This is only a preview of the Performance Electronics for Cars issue of Silicon Chip. You can view 38 of the 160 pages in the full issue, including the advertisments. For full access, purchase the issue for $20.00. Items relevant to "Smart Mixture Meter":
Items relevant to "Duty Cycle Meter":
Items relevant to "High Temperature Digital Thermometer":
Items relevant to "Versatile Auto Timer":
Items relevant to "Simple Voltage Switch":
Items relevant to "Temperature Switch":
Items relevant to "Frequency Switch":
Items relevant to "Delta Throttle Timer":
Items relevant to "Digital Pulse Adjuster":
Items relevant to "LCD Hand Controller":
Items relevant to "Peak-Hold Injector Adaptor":
Items relevant to "Digital Fuel Adjuster":
Items relevant to "Speedo Corrector":
Items relevant to "Independent Electronic Boost Controller":
Items relevant to "Nitrous Fuel Controller":
Items relevant to "Intelligent Turbo Timer":
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Chapter 8
Below: all the parts for the Smart
Mixture Meter are mounted on a
small PC board. This prototype uses
rectangular LEDs for the mixture
display but you can use round LEDs
if you prefer. The LEDs can also
be mounted remotely from the PC
board (see photo at right).
Above: in this installation, round LEDs
have been used for the display and
these have been mounted on the dashboard to mimic the response curve
of the oxygen sensor. This is a great
approach if there is sufficient room
available. [Michael Knowling]
Smart Mixture Meter
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.
T
HIS SMART MIXTURE METER
monitors your car’s oxygen sensor and air-flow meter outputs and
sounds a buzzer to warn if mixtures
go dangerously lean. It also uses 10
coloured LEDs to indicate the air/fuel
ratio while you drive – red for lean (red
is for danger!), green for mid-range
mixtures and yellow for rich.
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
42
PERFORMANCE ELECTRONICS FOR CARS
ECU is operating in closed loop or open
loop mode (more on this later).
An automatic dimming function
has been built into the unit so that the
10 mixture indicator LEDs are not too
bright at night. In addition, the unit is
very easy to build and set up.
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
the load sensor – usually the air-flow
meter. Most air-flow meters have an
analog output voltage that rises with
siliconchip.com.au
Fig.1: follow this parts layout diagram and the photo below to build the Smart Mixture Meter. Many of the
parts are polarised, so be sure to install them with the correct orientation. These parts include the piezo
buzzer, ICs, transistors, diodes (including zener diodes), LEDs and the electrolytic capacitor.
You can use round LEDs (instead of rectangular) for the mixture display if you wish but make sure they are
all orientated correctly. It’s easy to identify their leads – the anode lead will be the longer of the two. Note
that the LDR must be exposed to ambient light, otherwise the automatic display dimming function won’t work.
engine load, being around 1V under
light loads (eg, at idle) and close to 5V
under high loads. If the output voltage from the air-flow meter is high,
the meter knows that the engine load
must also be high.
LED will be on, at 0.2V the next red
LED will light up and so on. Of course,
this doesn’t give a precise indication of
air/fuel ratio (see the “Air/Fuel Ratio
Measurement and Oxygen Sensors”
LED Indicators
But what about the main section of
the Smart Mixture Meter – the LEDs?
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 (0.1V) coming from the
sensor, so at 0.1V the far lefthand red
siliconchip.com.au
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
RESISTOR COLOUR CODES
Value
4-Band Code (1%)
5-Band Code (1%)
1MΩ
220kΩ
10kΩ
2.2kΩ
680Ω
10Ω
brown black green brown
red red yellow brown
brown black orange brown
red red red brown
blue grey brown brown
brown black black brown
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
PERFORMANCE ELECTRONICS FOR CARS
43
How It Works
Fig.2 shows the circuit details for
the Smart Mixture Meter. 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 mid-range 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 between pin 7 of IC1
The exhaust gas oxygen sensor delivers
a mixture strength signal that 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]
output voltage is low (ie, there is a
lean mixture) at the same time as the
air-flow meter output is high (ie, a
high engine load), the on-board 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 LEDs as you drive along.
Dancing? Won’t the illuminated
LED stay constant if the air/fuel ratio
isn’t changing?
44
PERFORMANCE ELECTRONICS FOR CARS
and ground and this is varied to dim the
LEDs in darkness. In bright light, LDR1
(a light dependent resistor) has a low
resistance and so the base of transistor
Q1 is pulled low. As a result, Q1 turns
on and the LEDs operate at maximum
brightness.
Conversely, in darkness, LDR1 has a
high resistance and so 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. Rotating VR3’s wiper
anticlockwise brings the LEDs up to
full brightness in normal daylight, with
dimming occurring at progressively
lower ambient light levels.
Comparators
Op amps IC2a and IC2b are used as
comparators which monitor the load
and oxygen sensor signals respectively.
As shown in Fig.2, IC2b monitors the
oxygen sensor signal at its non-inverting
input (pin 5), while VR4 and its associated 10kΩ series resistor set the
threshold 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 non-inverting
input (pin 3). If the load voltage is above
the level set by VR5, pin 1 of IC2a goes
low and LED12 lights.
