Sun tach rebuild

A tachometer is a frequency to current converter. There are many ways with electronics to do that. There are circuits with NE555 timer and a few components. The circuit is basically a one shot, that is triggered by ignition pulses, more pulses more on time, hence more drive current, capacitance, and inductance of meter movement filters pulses to average value. Calibration done various ways, series resistance, pulse voltage, pulse width. Pulse width must be short enough to accommodate highest RPM without over run.

Taking signal from coil - can be troublesome if there is point bounce, it will cause over reading of tach. If input circuit loads coil - (too low of input resistance) it degrades spark energy, it also must be capable of handling 300 to 500V transient. Modern tachs get signals from position sensors, measure period between ignition events. Inverse of period is frequency, that number is used to drive display.

The modern tach Del writes about uses a stepper motor to drive needle, it increases needle deflection a specific number of degrees per pulse. Permanent magnets make poles of x degrees, coil drives rotor, to index next step. More complicated smart control electronics, but needle movement more reliable.