Some of the smaller cars available today does not have an RPM meter on them – this would be some trouble since you wont be able to determine the actual speed of the car’s engine, thus will have an impact on the fuel consumption since engines are efficient at a certain range of shaft speed.
This project makes use of an AVR microcontroller and a signal conditioning circuit that makes use of an LM324 operational amplifier (Op-Amp). The input signal was taken from the High Voltage (HV line) of the main distributor of the engine. This specific point was chosen since all (gasoline) engines have an HV line. The signal from the HV line is fed into the signal conditioning circuit before being fed to the microcontroller’s PB1 pin. The Timer/Counter1 External Counter Input mode of the PB1 pin is used as an external clocking source for the MCU’s timer1 making it able to do frequency measurements. While another timer (timer0) is used to measure the elapsed time.
If you wonder whats in the HV line that makes it a good measuring point, the HV line is where the spark plug gets its energy from. Thus a spark in the in one of the sparkplugs corresponds to a surge in the HV line – and since the engine’s revolution rate is directly proportional to the number of sparks we can easily calculate the engines rate of revolution by just counting the number of pulse surges in the HV line and how are they distanced from each other.
One thing to note about using the HV lines is that if they are come in contact with ground, or close to it, the engine could stop.