超声测距,外围用LM567音频解码器来锁定接受的声波频率，程序里用的是40KHZ,发送200ms，然后等待接受进行中断。里面只有程序，外围电路很简单，注意要把信号放大。-Ultrasonic ranging, external LM567 audio decoder to use to lock in acceptable sound wave frequency, the program inside using a 40KHZ, send 200ms, and then waiting fo