Sparkfun: Flex Sensor

Overview
The product. The datasheet.

This is a simple flex sensor that measures 4.5" in length. As the sensor is flexed, the resistance across the sensor increases. What you do with this resistance is totally up to whatever you can come up with.

Inputs and Outputs
As for inputs and outputs, the sensor is basically a resistor and thus should be read from the analog input pins. To work with the analog input, you must put 1k ohm resistor from analog ground to the analog input you choose then connect the sensor from the analog VDD to the analog input you choose. The input voltage can then be read by the processor to determine how far the flex sensor is being bent (flexed).

Bone Usage
To interface this with the BeagleBone you just need to connect the flex resistor from the analog VCC to an analog input and another resistor (I used 1.1kohms) from analog input to analog ground. From there you can read the voltage input and manipulate it using code however desired. In my case I basically took the number that the analog input gave me and manipulated it to average about 1000 samples per second, and display the averaged value. When you bend the resistor the value increases, and when you relax the resistor the value decreases. You can see this in the sample code provided.

Reading and Interpreting Analog Input Data
Reading the analog input data is pretty simple. Basically, with the way I described hooking the component up, as you bend the flex senor the value of its resistance increases. So, you receive a lower value for the voltage read from the ain pin. The resistance of the flex sensor is approximately 10kohms when flat, and increases to approximately 110kohms whe bent as far as possible.

Sample C Code
/****************************************************************
 * James Popenhagen
 * MiniProject02_FlexResistor.c
 * 9/23/12
 * 1) include 
 * 2) include 
 * 3) include 
 * 4) include 
 * 5) include 
 * 6) include 
 * 7) include 
 * 8) include 
 * 1) include 

/****************************************************************
 * Constants
 * 1) define MAX_BUF 64
 * 1) define MAX_BUF 64

/**************************************************************** int keepgoing = 1;
 * Global Variables

void signal_handler(int sig) {	printf( "Ctrl-C pressed, cleaning up and exiting..\n" ); keepgoing = 0; } /**************************************************************** * read_ain ****************************************************************/ int read_ain(char* ain){ FILE *fp; char ainPath[MAX_BUF]; char ainRead[MAX_BUF];

snprintf(ainPath, sizeof ainPath, "/sys/devices/platform/omap/tsc/%s", ain);

if((fp = fopen(ainPath, "r")) == NULL){ printf("Cannot open specified ain pin, %s\n", ain); return 1; }

if(fgets(ainRead, MAX_BUF, fp) == NULL){ printf("Cannot read specified ain pin, %s\n", ain); }	fclose(fp); return atoi(ainRead); }

/**************************************************************** * Main ****************************************************************/ int main(int argc, char **argv){

int ainPin; char ain[MAX_BUF]; float duty_cycle = 0; float avgDutyCycle = 0; int avgCount = 1; if (argc < 2){ printf("Usage: ./MiniProject02 "); exit(-1); }

signal(SIGINT, signal_handler); ainPin = atoi(argv[1]); snprintf(ain, sizeof ain, "ain%d", ainPin);

while (keepgoing) { usleep(1000); duty_cycle = read_ain(ain); duty_cycle = duty_cycle/4095 * 100; duty_cycle = 30 - duty_cycle; duty_cycle = duty_cycle*4; duty_cycle = duty_cycle - 28.6; if(duty_cycle <= 6) duty_cycle = 0; avgDutyCycle += duty_cycle; if (avgCount == 250) { printf("Calculating. \r"); fflush(stdout); }		if (avgCount == 500) { printf("Calculating . \r"); fflush(stdout); }		if (avgCount == 750) { printf("Calculating .\r"); fflush(stdout); }		if(avgCount >= 1000){ avgDutyCycle = avgDutyCycle/avgCount; avgDutyCycle = avgDutyCycle * 100; printf("                                \r"); printf("Calculating   %d\r",(int)avgDutyCycle); if(avgDutyCycle >= 9000) printf("Calculating   It's over 9000!!!!\r"); fflush(stdout); avgDutyCycle = 0; avgCount = 0; }		avgCount++; }	fflush(stdout); return 0; }

You must specify the ain pin number as the first argument when executing this code after it is compiled.