TACHOMETER APPLICATION

A measure of the operating level of any motor or engine is

the rotational velocity of its output shaft In the case of an

automotive engine the crankshaft speed is measured using

the units ‘‘revolutions per minute’’ (RPM) It is possible to

indirectly measure the speed of the crankshaft by using the

signal present on the engine’s ignition coil The fundamental

frequency of this signal is a function of engine speed and

the number of cylinders and is calculated (for a four-stroke

engine) from the formula

(Hz)

(5)

the crankshaft in RPM From this formula the maximum fre-

quency normally expected (for an 8 cylinder engine turning

4500RPM) is 300 Hz In certain specialized ignition systems

(motorcycles and some automobiles) where the coil wave-

systems are identified by the fact that multiple coils are used

in lieu of a single coil and distributor Also the coils have

two outputs instead of one

A typical automotive tachometer application is shown in

Fig-

ure 1 The coil waveform is filtered squared and limited by

the RC network and NPN transistor The frequency of the

pulse train at pin 9 is converted to a proportional voltage by

the Norton amplifier’s charge pump configuration The igni-

tion circuit shown in

Figure 5 is typical of automotive sys-

tems The switching element ‘‘S’’ is opened and closed in

synchronism with engine rotation When ‘‘S’’ is closed en-

ergy is stored in Lp When opened the current in Lp diverts

from ‘‘S’’ into C The high voltage produced in Ls when ‘‘S’’

is opened is responsible for the arcing at the spark plug

The coil voltage (see

Figure 6 ) can be used as an input to

the LM1819 tachometer circuit This waveform is essentially

constant

duty cycle D4 rectifies this waveform thereby pre-

venting negative voltages from reaching the chip C4 and

R5 form a low pass filter which attenuates the high frequen-

cy ringing and R7 limits the input current to about 25mA

R6 acts as a base bleed to shut the transistor OFF when

‘‘S’’ is closed The collector is pulled up to the internal regu-

pulse

Many ignition systems use magnetic hall effect or optical

sensors to trigger a solid state switching element at ‘‘S’’

These systems (see the LM1815) typically generate pulses

of constant

width and amplitude suitable for driving the

charge pump directly

The charge pump circuit in

Figure 7 can be operated in two

modes constant input pulse width (C1 acts as a coupling

capacitor) and constant input duty cycle (C1 acts as a differ-

entiating capacitor) The transfer functions for these two

modes are quite diverse However deflection is always di-

rectly proportional to R2 and ripple is proportional to C2

The following variables are used in the calculation of meter

deflection

symbol

description

n

number of cylinders

i

pointer deflection at redline degrees

e

charge pump input pulse width seconds

peak to peak input voltages volts

Di

maximum desired ripple degrees

k

function generator gain degreesvolt

Where the NPN transistor and regulator are used to create a

grees (a typical pointer is about 3 degrees wide) depending

on meter damping and the input frequency

The constant pulse width circuit is designed using the fol-

lowing equations

(1)

R1

k

3mA

(2)

10e

(3)

e

(4)

1

e

1

The constant duty cycle equations are as follows

The values in

270

degrees

1

ms

is

mode For distributorless ignitions these same equations will

4