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All the parts for the
Smart Mixture Display
are mounted on a small
PC board. This prototype
uses rectangular LEDs
for the 10-LED mixture
display but you can use
round LEDs if you prefer
– see text.
A Smart Mixture Display
For Your Car
Track your car’s fuel mixtures in real time, see the operating modes of
the ECU and be warned if a catastrophic high-load “lean out” occurs.
This Smart Mixture Display monitors your car’s oxygen sensor and
airflow meter outputs and gives an audible warning if mixtures go
dangerously lean.
T
By JULIAN EDGAR & JOHN CLARKE
HE SILICON CHIP Mixture Meter – first presented in 1995
– is one of the most popular
performance car electronic kits ever
produced. Literally thousands have
been built, each showing by means
of 10 coloured LEDs whether the air/
fuel ratio is rich or lean.
While such a design – which works
from the car’s standard oxygen sensor
– won’t give you an absolutely accurate
readout of the mixture strength, it’s far
better than having no indication at all
as to whether the car is running rich,
lean or at stoichiometric (the latter
means an air/fuel ratio of 14.7:1). As a
bonus, it also clearly shows if the car
is in closed or open loop mode
The new Smart Mixture Display pre60 Silicon Chip
sented here still displays the mixture
strength by means of 10 LEDs – red for
lean (red is for danger!), green for midrange mixtures and yellow for rich.
However, we’ve added three important
extra features with this new design:
(1). Better protection of the electronics
(in some cars, the old design was prone
to blowing its chip);
(2). An automatic dimming function
for night driving; and
(3). An audible lean-out alarm.
Lean-out alarm
The lean-out alarm is a great idea. It
monitors both the air/fuel ratio and the
engine load, sounding a buzzer if the
air/fuel ratio is ever lean at the same
time as the engine is developing lots
of power. So why is this important?
Well, if the engine – especially one
with a turbo – goes lean under high
loads, it’s almost certain that you’ll
instantly do damage. One Impreza
WRX that we know of lost part of an
exhaust valve this way.
What could cause this sudden and
catastrophic condition? Lots of things
– from a dying fuel pump to fuel starvation during cornering. Even a couple
of blocked injectors could cause a lean
condition. It’s not the complete answer
– there are some conditions that the
meter won’t register. However, in most
situations, it will act as an important
warning that things aren’t right.
The lean alarm works by also monitoring the voltage signal coming from
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Fig.1: the circuit is based on an LM3914 dot/bar display driver IC. This accepts the signal from the oxygen sensor and
directly drives a 10-LED display. Op amps IC2a & IC2b and their associated components (including Q2 and the piezo
buzzer) provide the “lean-out” alarm feature.
the load sensor – usually the airflow
meter. Most airflow meters have an
analog output voltage that rises with
engine load, being around 1V under
light loads (eg, at idle) and close to
5V under high loads. If the output
voltage from the airflow meter is high,
the meter knows that the engine load
must also be high.
LED indicators
But what about the main section of
the Smart Mixture Meter – the LEDs?
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How do they work?
In broad terms, the oxygen sensors
in most cars have an output voltage
that varies between 0-1V, with higher
voltages indicating richer mixtures.
The meter lights one LED for each
tenth of a volt coming from the sensor, so at 0.1V the far end red LED will
be on, at 0.2V the next red LED will
light up and so on. This doesn’t give a
precise indication of air/fuel ratio (see
the “Air/Fuel Ratio Measurement and
Oxygen Sensors” panel for the reasons)
but in practice, it’s still very useful.
So the oxygen sensor voltage is constantly displayed by means of the LEDs
and if the oxygen sensor output voltage
is low (ie, there is a lean mixture) at the
same time as the airflow meter output
is high (ie, a high engine load), the onboard piezo buzzer sounds.
However, most of the time (we hope
all of the time!), you won’t have to
worry about alarms sounding – instead
you’ll be able to glance at the dancing
LED as you drive along. Dancing?
