The circuit shown below diagrams how to make a digital speedometer which can measure up to 99kmph with a resolution of 1kmph for i.e LML Vespa, Kinetic Honda or some other vehicles using the LM324.
This digital speedometer circuit uses IC LM324 which is wired as comparator. It forms an opaque disc mounted on the spindle attached to the front wheel of the vehicle, the disc has a fixed infrared LED on one side, and a phototransistor in line with the IR LED on the other side.
See how to calculate the speed using this device in the completed How to Circuit a Digital Speedometer of pdf filetype (source: electronicsforu.com).
