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A FUEL INJECTOR
MONITOR FOR CARS
Have you ever wondered how much petrol
you use when you accelerate away from the
traffic lights? Perhaps you would like to know
how your fuel consumption increases as you
climb a hill. If you have a fuel injected car, this
project is for you.
By RICK WALTERS & LEO SIMPSON
Back before cars had engine management computers, they often had a
vacuum gauge which was supposed
to give an indication of fuel economy.
Low vacuum readings meant you were
using lots of juice while high vacuum
meant that you were driving with a
light throttle.
In practice, a vacuum gauge was
often a distraction as it fluctuated
wildly each time you depressed the
accelerator, as you moved up or down
through the gears. Some drivers even
24 Silicon Chip
went so far as to cover up the vacuum
gauge to avoid its distraction.
Now we’re in the 90s and vacuum
gauges are decidedly “old hat”. Most
modern cars have fuel injection and
the drive signal to the injectors can
be monitored to provide a very good
guide to fuel use. The amount of fuel
provided by the injectors is controlled
by the amount of time they are open.
When your car is at idle, the injectors
are open only about 5% of the time.
During normal driving, the injectors
are open between 10% and 20% of the
time. And when you are accelerating
absolutely flat out, with the engine
wound out to 5000 RPM or more and
the accelerator fully open – “pedal to
the metal” – the injectors will be open
for more than 90% of the time.
Since the injectors are fed from
the fuel rails at essentially constant
pressure, the fuel used by the motor
is directly proportional to the injector
opening time.
The Fuel Injector Monitor is housed
in a compact case, allowing it to be
conveniently placed on your car’s
dashboard at eye level. The straightline display consists of 20 light emit
ting diodes (LEDs), 18 green, one
orange and one red. The display is
semi-logarithmic, with the first 10
LEDs showing 10 steps of 1% from
0-9%, while the second group of LEDs
covers from 10% to 100%.
The LED display takes the form of
a bargraph which shows the average
+15V
0.1
10k
D1
1N914
LK2a
10k
INPUT
IC1b
LK2b CA3260E
LK1b
6
LK1a
D2
1N914
16
8
5
2
VR1
10k
7
2 bx
3
cy
13
ay
1
IC1a
3
4
47k
D3
1N914
4.7
2.2M
IC2
5
4053
cx
12
ax
10
10
4.7k
10k
+15V
1
C 9
11
A
10
B
c 4
14
a
15
b
by
6
7
8
+12V
LED9 LED10
GRN GRN
A
LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8
GRN GRN GRN GRN GRN GRN GRN GRN
A
A
A
A
10
A
K
1
K 18
K
A
K
17 K 16
A
K
15 K 14
22k
A
13 K 12
A
K
11 K 10
6
7
4
8
2
K 18
A
K
1
9
A
K
IC3
LM3914
5
LED11LED12 LED13LED14 LED15 LED16LED17 LED18LED19LED20
GRN GRN GRN GRN GRN GRN GRN GRN YEL RED
A
A
A
A
A
K
17 K 16
A
K
15 K 14
10
A
K
13 K 12
A
11 K 10
IC4
LM3914
5
3
3
6
7
820
820
8
4
2
9
6.8k
680
ZD1
5.6V
400mW
+12V
+12V
0.1
33k
7
0V
47k
4
8
IC5
555
6
2
3
5
1
D5
IN914
100
IN
D4
1N914
REG1
7815
GND
100
OUT
+15V
4.7k
10k
10
B
.01
Q1
BC327
C
+12V
.01
E
B
A
K
E
A
K
100k
C
VIEWED FROM
BELOW
I GO
FUEL INJECTOR MONITOR
Fig.1: the 4053B multiplexer (IC2) enables the LM3914 LED drivers to give a
dot and bar display to indicate the average and peak injector duty cycles. The
555 timer (IC2) controls the switching of the 4053 and also steps up the battery
voltage to provide for a +15V regulated supply.
opening times, combined with a
brighter “peak” LED which shows
more rapid fluctuations of the injector
openings, as can happen, for example,
when you blip the throttle. The peak
LED is actually a “peak-hold” display
which captures the rapid transients
and “holds” them so that they can be
more easily seen.