April 2004 61
One of the most common causes of
turbo engine damage (along with
detonation) is a high load lean-out.
That’s what happened to this Impreza
WRX motor – and in just a moment
part of an exhaust valve was gone.
[Michael Knowling]
The exhaust gas oxygen sensor
delivers a mixture strength signal
than can be monitored by the 10-LED
Smart Mixture Meter. All cars made
in at least the last 15 years use an
oxygen sensor. [Bosch]
Won’t the illuminated LED stay constant if the air/fuel ratio isn’t changing?
One of the beauties of the meter is
that it will show when the ECU is in
closed loop operation, with the mixtures hovering around 14.7:1. This air/
fuel ratio – called stoichiometric – allows the catalytic converter to work
best, so at idle and in constant-speed
cruise, the air/fuel ratio will be held
around this figure.
To achieve this, the ECU monitors the oxygen sensor output. If the
mixtures are a bit richer than 14.7:1,
it leans them out a little. Conversely,
if the mixtures are a bit leaner than
14.7:1, it makes them slightly richer.
This constant cycling of mixtures
around the 14.7:1 point is called
“closed loop” and will cause the lit
LED to dance back and forth across
the meter – as much as two or three
LEDs either side of centre.
When some people see the LEDs
flashing back and forth in closed
62 Silicon Chip
loop operation, they quickly decide
that the meter is useless. After all,
the indication is “all over the place”!
However, it’s showing the very fast
oscillations that are actually occurring
in the mixture. By contrast, most aftermarket tail-pipe air/fuel ratio meters
aren’t sensitive enough to “see” this
behaviour.
Closed loop operation does not
occur in the following driving conditions: (1) during throttle lift-off; (2)
when the engine is in warm-up mode;
and (3) at wide throttle openings.
At these times, the ECU ignores the
output of the oxygen sensor, instead
picking the injector pulse widths
solely on the basis of the data maps
programmed into it.
When the throttle is opened wide,
the air/fuel ratio becomes richer, holding at that level. For example, the green
LED second from the end may light and
stay on. If you accelerate even harder,
then the very end green LED may light.
On the other hand, back right off and
it’s likely that all the LEDs will go out.
That’s because the injectors have been
switched off on the over-run and the
air/fuel ratio is so lean that it’s off the
scale. Watching the behaviour of a LED
mixture meter really is a fascinating
window into how an ECU is operating!
The mixture meter is also a vital
tool when undertaking engine modifications. For example, if a particular
LED lights at full throttle before and
after making engine modifications
(eg, to increase power), then you can
be fairly confident that the mixtures
haven’t radically changed (under the
same conditions, that is). Conversely,
if the lit LED shifts two along after the
modifications have been done, you
can be fairly sure that the mixtures
are different!
A word of warning though – the
Smart Mixture Display shouldn’t be
relied on when making major engine
modifications and/or working on expensive cars,
In summary, fitting the Smart Mixture Display to your car has three
major benefits – you can roughly track
your mixtures in real time, you can
see the operating modes of the ECU
and you can be warned if there is an
unexpected catastrophic high-load
lean out. Sounds good to us!
How it works
OK, let’s take a look at the circuit
details – see Fig.1. IC1 is an LM3914
dot/bar display driver. In dot mode, it
drives the LEDs so that as the voltage
at its pin 5 input increases, it progressively turns on higher LEDs. For example, at the lowest input voltage, LED1
is lit. At midrange voltages, LED4 or
LED5 might be lit and at the highest
input voltage, LED10 will be lit.
Trimpots VR1 and VR2 set the
voltage range for the LED display.
Normally, VR2 is set so that its wiper
is at ground and VR1 is set so that its
wiper is at 1V. Thus, the LED display
covers a 0-1V range which is the normal output variation of an automotive
oxygen sensor.
The LED brightness is set by the total
resistance from pin 7 to ground and we
vary this to dim the LEDs in darkness.
In bright light, the Light Dependent
Resistor (LDR1) is a low resistance
and this provides current to the base
of transistor Q1 which switches it on
to set the LED brightness at maximum.
