Generally a tachometer may be considered an expensive equipments as these are highly accurate and intended for obtaining correct RPM rates of the concerned engine under test.
The conventional units are therefore very sophisticated and generate highly accurate results while testing.
However it doesn’t mean that a simpler version cannot be built at home. With electronics at its best today, making a tachometer circuit at home isn’t at all difficult. What’s more the results obtained from such circuits are fairly accurate and provides the required data for assessing the overall working condition of the system.
The circuit diagram shows a simple configuration utilizing the IC 555. The IC is basically configured as a monstable multivibrator.
The pulse is derived from the spark plug and fed to the end of R6.
The transistor responds to the pulses and conduct in accordance with triggers.
The transistor activates the monostable with every rising pulse of the input.
The monostable stays ON for a particular moment each time its triggered and generates an average ON time at the output which is directly proportional to the average trigger rate.
The capacitor and the resistor at the output of the IC integrate the result so that it can be directly read over a 10V FSD voltmeter.
The pot R3 should be adjusted such that the output generates the exact interpretations of the fed RPM rates.
The above setting up must be done with the help of a good conventional tachometer unit,
The above circuit can be further simplified as expressed by Mr. Abu Hafss through the following diagram:
Please note that since the image color is inverted, the electrolytic capacitor’s white bar signifies negative and the black bar denotes positive terminal of the capacitors.
C5 in the above diagram could be any value between 3.3uF and 4.7uF @ 25V