How I made a digital engine tachometer for my vehicle.
I own a 2000 Chevy S10 with an automatic transmission. In order to, I assume, cut production cost, the stock version did not come with an engine tachometer. Being an automatic, it not needed, and so, it doesn't have one. This gave me an excuse to attempt to build my own.
I wanted it to be a challenge, however, so I decided to do it without using a microcontroller (for example, an Arduino).
Schematic of the main circuit for the tachometer.
Schematic of the optical encoder.
Schematic of the 0.25 second real-time-clock built using a 555 timer.