Unlike some car circuits, installation of the Fuel Injector Monitor
is quite straightforward: one lead to
ground (chassis) and two leads to the
injector leads (one switched and the
other battery positive) – more about
that later.
Circuit details
The circuit of Fig.1 consists of five
ICs plus a regulator, the 20 LEDs and
a few other minor components.
In most modern cars, all the injector
solenoid coils are wired in parallel
with one side connected to the battery
positive, through the ignition switch.
The coils are switched to ground via a
transistor when fuel is to be injected.
This means that the pulse waveform
fed from the injectors to our monitor
is a +12V signal going to ground.
While most cars have negative-going
pulse injector wave
forms, we have
provided for vehicles with the opposite waveform polarity. This is done
via two links to allow the selection
of either system. The input circuit
consists of IC1, a dual opera
tional
August 1995 25
amplifier. IC1b is used as a comparator
while IC1a is used as a peak detector.
The injector signal is applied via
a 10kΩ isolation resis
tor to diodes
D1 and D2. These diodes provide
transient protection for the following
op amp by clamping any input signal
between ground and +15V (more pre-
LED1-LED20
0.1
820W
VR1
IC1
CA3260E
+12V
4.7k
1
LK1a
LK2b
LK2a
D3
1
D4
.01
.01
0.1
IC5
555
100uF
4.7uF
47k
33k
1
10uF
2.2M
10uF
0V
IC2
4053
10uF
10k
680
100k
ZD1
100uF
LK1b
6.8k
47k
D1
820W
10uF
D2
INPUT
Q1
1
1
10k
IC4
LM3914
4.7k
IC3
LM3914
10k
22k
10k
K
10uF
A
D5
REG1
7815
Fig.2: install the parts on the board as shown here. The electrolytic capacitors
must all lie flat on the board, otherwise it will not fit into the plastic case.
Fig.3: this is the full-size etching pattern for the PC board.
26 Silicon Chip
cisely, to between -0.6V and +15.6V).
IC1 can accept signals in this range
without damage.
Our circuit description will apply
to cars with a negative-going injector
signal (the most common situation)
and so links LK1a and LK2a will be
installed. Ignore the links LK1b and
LK2b which are shown dotted. Hence,
the injector signal is applied via a 10kΩ
resistor to pin 6 of IC1b. Pin 5 of IC1b
is held at approximately +5V via a
voltage divider consisting of 10kΩ
and 4.7kΩ resistors. Thus, whenever
the injector voltage falls below +5V,
the output (pin 7) of IC1b will go high.
The output of IC1b is fed to trimpot
VR1, a 10kΩ pot wired as a variable
resistor. VR1, in conjunction with the
4.7kΩ resis
tor to ground, provides
calibration for the circuit. The output
of IC1b is used to charge the 220µF
capacitor. This becomes the “average”
value of the pulse signal and is used to
drive the bargraph portion of the LED
display. The “average” signal from the
220µF capacitor is fed to pin 3 of IC1a
and to pins 5 & 12 of IC2.
IC1a and diode D3 function as a peak
detector to charge a 4.7µF capacitor
to the “peak” value of the voltage appearing at pin 3. The 4.7µF capacitor
is slowly discharged by the 2.2MΩ
resistor and so it provides the “peak
hold” value for the peak DOT on the
LED display.
So now we have two voltages, the
peak and average values of the injector
pulse widths which must be shown on
the same 20-LED bargraph. How do
we do this? It is done by a technique
known as multiplexing whereby two
values are alternately flashed onto
the LEDs, each value being shown for
part of the time. This switching of the
signals happens very rapidly so that
our eyes are not aware of it.