Conversely, in darkness, LDR1 is a
high resistance and so transistor Q1
is off. This sets the LED brightness to
minimum.
Trimpot VR3 adjusts the dimming
threshold. If it’s set fully clockwise
(ie, to minimum resistance), the LEDs
will be dimmed at a relatively high
ambient light level. As VR3’s wiper
is rotated anticlockwise, the dimming
begins at progressively lower ambient
light levels until eventually, the LEDs
are at maximum brightness in normal
daylight.
Op amps IC2a and IC2b are used as
comparators to monitor the load and
oxygen sensor signals respectively.
As shown in Fig.1, IC2b monitors
the oxygen sensor signal at its noninverting input (pin 5), while VR4 and
its associated 10kΩ series resistor set
the voltage at the inverting input (pin
6). If the oxygen sensor signal level
is below the voltage on the inverting
input, then IC2b’s output (pin 7) goes
low and lights LED11.
Comparator IC2a operates in reverse
fashion. It monitors the load signal at
its inverting input (pin 2), while VR5’s
wiper sets the threshold for the noninverting input (pin 3). If the load voltage is above the level set by VR5, pin
1 of IC2a goes low and LED12 lights.
When the outputs of IC2a and IC2b
are both low, transistor Q2 is switched
on due to the base current through
5.6V zener diode ZD4 and the 2.2kΩ
resistor to ground. Q2 then drives the
piezo buzzer.
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Fig.2: this diagram
shows where each
of the components is
placed on the main
PC board. Use this
diagram, the photos
of the completed
board and the parts
list to help you
assemble it correctly.
Now consider what happens if one
of IC2’s outputs goes high – ie, if the
oxygen sensor signal goes above VR4’s
wiper or if the load input signal goes
below the VR5’s wiper. In that case,
ZD4’s anode is pulled high via either
diode D2 or D3 (depending on which
op amp output is high). This causes
transistor Q2 to turn off and so the
alarm stops sounding.
This means that the outputs of IC2a
& IC2b must both be low for Q2 to
switch on and sound the alarm.
Note the 1MΩ input resistors in
series with the oxygen sensor and
load inputs. These prevent loading of
the circuits they are connected to and
ensure that the car’s ECU operation is
not affected in any way by the addition of the Smart Mixture Display. The
associated 10nF capacitors to ground
are included to filter voltage transients
on the inputs.
Power for the circuit is derived from
the vehicle’s +12V ignition supply.
Diode D1 prevents damage if the battery supply connections are reversed,
while the 10Ω resistor and 470µF
capacitor provide decoupling and
filtering. As a further precaution, 16V
zener diode ZD1 is included to prevent
voltage spikes from damaging the ICs.
Construction
The Smart Mixture Meter is straightforward to build, with all the parts installed on a PC board coded 05104041.
Fig.2 shows the assembly details.
Begin the assembly by installing the
wire links and resistors first. Table 1
shows the resistor colour codes but
it’s advisable to check each one with a
digital multimeter as well, as some of
the colours can be difficult to decipher.
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The assembled PC board should look like this! Make sure that you observe the
orientation of the 12 LEDs, two ICs, seven diodes and the electrolytic capacitor.
Our prototype has rectangular LEDs for the mixture display but round ones are
generally easier to mount in a panel. They can also be mounted remotely from
the PC board to make it easier to package the meter in your car. Note that the
LDR must be able to see ambient light, otherwise it won’t work!
The diodes, capacitors and trimpots
can go in next, along with the two ICs.
Follow these with the two terminal
blocks and the piezo buzzer. Make
sure that you install the polarised
components the correct way around.
These parts include the diodes, ICs,
transistors, piezo buzzer and the 470µF
electrolytic capacitor. Follow the overlay diagram and the photo closely to
avoid making mistakes.
Finally, install the LDR and the
LEDs. The LDR can go in either way,
but the 10 bargraph LEDs must all be
installed with their anodes (the longer
of the two leads) to the left. LEDs 11
& 12 are installed with their anodes
towards the top – see Fig.2.
