was helpful in getting your MPR121 up and running with your Arduino, and if you have any questions, please use the discussion link.
Video
Article taken from bildr.org with minor changes - I am the original author of this content