IC2, a 4053, does the multiplexing
and is described as a triple 2-channel
analog multiplexer. It alternately
switches the bar signal (pins 5 & 12)
and the dot signal (pins 3 & 13) to the
LED display drivers (IC3 & IC4).
IC5 controls the switching of IC2
and serves another purpose – to step
up the car’s battery voltage. The vol
tage step-up is necessary to enable
the display drivers to handle the full
range of signal voltage from IC1. We’ll
explain more about this later.
The 555 timer is arranged as an
astable oscillator, with a frequency of
about 1kHz. Its pulse output waveform
All the LEDs are arranged to sit flat along the edge
of the PC board but because of the pin layout of the
LM3914 drivers, the display reads from right to left.
Consequently, the board hangs upside down in the
case to make the display read from left to right.
is fed to a voltage doubler consisting
of diodes D4 & D5 together with two
100µF electrolytic capacitors. The
resulting voltage of about +19V is fed
to the 7815 regulator which delivers
a stable +15V.
Multiplex operation
We have already referred to multiplex operation so let’s now look at it in
more detail. As noted above, we need
to display two signals (the “average”
and “peak” values) and at the same
time we need to switch the display
drivers, IC3 & IC4, between dot and bar
modes. IC2, the multiplexer, has three
internal switches and while these are
not shown on the circuit, they can be
identified in the following way.
Switch A involves pins 11, 12, 13
& 14; switch B involves pins 1, 2, 10
& 15 and switch C involves pins 3, 4,
5 & 9. Pins 9, 10 and 11 control the
position of each associated switch; eg,
if pin 9 (the C switch control input)
is high, pin 4 (c) is connected to pin
3 (cy) while if pin 9 is low, pin 4 is
connected to pin 5 (cx).
Returning now to IC5, which provides the switching signal, when pin
3 is low, pins 9 & 11 of IC2 switch the
“average” signal to the pin 5 inputs
of the display drivers IC3 and IC4.
At the same time, pins 9 of IC3 & IC4
are pulled low to select the bar mode.
Conversely, when pin 3 of IC5 is high,
the “dot” signal at pins 3 & 13 of IC2 are
switched to pins 5 of IC3 & IC4 which
are then switched into the dot mode.
Just to reiterate, the bar mode displays the average signal while the
dot mode displays the peak which is
RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 1
❏ 1
❏ 4
❏ 1
❏ 3
❏ 2
❏ 1
Value
2.2MΩ
100kΩ
33kΩ
22kΩ
10kΩ
6.8kΩ
4.7kΩ
820Ω
680Ω
4-Band Code (1%)
red red green brown
brown black yellow brown
orange orange orange brown
red red orange brown
brown black orange brown
blue grey red brown
yellow violet red brown
grey red brown brown
blue grey brown brown
5-Band Code (1%)
red red black yellow brown
brown black black orange brown
orange orange black red brown
red red black red brown
brown black black red brown
blue grey black brown brown
yellow violet black brown brown
grey red black black brown
blue grey black black brown
August 1995 27
Fig.4: this fuel injector waveform was taken from a Ford
Laser S with a 1.8 litre engine. The duty cycle is under
10% at around 2000 RPM with the car stationary. The
lower waveform is taken directly from the injector, while
the upper waveform is the output of IC1b at pin 7.
always equal to or higher than the average. To make the peak (dot) display
brighter than the average, it is turned
on for longer than the average and
this is arranged by giving the pulse
signal from IC5 a duty cycle of more
than 50%.
Part of the switching function controlled by IC5 is performed by transistor Q1 but because IC5 runs from
12V rather than 15V, its output cannot
swing to the +15V necessary to ensure
that Q1 is turned off. Therefore, zener
diode D4 is included to allow Q1 to
turn off when IC5’s output is high.