Note that you can use high intensity
LEDs if you want but because these
are more directional, they may in fact
not be any easier to see than normal
LEDs. You may also used round or
rectangular LEDs – the choice is yours.
We used rectangular LEDs in our
prototype for the 10-LED mixture
display and these were installed with
their leads bent through 90°, so that
they were in line with the edge of the
PC board – see photo. Alternatively,
April 2004 63
Air/Fuel Ratio Measurement & Oxygen Sensors
TOPIC OF measuring the voltage
TtheHEoutput
of an oxygen sensor to quantify
air/fuel ratio is surrounded by misin-
formation. This is especially the case when
people are attempting to perform critical
tuning of modified engines while working
within a budget that calls for the use of a
low cost sensor.
Most exhaust gas oxygen sensors have
an output voltage of approximately 0–1V,
depending on the mixture strength (or
air-fuel ratio). In most cars, the oxygen
sensor is used in a closed loop process
to maintain an air/fuel ratio of about
14.7:1 (“stoichiometric”) during idle, light
load and cruise conditions. In this way,
emissions are reduced and the catalytic
converter works most effectively.
However, this project attempts to quan-
Fig.3: the output voltage from an
oxygen sensor changes rapidly as
the air/fuel ratio passes through
14.7:1. The degree to which the
response curve flattens on either side
of this ratio determines how useful
the sensor is at measuring mixture
strengths away from 14.7:1. [Ford]
Fig.4: the operating temperature
dramatically affects the output of
an oxygen sensor. Sensors mounted
close to the engine are particularly
affected by temperature variations.
[Bosch]
64 Silicon Chip
tify air/fuel ratios on the basis of the sensor
output, which can be well away from the
stoichiometric point. Commercially available air/fuel ratio meters utilising oxygen
sensors - now widely used in automotive
workshops – do the same thing. However,
they use what are known as “wide-band”
sensors, as opposed to the “narrow-band”
sensors used in nearly all cars.
So what are the performance differences when it comes to wide-band sensors
and can narrow-band sensors still be used
to provide useful information?
The most common type of oxygen sensor is the zirconium dioxide design. In this
sensor, part of the ceramic body is located
such that exhaust gases impinge on it. The
other part is located so that it has access to
the atmosphere. The surface of the ceramic
body is provided with electrodes made of
a thin, gas-permeable layer of platinum.
Above about 350°C, the ceramic material begins to conduct oxygen ions. If the
proportions of oxygen at the two ends of
the sensor differ, a voltage proportional to
the difference in the oxygen concentrations
is generated. The residual exhaust gas
oxygen component is largely dependent on
the engine’s instantaneous air/fuel ratio –
thus the output voltage of the sensor can
be correlated with the air/fuel ratio.
Fig.3 shows the typical output characteristic of a zirconia oxygen sensor. As can
be seen, the output voltage varies rapidly
either side of the 14.7:1 stoichiometric
point. This is the characteristic curve
output of a narrow-band oxygen sensor,
as used in most cars. What is generally
not realised is that a so-called wide-band
sensor also has a very similar output, with
just a little more linearity in its response at
both ends of the air/fuel ratio scale!
In addition to the air/fuel ratio, the
output voltage of a sensor is heavily
dependent on its temperature. At very
low temperatures – below about 350°C
– the ceramic material is insufficiently
conductive to allow the sensor to function
correctly. As a result, the output signal of
a “cold” sensor will be either non-existent
or incorrectly low in voltage (note: the
minimum operating temperature varies a
little from sensor to sensor).
To overcome this problem, a resistive
heating element is often placed inside the
sensor to quickly bring it up to minimum
operating temperature. Once this occurs,
the heater is the usually switched off, with
the flow of exhaust gases then responsible
for heating the sensor.
The temperature of the sensor has a
major bearing on the output voltage, even
in the normal working range of 500-900°C.
