Jordan Ford
Measure capacitances by counting rising edges of a 555 astable oscillator.
Dependencies: 4DGL-uLCD-SE mbed
Revision 0:292d0c987fe3, committed 2014-03-24
- Comitter:
- jford38
- Date:
- Mon Mar 24 19:47:55 2014 +0000
- Commit message:
- Capacitive Sensing using 555 astable oscillator
Changed in this revision
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/4DGL-uLCD-SE.lib Mon Mar 24 19:47:55 2014 +0000 @@ -0,0 +1,1 @@ +http://mbed.org/users/4180_1/code/4DGL-uLCD-SE/#e39a44de229a
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/capSense.h Mon Mar 24 19:47:55 2014 +0000
@@ -0,0 +1,67 @@
+#ifndef CAP_SENSE_H
+#define CAP_SENSE_H
+
+#include "mbed.h"
+
+class CapSense {
+
+ public:
+ int _Ra;
+ int _Rb;
+ int _Period;
+
+ CapSense(int, int, int, PinName);
+ ~CapSense();
+ float measure();
+
+ //private:
+ volatile int _count;
+ volatile float _partial;
+ float _coeff;
+
+ void atInterrupt();
+
+ Timer _t;
+ InterruptIn* _event;
+};
+
+Serial pc(USBTX, USBRX);
+
+CapSense::CapSense(int Ra, int Rb, int Period, PinName pIn) {
+ _Ra = Ra;
+ _Rb = Rb;
+ _Period = Period;
+ _event = new InterruptIn(pIn);
+
+ _coeff = (_Ra*_Rb)/(float)(_Ra+_Rb) * log( (float)(_Rb-2*_Ra)/(2*_Rb-_Ra) );
+}
+
+CapSense::~CapSense() {
+ delete _event;
+}
+
+void CapSense::atInterrupt(void) {
+ static float lastTime = 1;
+ float time = _t.read_us();
+ _t.stop();
+ _t.reset();
+ _partial = (time < lastTime) ? time/lastTime : 1;
+ _count++;
+ lastTime = time;
+ _t.start();
+}
+
+float CapSense::measure() {
+ _event->rise(this, &CapSense::atInterrupt);
+
+ _count = 0;
+ _partial = 0;
+ wait_us(_Period);
+ float read = _count + _partial;
+
+ _event->rise(NULL);
+ _t.stop();
+ return (float)_Period / ((.693*_Ra + _coeff) * read);
+}
+
+#endif
\ No newline at end of file
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp Mon Mar 24 19:47:55 2014 +0000
@@ -0,0 +1,22 @@
+#include "mbed.h"
+#include "capSense.h"
+#include "uLCD_4DGL.h"
+
+
+#define RA 19770 // ohms
+#define RB 9053 // ohms
+#define PERIOD 2000000 //us
+
+uLCD_4DGL uLCD(p9, p10, p11); // lcd connection
+CapSense cap(RA, RB, PERIOD, p21);
+
+int main() {
+ uLCD.locate(3,5); //setting the cursor to the middle
+
+ while(1) {
+ uLCD.locate(0,6); // setting cursor
+ float capacitance = cap.measure();
+ uLCD.printf("Capacitance:\n%f uF \n\n", capacitance);
+ uLCD.printf("Frequency:\n%f Hz ",(float)cap._count/PERIOD*1000000);
+ }
+}
\ No newline at end of file
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/mbed.bld Mon Mar 24 19:47:55 2014 +0000 @@ -0,0 +1,1 @@ +http://mbed.org/users/mbed_official/code/mbed/builds/e2ed12d17f06 \ No newline at end of file