Drive Safely wth Speed Alarm

At high speed, visually checking your speedometer to keep under maximum speed can be dangerous. It will be much safer if we put an audio alarm system to give a warning if the speed reach a certain threshold. To build a speed alarm circuit, two IC timers are needed. The alarm will beep if the input frequency is greater than reference frequency. It indicates there is a over-speed occurs. This circuit is designed for frequency of 1 kHz. The fixed-width positive pulse is generated by mono multi-vibrator A1. The input pulse train vary the average voltage of pulse. If the output of C1-R3 on pin 5 goes down below 2V or up 2V A2’s voltage threshold, the state of comparator A2 will be changed.┬áThe reference frequency of this circuit can be adjusted below 1KHz by connecting A2’s input logic to reference voltage instead of to ground. Here is the schematic diagram of this circuit: