Two and four cycle digital tachometer

A digital tachometer (2) is provided for two and four cycle internal combustion engines. A transformer (14) senses ignition pulses on the primary lead (6) of the ignition coil (4) of the two cycle engine, and an electrical connector (64) senses ignition pulses at the primary lead post terminal (66) of the ignition coil (54) of the four cycle engine. First and second signal conditioning circuits (50 and 96) are provided for each ignition pulse sensor for polarity immunity, RFI suppression, and transient and overvoltage protection. A precision timer (166) responds to pulses from the signal conditioning circuitry and outputs an output pulse of given delay, and ignores other pulses from the signal conditioning circuitry during the given delay, to protect against transient false triggering, especially in breaker point ignition systems. A phase-locked loop (186) responds to the delay pulses and outputs a pulse train having a frequency which is a function of frequency of the delay pulses. A counter and display circuit (190) counts the pulses in the pulse train as a function of time and displays same to indicate revolutions of the engine per unit time. A battery saver circuit (112) generates a reference voltage from the battery (110) and compares it against the voltage from the signal conditioning circuitry for outputting a turn-on signal to a transistor (154) to connect the battery (110) to various circuit components only above a threshold engine speed.

BACKGROUND AND SUMMARY 
The present invention provides a digital tachometer for an internal 
combustion engine having an ignition coil with an input primary low 
voltage lead and an output secondary high voltage lead. 
The invention arose from efforts to develop a reliable and highly accurate 
tachometer for marine drive engines. It was further desired to provide a 
universal tachometer usable for both a two cycle engine, such as an 
outboard drive, and for a four cycle engine, such as a stern drive. 
The tachometer of the invention senses pulses on the primary of the 
ignition coil, to avoid the high voltage and inference present on the 
secondary lead of the coil. Signal conditioning circuitry is provided to 
further isolate and condition both four cycle primary pulses and two cycle 
primary pulses. Further signal conditoning circuitry is provided for 
ignoring transients and the like, particularly with breaker point type 
ignitions. A precision timer provides an accurately controlled delay for 
ignoring false or transient pulses and the like. A phase-locked loop 
responds to the pulses and outputs a pulse train having a frequency as a 
function thereof. A counter and display circuit counts the pulses in the 
pulse train as a function of time and displays same to indicate 
revolutions of the engine per unit time. 
A battery saver circuit is provided for supplying battery power to various 
components only when the primary pulses provide a voltage above a give 
threshold corresponding to a given engine speed. This extends battery 
life. A simple single switch controls both connection/disconnection of the 
battery in the battery saver circuit and connection of the timing 
circuitry to either of the two or four cycle conditioning circuitry or to 
an OFF position. This facilitates ease of user operation.

DETAILED DESCRIPTION 
There is shown in the drawing a digital tachometer 2 for an internal 
combustion engine having an ignition coil such as 4 with an input primary 
voltage lead 6 and an output secondary high voltage lead 8 connected to a 
spark plug 10. The coil is typically grounded as at 12. For a two cycle 
outboard marine engine, a toroid transformer and clip 14 senses the signal 
on primary lead 6. Clip 14 is a standard type clamp with separable jaws 
loosely enclosing lead 6, and including for example 34 turns of number 24 
wire. Sensing transformer 14 is connected by a shielded cable 16, such as 
a Belden 8412 shielded connector, providing conductors 18 and 20 connected 
to each end of the transformer coil, and shield conductor 22. These 
conductors are connected through respective plug-in type connectors 24, 26 
and 28, such as Amp 206153-1, to conductors 30, 32 and 34, respectively, 
with the latter being grounded. 
Conductors 30 and 32 are connected in a signal conditioning circuit 
including a pair of grounded bypass capacitors 36 and 38 for passing high 
frequency signals such as RFI out of the system, and hence provide a low 
pass filter. Another capacitor 40 is provided for conditioning and 
stabilizing the signal, and eliminating transients and the like. The 
signal is then passed to a full wave rectifier bridge 42 to provide a 
positive output regardless of negative or positive polarity input from 
sensing coupling transformer 14. The signal is then further filtered by 
resistor 44 and capacitor 46, and the voltage dropped across resistor 48. 
The signal conditioning circuit is blocked in dashed line as shown at 50, 
and its output is provided at 52. 