Fig.4 shows the change in output voltage
characteristics of a sensor when it is at
550°C, 750°C and 900°C. (Note that here
the air/fuel ratio is expressed as Lambda
numbers – Lambda 0.75 is an air/fuel
ratio of 11:1).
As can be seen, temperature variations
can cause the output signal to vary by as
much as one third of the full scale! It is also
important to note that as the temperature
of the sensor increases, its reading for the
same air/fuel ratio decreases. Specifically,
one tested sensor had an output of 860mV
at 900°C, which corresponds to an air/fuel
ratio of 11:1 (which is very rich). The same
output voltage at 650°C would indicate
an air/fuel ratio of 14:1 (ie, much leaner).
The temperature of the sensor also has
a major effect on its response time. The
response time for a voltage change due to
a change in mixture can be seconds when
the sensor is below 350°C, or as short as
50ms when the sensor is at 600°C.
These temperature-dependent variations occur in all zirconia-based oxygen
sensors – wide-band and narrow-band.
So where does this leave us when we
want to source a cheap sensor for use in
measuring air/fuel ratios during tuning?
First, an oxygen sensor which still has a
variation in output well away from stoichiometric is required. Once that sensor
is found, its temperature should be kept as
stable as possible, while being maintained
above 350°C during the testing.
As part of a general research project
into the characteristics of common oxygen sensors, mechanic Graham Pring (a
modification enthusiast) and the author
(Julian Edgar) conducted an extensive
series of tests on professional air/fuel ratio
meters and sensors, both (supposedly)
wide-band and narrow-band. We found
that there were major variations between
the readings of professional air/fuel ratio
meters and that the use of a slightly used
sensor could make a dramatic difference
to the reading.
In short, when using zirconia oxygen
sensors away from stoichiometric ratios,
the professional meters were often not
accurate to even one full ratio, let alone
the one-tenth of a ratio shown on the
digital displays.
The best low-cost probe that we found
was the heated NTK-manufactured Ford
E7TF 9F472 DA sensor, which gave excelwww.siliconchip.com.au
Parts List
1 PC board, code 05104041,
121 x 59mm
1 plastic case, 130 x 68 x 42mm
2 PC mount 2-way screw terminals with 5mm pin spacing
1 12V piezo alarm siren with
7.6mm pin spacing
1 Light Dependent Resistor
((Jaycar RD3480 or equiv.)
(LDR1)
1 100mm length of 0.8mm tinned
copper wire
Fig.5: this diagram shows the relationship between the air/fuel ratio and the
voltage output at different exhaust gas temperatures for the heated Ford E7TF
9F472 DA oxygen sensor (the best low-cost sensor we have found). This sensor
is sufficiently wide-band that it can be used in conjunction with a digital
multimeter to give a more accurate indication of mixture strength than is
achievable with the 10-LED meter.
lent results, even when compared with a
new Bosch wide-band sensor. The E7TF
9F472 DA is the standard sensor from
the Australian Ford Falcon EA, EB and
ED models.
To gain the best results from this sensor,
it should be mounted at the tailpipe with its
12V heater active. Any testing should be
consistent in approach so that the actual
temperature of the sensor (due to both
the internal heater and the exhaust gas)
remains similar during each procedure. For
example, the same warm-up and engine
loading sequence should be undertaken for
each test. By using the Ford sensor in this
way, results are sufficiently accurate and
a fast-response multimeter can be used
to monitor the sensor output. However,
realistically, an air/fuel ratio accuracy of
only about 1-1.5 can be expected.
With this warning kept in mind, Fig.5
gives an indication of the response curves
of the Ford sensor, measured at three different exhaust gas temperature ranges:
250–300°C, 300-450°C and 450–650°C.
However, tapping into the car’s standard oxygen sensor and using the 10-LED
Smart Mixture Display as described in the
main text will still give data that is very
useful. In fact, the lack of a digital readout
is actually an advantage, as it stops people
putting too much faith in numbers which
in all likelihood are not accurate to even
a full ratio.
