Project 17 - ELECTRONIC TACHOMETER

Abstract

This chapter focuses on the electronic tachometer. This versatile and highly accurate electronic revolutions per minute (r.p.m.) meter can be fitted to any vehicle that uses a 12 V ignition system. It works equally well on all 2-stroke and 4-stroke engines having from 1 to 12 cylinders, and using either conventional or electronic positive or negative ground ignition. The accuracy is virtually unaffected by changes in battery voltage and ambient temperature, and is totally unaffected by contact breaker point bounce, switching transients, and dwell time variations. The unit's input is connected to the vehicle's contact breaker, which generates a driving waveform that has a rate directly proportional to engine r.p.m. Therefore, to prevent the ringing high-voltage contact breaker signal damaging the semiconductor circuitry of the tachometer, the contact breaker signal is first passed through a low-pass filter, which eliminates the high-frequency high-voltage components of the basic waveform, and gives a modified low-voltage output waveform.