Sensing coupling circuitry is also provided for a four cycle marine drive 
engine, such as in a stern drive having an ignition coil 54 with a primary 
low voltage lead 56 and an output secondary high voltage lead 58 connected 
to spark plug 60. The coil is grounded by the other primary lead 62 
through an electronic switch or points 63. An alligator type clip 64 is 
connected to the primary circuit on the point side at terminal post 66 of 
the coil in conventional manner providing a direct electrical connection. 
Another alligator type clip 68 is grounded, as by connection to the frame 
or the like. Conductors 70 and 72 from respective clips 64 and 68 are 
connected by a cable connector 74, such as a Belden 8412 cable, to 
respective conductors 76 and 78 which are in turn connected through 
respective plug-in type connectors 80 and 82, such as Amp 206153-1, to 
respective conductors 84 and 86, with conductor 84 being grounded. The 
signal pulses on conductor 86 are half wave rectified by a diode 88, and 
signals below a given threshold voltage are blocked by reverse zener diode 
90 to prevent unwanted signals from entering the circuit. The voltage is 
then dropped across resistor 92 in a voltage dividing network with 
resistor 94. The signal conditioning circuit described is shown as blocked 
in at 96, and provides an output at 98. 
A user controlled switch 100 has a pair of inputs 102 and 104 and an output 
106. The switch is settable by the user to selectively connect either of 
inputs 102 and 104 to output 106. The switch also has an OFF position at 
108 wherein neither input 102 nor 104 is connected to output 106. Signal 
conditioning circuit 96 is connected between coupling sensor 64 and switch 
input 102. Signal conditioning circuit 50 is connected between coupling 
sensor 14 and switch input 104. 
Various of the components in tachometer 2, to be described, are powered by 
a battery 110. A battery saver circuit is shown at dashed line 112. A user 
controlled switch 114 selectively connects the battery to either of inputs 
116 and 118, each of which is connected to a node 120, or switch 114 may 
be in an OFF position at 122, whereby switch 114 selectively connects and 
disconnects battery 110 to node 120. Voltage regulating means are provided 
between node 120 and a comparator 124 provided by an operational amplifier 
such as a 3130 IC chip. The voltage at node 120 is dropped across resistor 
126 and then clamped to a given value by zener diode 128 and then further 
dropped through the voltage dividing network provided by resistors 130 and 
132, and filtered by capacitor 134 to thus provide a reference voltage for 
operational amplifier 124. Manufacturer assigned pin designations are 
shown for the 3130 chip to facilitate clarity. The zener diode clamping 
voltage is provided as the supply voltage at pin 7, and a compensating 
capacitor 136 is provided between pins 1 and 8, as is standard for such 
3130 chip. 
The reference input of operational amplifer 124 is provided at pin 2, as 
noted. The comparing input of operational amplifier 124 is provided at pin 
3 which is connected to the signal conditioning circuitry 96 or 50 at the 
output 106 of switch 100. Output 106 is connected through conductor 138, 
diode 140 and an RC charging network, provided by resistor 142 and 
capacitor 144, to the comparing input at pin 3 of operational amplifier 
124. When the frequency and magnitude of pulses through diode 140 are 
sufficient to charge the voltage of capacitor 144 above the reference 
voltage at pin 2 of operational amplifier 124, then output pin 6 at 
conductor 146 goes high, which output is fed through diode 148 and the 
voltage dividing network provided by resistors 150 and 152 to provide a 
turn-on signal at the base of transistor 154 to render the latter 
conductive. The base of transistor 154 is connected by conductor 146 to 
the output of operational amplifier 124, the collector of transistor 154 
is connected through resistor 156 to node 120, and the emitter of 
transistor 154 is connected through a filtering and clamping network 
provided by zener diode 158 and capacitors 160 and 162 to provide an 
output supply voltage at conductor 164, for example +5 volts for powering 
various components in the tachometer, to be described. 
With the user controlled switch 114 in either its up or down ON position, 
and with the electronic switch provided by transistor 154 being triggered 
into conduction, current can flow from battery 110 through switch 114 and 
switch 154 to output 164 to provide supply voltage. With switch 114 it is 
central OFF position at 122, no battery power is supplied. With transistor 
switch 154 in its nonconductive OFF state, regardless of the state of 
switch 114, no battery power is supplied at output 164 to various circuit 
components, to be described. Transistor 154 is in its nonconductive OFF 
state when there is no base drive turn-on signal thereto, i.e., when the 
signal on conductor 146 at output pin 6 is low. Output pin 6 is low when 
the voltage at comparing input pin 3 is below the voltage at reference 
input pin 2, which in turn occurs at engine speeds or rpm's below a given 
threshold. 