The temperature of
the exhaust reduces
as it gets further
from the engine.
As this computer
simulation shows,
by the time it
reaches the tailpipe it is typically
only at about 200°C
whereas close to the
exhaust valves, the
gas temperatures
can be over 800°C!
[Network Analysis]
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Semiconductors
1 LM3914 display driver (IC1)
1 LM358 dual op amp (IC2)
2 BC327 PNP transistors
(Q1, Q2)
3 16V 1W zener diodes (ZD1ZD3)
1 5.6V 400mW zener diode (ZD4)
1 1N4004 1A diode (D1)
2 1N914 diodes (D2,D3)
4 5mm red LEDs (LED9-12)
2 5mm yellow LEDs (LED1,2)
6 5mm green LEDs (LED3-8)
Capacitors
1 470µF 16V PC electrolytic
2 10nF (.01µF) MKT polyester
Trimpots
1 200kΩ horizontal trimpot (VR3)
2 100kΩ horizontal trimpots
(VR4,VR5)
2 5kΩ horizontal trimpot
(VR1,VR2)
Resistors (0.25W, 1%)
2 1MΩ
3 2.2kΩ
1 220kΩ
2 680Ω
4 10kΩ
1 10Ω
you can mount the LEDs vertically so
that they later protrude through a slot
(or a row of holes in the case of round
LEDs) in the lid of the case. Another
alternative is to use round LEDs which
are mounted remotely from the board,
to mimic the response curve of the
oxygen sensor – see photo.
Installing it in a case
It’s up to you what type of case you
mount the PC board assembly in. As
it stands, the board is designed to clip
into a standard plastic case measuring
130 x 68 x 43mm. Note that if your car
is very noisy, you may want to mount
the piezo buzzer external to the box
– or even fit a louder one. The buzzer
April 2004 65
up) and that the signal coming from the
airflow meter rises when the throttle
is blipped.
Note that the 0V connection for the
Smart Mixture Meter should be made
at the ECU.
Setting up
The step-by-step setting up procedure is as follows:
(1). Make sure that the “High” trimpot (VR1) is set fully clockwise and
that the “Low” trimpot (VR2) is fully
anticlockwise.
(2). Start the car, let the oxygen sensor warm up and confirm that the LED
display shows one illuminated LED. It
will probably move around, perhaps
quite quickly.
(3). Go for a drive and briefly use full
throttle. The end yellow LED should
light up. Back off sharply – the end red
LED should light and then the display
should blank for a moment before
resuming normal operation (ie, the
over-run injector shut-off is visible).
(4). Check that the illuminated LED
travels back and forth when the engine
is at idle (ie, the engine is in closed
loop mode).
Cars like this Ford XR6 Turbo are especially vulnerable to engine damage if
the mixtures go lean under load. The Smart Mixture Meter sounds an alarm
the instant there is a high-load lean-out, allowing the driver to back off.
can draw up to 60mA without causing
any problems to the circuit.
Fitting
You will need to make four wiring
connections to your car. It’s easiest to
do that at the ECU, so you will need
to have a wiring diagram showing
the ECU pin-outs. The four connections are:
(1). +12V ignition switched;
(2). chassis (0V);
(3) oxygen sensor signal; and
(4) airflow meter signal.
Use the car’s wiring diagram to
find these connections and then use
your multimeter to check that they’re
correct. For example, when you find
the +12V supply, make sure that it
switches off when you turn off the ignition. In addition, you have to confirm
that there is a fluctuating signal in the
0-1V range on the oxygen sensor lead
(the car will need to be fully warmed
Adjusting the display to suit
your oxygen sensor
(1). If the end yellow LED never
lights, even at full throttle, adjust VR1
so that it lights when the mixtures are
fully rich.
(2). In closed loop, the moving LED
should move back and forth around
the centre LED. If the oscillations are
all down one end after adjusting VR1,
adjust the “Low” pot (VR2) again to
centre the display.
