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 }