Switches 100 and 114 are provided together in a double pole double throw 
user controlled manual switch. Battery 110 is connected and disconnected 
from the battery saver circuit at node 120 in unison with connection and 
disconnection of first switch output 106 with signal conditioning 
circuitry 96 and 50 at inputs 102 and 104. The double pole double throw 
switch has a central OFF position with switches 114 and 100 connected 
respectively to points 122 and 108. The double pole double throw switch 
has an upward ON position with switch 114 connected to point 116 and 
switch 100 connected to point 102. The double pole double throw switch has 
a downward ON position with switch 114 connected to point 118 and switch 
100 connected to point 104. This operation in unison with a single manual 
switch facilitates ease of user operation. 
A precision timer 166, such as provided by a 2905 IC chip as shown with 
manufacturer assigned pin designations, is connected to the signal 
conditioning circuitry 96 or 50 through switch 100 at output 106. A third 
signal conditioning circuit 168 is connected between the switch output 106 
and timer 166, and includes a filter capacitor 170 and a clamping zener 
diode 172 for voltage protection. Supply voltage for chip 166 is provided 
at pins 5 and 6 from output 164. A timing constant is provided by the RC 
network having resistor 174 and capacitor 176, and RFI filtering is 
provided by capacitor 178. In one particular embodiment, the timing 
constant is set for two microseconds to eliminate unwanted signals from 
affecting the remaining counting circuitry. Transient or other unwanted 
signals may be a particular problem in breaker type ignition systems. 
In accordance with the operation of a 2905 chip, when input pin 1 at 
conductor 180 goes high, for an incoming pulse, then output pin 7 at 
conductor 182 goes low and stays low for two microseconds, and then goes 
back high again. During the two microsecond delay interval, output pin 7 
stays low regardless of other pulses at input pin 1, i.e., regardless of 
whether conductor 180 goes low and then back high again. Precision timer 
166 thus responds to a pulse from the signal conditioning circuitry and 
outputs an output pulse of given delay, and ignores other pulses from the 
signal conditioning circuitry during the given delay, and then outputs 
another output pulse of given delay in response to the next pulse from the 
signal conditioning circuitry following the given delay of the first 
mentioned output pulse. In the particular embodiment in marine 
applications, transient ignition spikes may be present for up to 1 or 1.5 
microseconds after a sensed pulse, especially breaker point ignitions. 
Precision timer 166 ignores such transients. 
The output of timer 166 on conductor 182 is supplied to a monostable 
multivibrator 184, such as a 4538 IC chip, providing a one shot clean 
square wave for every input pulse thereto on conductor 182. Phaselocked 
loop 186 responds to the pulses from monostable multivibrator 184 and 
outputs a pulse train having a frequency which is a function of the 
frequency of the delay pulses on conductor 182 through monostable 
multivibrator 184. Phase-locked loop 186 may be provided by a 4046 IC chip 
and may include a 40102 divide by n downcounter IC chip, as is known, for 
example "Design Of Phase-Locked Loop Circuits", Howard M. Berlin, Howard 
W. Sams and Co., 4300 West 62, Indianapolis, Ind., 1978, pgs. 360-367, and 
FIGS. 7-22. 
As is standard, a different value of n can be programmed to calibrate for 
different output pulse train frequencies, for example for calibration 
between two and four cycle engines, and for calibration according to the 
number of cylinders of a four cycle engine, and so on. The output of 
phase-locked loop 186 is provided through a buffer gate 188, such as 4011 
IC chip providing transition isolation, and then fed to a counter and 
display circuit 190, such as a Red Lion Controls Ditak V DT500000 LCD 
display head. Counter and display circuit 190 is a precision counter and 
crystal control time base with a liquid crystal display, and accumulates 
incoming pulses in the pulse train through buffer gate 188 from 
phase-locked loop 186 for a precise interval, for example exactly one 
second. Immediately at the end of this interval, the accumulated count is 
transferred and latched into the LCD display. Immediately after 
transferring the count to the display, the internal counter is reset to 
zero and begins accumulating a new count. Display head 190 is powered by a 
supply voltage on conductor 192 from the zener clamped battery voltage 
reference. The counter and display circuit 190 counts the pulses in the 
incoming pulse train as a function of time and displays same to indicate 
revolutions of the engine per unit time. 
It is recognized that various alternatives and modifications are possible 
within the scope of the appended claims.