51单片机的变频声救护车报警信号输出试验，单片机教程：http://www.ipbb.cn，主要利用５１单片机的定时器输出控制蜂鸣器，发出救护车报警信号！-Variable frequency sound of an ambulance alarm signal output test, 51 single-chip microcontroller tutorial: http://www.ipbb.cn, the main use of the 51 single-chip timer output control buzzer, issued by the ambulance alarm signal.