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,
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.
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
comparator 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 Meter. The
associated 10nF capacitors to ground
are included to filter voltage transients
on the inputs.
Power Supply
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.
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 loop
operation, they quickly decide that the
meter is useless. After all, the indication is “all over the place”! However,
siliconchip.com.au
Fig.2: the circuit is based on an LM3914 dot/bar display driver IC. This accepts the signal from the oxygen sensor and directly drives
the 10-LED display. Op amps IC2a & IC2b, together with transistor Q2 and the piezo buzzer, provide the “lean-out” alarm feature.
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 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 setting the injector
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pulse widths solely on the basis of the
data maps programmed into it.
When the throttle is opened, the air/
fuel ratio becomes richer, holding at
that level. For example, the green LED
second from the end (LED7) may light
and stay on. If you accelerate even
harder, then the very end green LED
(LED8) 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!
Engine Modifications
The Smart 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
PERFORMANCE ELECTRONICS FOR CARS
45
Air/Fuel Ratio Measurement & Oxygen Sensors
The topic of measuring the voltage
output of an oxygen sensor to quantify
the air/fuel ratio is surrounded by misinformation. 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
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 for measuring mixture
strengths away from 14.7:1.
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]
46
PERFORMANCE ELECTRONICS FOR CARS
quantify 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. The reason for this is that 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 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 its
minimum operating temperature. Once
this occurs, the heater is then 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 narrowband.
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 oxy-
siliconchip.com.au
Parts List
1.2
1.0
OUTPUT VOLTAGE (V)
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 lowcost sensor we have
found).
0.8
0.6
0.4
0.2
0
10
11
gen 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 even 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 excellent 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
12
13
14
AIR/FUEL RATIO
15
16
17
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
Smart Mixture Meter 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.
Fig.6: the exhaust
gas temperature
reduces as it gets
further from the
engine, as this
computer simulation
shows. By the time it
reaches the tail-pipe,
it is typically at about
200°C, whereas
close to the exhaust
valves, the gas
temperatures can be
over 800°C! [Network
Analysis]
siliconchip.com.au
1 PC board coded 05car011 or
05104041, 121 x 59mm
1 plastic case, 130 x 68 x 42mm
(optional, not included in kit)
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 equivalent) (LDR1)
1 100mm length of 0.8mm tinned
copper wire
Semiconductors
1 LM3914 display driver (IC1)
1 LM358 dual op amp (IC2)
2 BC327 PNP transistors
(Q1, Q2)
3 16V 1W zener diodes (ZD1-ZD3)
1 5.6V 400mW zener diode (ZD4)
1 1N4004 1A diode (D1)
2 1N914 diodes (D2,D3)
2 5mm yellow LEDs (LED1,2)
6 5mm green LEDs (LED3-8)
4 5mm red LEDs (LED9-12)
Capacitors
1 470µF 16V PC electrolytic
2 10nF MKT polyester (code 103
or 10n)
Trimpots
1 200kΩ horizontal trimpot (VR3)
2 100kΩ horizontal trimpots
(VR4,VR5)
2 5kΩ horizontal trimpot (VR1,VR2)
Resistors (0.25W, 1%)
2 1MΩ
1 220kΩ
4 10kΩ
3 2.2kΩ
2 680Ω
1 10Ω
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
PERFORMANCE ELECTRONICS FOR CARS
47
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,
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.
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
can draw up to 60mA without causing
any problems to the circuit.
Fitting
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]
and you can be warned if there is an
unexpected catastrophic high-load
lean out. Sounds good to us!
Construction
The unit is straightforward to
build, with all the parts installed on
a PC board coded either 05car011 or
05104041. Fig.1 shows the assembly
details.
Begin by installing the wire links
and resistors. The accompanying table
shows the resistor colour codes but
it’s also advisable to check them with
a digital multimeter, as some colours
can be difficult to decipher.
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.1.
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 use round or rectangular LEDs – the choice is yours.
We used rectangular LEDs in our
prototype for the 10-LED mixture
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).
48
PERFORMANCE ELECTRONICS FOR CARS
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) air-flow 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
up) and that the signal coming from
the air-flow 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.
siliconchip.com.au
Engines with turbocharging are especially vulnerable to 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.
(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).
Adjusting For The O2 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 mode, 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) 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 boost
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starts showing on the gauge.
(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 (LED3) before the
red (LED2).
(3). When LEDs 11 and 3 come on
together, the alarm sounds. If this
occurs when there’s no obvious prob-
lem, 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
night when you’re setting it!).
Uhh, Ohhh – Check Your Car First
In some cars, this Smart Mixture
Meter simply won’t work and there can
be several reasons for this.
First, it needs an oxygen sensor that
outputs a signal voltage from 0–1V, with
the 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 also use an
air-flow meter which has an output
signal varying from about 1–5V, with
the higher voltages corresponding to
higher engine loads. However, some airflow meters have a variable-frequency
output signal and the Smart Mixture
Meter won’t work with that type of
air-flow meter. 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.
PERFORMANCE ELECTRONICS FOR CARS
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