IC3 and IC4, the LM3914 dot/
bar display drivers, which accept
analog input signals from IC1, have
10 internal comparators which drive
10 external LEDs. The input range is
determined by one or two resistors.
IC3 is set by the 820Ω resistor between
Fig.5: taken from a VP Holden Statesman with a 5-litre V8
engine, these injector waveforms are again at around 2000
RPM and the duty cycle is under 20%. The lower waveform
is the fuel injector driving voltage, while the upper
waveform is the output of IC1b at pin 7.
pins 6 and 7 and ground, to accept
0.125-1.25V and dis
play 10 output
steps from 0-9%. IC4 with its extra
resistors accepts 1.265-12.65V for its
10 outputs, from 10% to 100%.
Actually, these display steps should
not be thought of as being absolutely
precise. For example, if the 10% LED
is lit, the injector pulse width can only
be regarded as being above 10% but
less than 20%. Similarly, if the 30%
LED is lit, the injector pulse width is
above 30% but below 40%.
Construction
All the components for this project,
including the 20 LEDs, are mounted on
a small PC board coded 05108951 and
measuring 120 x 102mm– see Fig.2.
The PC board is mounted in a small
plastic case measuring 141mm wide,
36mm high and 110mm deep. The case
Fig.6: this is
another injector
waveform, taken
with a Tektronix
TDS744A digital
oscilloscope from
a Ford Laser
S at idle. Note
the very narrow
pulse width.
28 Silicon Chip
splits into two sections, upper and
lower, with two removable pieces for
the front and back sections.
The lower section has four integral
pillars for the PC board but because
of a layout constraint caused by the
LM3914 display drivers, the PC board
has had to be designed so that the
LEDs run from right to left (to minimise the number of links required).
To make the display read from left
to right as it should, the PC board is
mounted on the base of the case and
then it is inverted, so that it “hangs
from the roof”.
Before you begin assembly, carefully check the PC board for broken
or shorted tracks, especially between
the pads on IC2 and IC4.
First, install the six links, diodes
and resistors. The capacitors are next.
Be sure to lie the electrolytics flat, as
the board will not fit into the case if
you stand them up. Be sure to bolt the
regulator down flat onto the PC board.
Lastly, fit the LEDs, ICs, trimpot and
transistor.
The LEDs should be mounted so that
they are flush with the front edge of the
PC board. We could not obtain a 5mm
square orange LED for our prototype
so we fitted a 5mm round one in that
position.
We used a thin piece of tinted
plastic for our front panel and made
an adhesive front-panel label with a
rectangular cutout for the LEDs. The
PC board is mounted to the integral
pillars using 6mm spacers and 12mm
long self-tapping screws.
After you have carefully checked
all your assembly work and soldering,
you are ready to do an initial power
check. If you don’t have a 12V power
supply, you could apply power from
a 12V car battery or from your car’s
cigarette lighter socket. Make sure
that you connect the 12V leads the
right way around otherwise you will
damage the circuit, with IC5 (the 555
timer) the most likely casualty.
Just connect the 12V supply at first,
without connecting the input lead
from the injectors. All the LEDs should
flash once and then the peak LED
moves slowly from right to left. Now
connect the injector input lead to 0V
and most, if not all, LEDs should come
on and stay on. If that checks out OK,
you can move to the next step which
is calibration.
be slightly less, at around 13.8V. This
latter lead is the one we’re looking for
and is the one which we will make the
permanent connection to.
Now remove the pin from the other
injector lead. To make a permanent
connection, again the easiest method
is to use a pin. This time, push the pin
right through the centre of the injector
lead and bend it over and twist the
ends together. This way, the integrity
of the injector lead itself is preserved.
Now solder a lead to the pin while
making sure that you don’t damage
the injector lead insulation. (Perhaps
you might like to practice soldering to
a sample pin before you do the actual
job on your car!)
Having made the connection, carefully wrap it with insula
tion tape.
