The over speed for any vehicle is indicated using this Speed alarm circuit using 555 IC.
In this circuit the input Vi is taken from any vehicles spark plug. In figure B, Vi is the normal speed output. When vehicle goes at a normal speed, the pulse is normal. When vehicle increases its speed, The pulse is fast. It is shown as a representation as 2VDD. When the pulse becomes fast from IC1, ie, VC1, the IC2 gets triggered. So, it delivers an output of VC2. At a speed mode, the pulse of VC2 is low. When VC2 becomes low, then, the LED goes ON, which indicates the over speed of the vehicle.