Yihui Xiong
Simplest capacitive touch implementation
Capacitive touch is widely used in our dairly life. Curious about how it works? Let's build a capacitive button.
The capacitance of a capacitive button will be changed when with a human touch. There are several ways to measure the change of the capacitance. Here we use the charging/discharging time of a simple RC circuit to correlate the capacitance.
Hardware
- An mbed board (Arch is used here)
- An Apple with a bite
- A wire
Use the wire to connect the Arch's P0_11 with the apple.
Software
The pull-down feature of a general porpuse I/O is used to discharging the capacitor of the touch button. A Ticker is used to measure the discharging time of the capacitor every 1/64 seconds.
Import program
00001 #include "mbed.h" 00002 00003 DigitalOut led(LED1); 00004 DigitalInOut touch(P0_11); // Connect a wire to P0_11 00005 Ticker tick; 00006 00007 uint8_t touch_data = 0; // data pool 00008 00009 void detect(void) 00010 { 00011 uint8_t count = 0; 00012 touch.input(); // discharge the capacitor 00013 while (touch.read()) { 00014 count++; 00015 if (count > 4) { 00016 break; 00017 } 00018 } 00019 touch.output(); 00020 touch.write(1); // charge the capacitor 00021 00022 if (count > 2) { 00023 touch_data = (touch_data << 1) + 1; 00024 } else { 00025 touch_data = (touch_data << 1); 00026 } 00027 00028 if (touch_data == 0x01) { 00029 led = 1; // touch 00030 } else if (touch_data == 0x80) { 00031 led = 0; // release 00032 } 00033 } 00034 00035 int main() 00036 { 00037 touch.mode(PullDown); 00038 touch.output(); 00039 touch.write(1); 00040 00041 tick.attach(detect, 1.0 / 64.0); 00042 00043 while(1) { 00044 // do something 00045 } 00046 }
Revision 0:090d8059cc10, committed 2014-04-28
- Comitter:
- yihui
- Date:
- Mon Apr 28 06:00:18 2014 +0000
- Child:
- 1:a1a6d30c41d1
- Commit message:
- initial
Changed in this revision
| main.cpp | Show annotated file Show diff for this revision Revisions of this file |
| mbed.bld | Show annotated file Show diff for this revision Revisions of this file |
--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/main.cpp Mon Apr 28 06:00:18 2014 +0000
@@ -0,0 +1,46 @@
+#include "mbed.h"
+
+DigitalOut led(LED1);
+DigitalInOut touch(P1_5); // Connect a wire to P1_5
+Ticker tick;
+
+uint8_t touch_data = 0; // data pool
+
+void detect(void)
+{
+ uint8_t count = 0;
+ touch.input(); // discharge the capacitor
+ while (touch.read()) {
+ count++;
+ if (count > 4) {
+ break;
+ }
+ }
+ touch.output();
+ touch.write(1); // charge the capacitor
+
+ if (count > 2) {
+ touch_data = (touch_data << 1) + 1;
+ } else {
+ touch_data = (touch_data << 1);
+ }
+
+ if (touch_data == 0x01) {
+ led = 1; // touch
+ } else if (touch_data == 0x80) {
+ led = 0; // release
+ }
+}
+
+int main()
+{
+ touch.mode(PullDown);
+ touch.output();
+ touch.write(1);
+
+ tick.attach(detect, 1.0 / 64.0);
+
+ while(1) {
+ // do something
+ }
+}
--- /dev/null Thu Jan 01 00:00:00 1970 +0000 +++ b/mbed.bld Mon Apr 28 06:00:18 2014 +0000 @@ -0,0 +1,1 @@ +http://mbed.org/users/mbed_official/code/mbed/builds/8e73be2a2ac1 \ No newline at end of file