Having done that, the most convenient
place to pick up +12V to power the
circuit is from the other injector lead,
so repeat the pin soldering to the other
injector lead.
Now anchor the two leads running
away from the injector harness with a
plastic cable tie to a convenient point
on the engine so that vibration is unlikely to dislodge them.
You will need to pass the two leads
through the firewall into the passenger
compartment. You will then need to
make a connection to chassis for the
0V lead. It would also be prudent to
install an in-line 1A fuse in the +12V
line from the injector harness.
Now make your connections to the
Fuel Injector Monitor and turn on the
ignition. With the engine stopped,
all LEDs should be alight. When the
engine is started, the LEDs will light
up to about 60% or higher and then
gradually drop back to the normal
idle value of around 5% or 6% as the
engine warms up.
Calibration
This will be the easiest calibration
you have ever done. With the input
lead connected to the 0V terminal,
carefully adjust the trimpot until the
red LED just comes on. You will need
to wind the trimpot anticlockwise
initially and then clockwise until the
red LED just comes on. This calibrates
the unit to correctly display an injector
opening of 100%.
Installation
The trickiest part of the installation
is to identify which of the two injector
leads to make the connection to. Unless you have a wiring diagram for your
car, you will need to make a voltage
measurement on the two leads while
the engine is running.
In practice, the easiest way to make a
temporary connection to your injector
leads is to push a pin right through
the centre of each of the wires. Now
start the car and let it idle for a couple
of minutes to let the battery voltage
stabilise. Now measure the voltage
between each injector lead and chassis. One injector lead will be at the
same voltage as the battery (eg, 14.4V)
while the other injector lead will
Fault finding
If you have a problem, the first thing
to check is the +15V rail. There should
be about +19V into the 7815 regulator
and +15V at its output. If the input
voltage is 0V to the 7815, then IC5 is
FUEL INJECTOR MONITOR
0
1
2
3
4
5
6
7
8
9 10 20 30 40 50 60 70 80 90 100
Fig.7: this is the full-size front panel artwork.
PARTS LIST
1 PC board, code 05108951, 120
x 102mm
1 plastic case, 141 x 36 x 110mm
1 front-panel label, 132 x 28mm
1 10kΩ horizontal trimpot (VR1)
1 3mm x 8mm roundhead screw
1 3mm nut
1 3mm SP washer
4 6mm spacers
4 12mm self-tapping screws
Semiconductors
1 RCA CA 3620E dual op amp
(IC1)
1 4053B triple 2-channel analog
multiplexer (IC2)
2 LM3914 dot/bar display drivers
(IC3,IC4)
1 555 timer (IC5)
1 7815 15V regulator (REG1)
1 BC327 transistor (Q1)
5 1N914 diodes (D1-D3, D5, D6)
1 5.6V 400mW zener diode (ZD1)
18 LTL9234A 5mm square green
LEDs or equivalent (LED1-18)
1 5mm square or round orange
LED (LED19)
1 LTL4223A 5mm square LED or
equivalent (LED20)
Capacitors
2 100µF 25VW PC electrolytic
5 10µF 50VW PC electrolytic
1 4.7µF 50VW PC electrolytic
2 0.1µF MKT polyester
2 .01µF MKT polyester
Resistors (0.25W 1%)
1 2.2MΩ
4 10kΩ
1 100kΩ
1 6.8kΩ
4 4.7kΩ
1 33kΩ
2 820Ω
1 22kΩ
1 680Ω
not oscillating. Check the component
values and soldering around this IC.
With the input connected to 0V (as
explained in the cali
bration procedure), pin 7 of IC1 should measure
around +14.5V. When the injector
input is not connected, pin
7 should be near 0V.
If your monitor reads
100% at idle and falls as
you accel
e rate, it means
your injector signal is the
wrong polarity. Remove
links LK1a and LK2a and
replace them in positions
SC
LK1b and LK2b.
August 1995 29
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