A very simple tachometer circuit can be designed using a 555 timer, like in this circuit diagram . An electrical signal taken from the low tension side of the distributor is converted into a voltage proportional to engine rpm and this voltage is displayed on a meter calibrated accordingly .
As you can see in the circuit diagram , this electronic project is based on a 555 timer circuit and some other electronic parts . The 555 timer IC is used as a monostable which converts the signal pulse from the breaker points to a single positive pulse the width of which is determined by value of R4 , RV4 and C2 . R2 and R3 resistors set a voltage of about 4 volts at pin 2 of IC2 .
The IC is triggered if this voltage is reduced to less than approximately 2.7 volts (1/3 of supply voltage ) , and this occurs due to the voltage swing when the breaker points open. Rv1 potentiometer enables the input level to be set to avoid false triggering .
ZD1 and 180 ohms resistor stabilize the unit against voltage variations .
This tachometer electronic project must be powered from a 12 volts Dc power supply circuit .