Adjusting the Lean Alarm
(1). Adjust the Load Threshold pot
(VR5) until LED12 comes on at reasonably heavy loads. For example, in a
turbo car, the pot should be set so that
LED12 first lights when there’s a little
boost showing on the gauge.
Fig.6: this is the full-size etching pattern for the PC board.
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
No.
2
1
4
3
1
1
66 Silicon Chip
Value
1MΩ
220kΩ
10kΩ
2.2kΩ
680Ω
10Ω
4-Band Code (1%)
brown black green brown
red red yellow brown
brown black orange brown
red red red brown
blue grey brown brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
red red black orange brown
brown black black red brown
red red black brown brown
blue grey black black brown
brown black black gold brown
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Silicon Chip
Binders
REAL
VALUE
AT
$14.95
PLUS P
&
P
In this installation, round LEDs have been used for the mixture display, mounted
remotely from the PC board. Note how the owner has chosen to arrange the
LEDs to mimic the response curve of the sensor. This is a great approach if there
is sufficient room available. [Michael Knowling]
(2). Adjust the Oxygen Level Threshold pot (VR4) until LED11 comes on
for what would be regarded as a lean
condition at the above load; eg, so that
LED11 lights when the unit is showing
the last green LED (LED8) before the
red (LED9).
(3). When LEDs 11 & 8 come on together, the alarm sounds. If this occurs
when there’s no obvious problem, adjust VR4 until the alarm just no longer
sounds when running high loads.
Adjusting the dimmer
(1). Turn the dimmer sensitivity
pot (VR3) until the display dimming
matches your preferences – clockwise
will give a brighter display at night (so
you need to cover the LDR to simulate
SC
night when you’re setting it!).
Lambda vs
Air/Fuel Ratio
The ratio of the mass of air to the
mass of fuel is the most common
method of describing the mixture
strength. So an air/fuel ratio of 13:1
means that there is a mass of 13kg
of air mixed with 1kg of fuel.
However, sometimes mixture
strength is quoted as a Lambda
(or excess air) value (λ). This is
defined as the air/fuel ratio divided
by the stoichiometric ratio (ie, on
typical road fuels, 14.7:1). So an
air/fuel ratio of 12:1 (rich) is 0.82
Lambda (12/14.7 = 0.82).
Uhh, Ohhhh – Check Your Car First!
In some cars, this meter simply
won’t work and there can be several
reasons for this.
First, it needs an oxygen sensor
that outputs a voltage between 0-1V,
with higher voltages corresponding
to richer mixtures. The vast majority
of cars produced over the last 15
years use this type of sensor but there
are exceptions, so be sure to use
your multimeter to check the oxygen
sensor output signal before buying
a kit.
Second, the car must use an
airflow meter with an output voltage
www.siliconchip.com.au
varying between about 1-5V, with
the higher voltages corresponding to
higher engine loads. However, some
airflow meters use a frequency output
signal and this circuit won’t work with
that type of design. Also, in non-turbo
cars using a MAP sensor, the sensor
voltage will go high whenever the
throttle is snapped open. This may
cause false alarms, as the air/fuel
ratio won’t immediately go rich.
By contrast, this design should be
fine in turbo cars using a MAP sensor.
Again, check the output of the load
sensor with a multimeter first.
These binders will protect your
copies of S ILICON CHIP. They
feature heavy-board covers & are
made from a dis
tinctive 2-tone
green vinyl. They hold 12 issues &
will look great on your bookshelf.
H 80mm internal width
H SILICON CHIP logo printed in
gold-coloured lettering on spine
& cover
H Buy five and get them postage
free!
Price: $A14.95 plus $A10.00 p&p
per order. Available only in Aust.
Silicon Chip Publications
PO Box 139
Collaroy Beach 2097
Or call (02) 9939 3295; or fax (02)
9939 2648 & quote your credit
card number.
Use this handy form
Enclosed is my cheque/money order for
$________ or please debit my
Visa Mastercard
Card No:
_________________________________
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Name ____________________________
Address__________________________
__________________ P/code_______
April 2004 67
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