This is only a preview of the November 2008 issue of Silicon Chip. You can view 29 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "12V Speed Controller/Lamp Dimmer":
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Wideband Air-Fuel
Mixture Display
By JOHN CLARKE
Monitor your car’s air/fuel ratio as you drive
This Wideband Oxygen Sensor Display can show your car’s airfuel ratio as you drive. It’s designed to monitor a wideband oxygen
sensor and its associated wideband controller but could be used to
monitor a narrowband oxygen sensor instead. Alternatively, it can
be used for monitoring other types of engine sensors.
W
HY WOULD YOU want to
monitor the air/fuel ratio as you
drive? Well, for starters, it will allow
you to save fuel since the display
clearly indicates when the engine is
running rich.
When used in conjunction with a
wideband oxygen sensor and controller, the air/fuel ratios shown on this
unit are more accurate than can be
obtained from the narrowband sensors that are typically used in cars and
which are really only accurate close to
the “stoichiometric” point (ie, the air/
fuel ratio at which there is just enough
oxygen in the air to ensure complete
combustion).
Under normal driving, most engine
management systems operate under
“closed loop” control. This is where
the air/fuel ratio from an oxygen sensor
58 Silicon Chip
is monitored and controlled by the car’s
engine control unit (ECU) to maintain
a predetermined fuel mixture. This is
usually stoichiometric but under light
cruise conditions the mixture can go
lean to improve fuel economy.
Conversely, during acceleration,
the air/fuel mixture in many cars is
allowed to go rich to improve performance and is not under the control of
the ECU. This is called “open loop”
and the richness of the mixture depends on other factors such as the
throttle setting and the injector opening period.
By monitoring the air/fuel mixture display as you drive, you will
quickly learn how to obtain the best
economy. When climbing a hill, for
example when the car would normally
be running rich, you can ease off on
the throttle just enough to return the
ECU to closed-loop control and run at
stoichiometric mixtures to reduce the
amount of fuel used.
In addition, when gear changes are
required, you may find that changing
earlier or later than normal will keep
the engine running leaner for longer.
Similarly, when travelling downhill
without throttle, most cars shut off the
injectors above a certain RPM limit, so
that no fuel is used at all. When this
happens, the display will show a very
lean air/fuel ratio.
Note, however, that the injectors
are usually partially open below
this RPM limit, to ensure a smooth
engine response when the throttle is
opened. This means that when travelling downhill, it may be better to
drop down a gear to ensure complete
siliconchip.com.au
injector shut-off (and thus reduced
fuel usage), rather than stay in a higher
gear with the injectors slightly open.
Diagnosing problems
Once you’ve used this unit for
awhile, you will soon learn what sort
of readings to expect in every-day driving. Any subsequent variations from
“normal” can then be interpreted as indicating a problem. For example, there
could be a fault with the oxygen sensor,
the wideband controller or the engine
management unit. A problem with fuel
delivery is another possibility.
Oxygen sensors do wear out eventually, due to an accumulation of
contaminants on the sensor tip. As a
result, car manufacturers recommend
that they be replaced after a specified number of kilometres (typically
around 100,000km for a heated sensor type). A worn-out oxygen sensor
becomes sluggish in its response and
causes a number of problems including excessive fuel consumption, poor
engine performance, accelerated catalytic converter damage and increased
emissions.
By monitoring your car’s air/fuel
ratio as you drive, you can quickly
discover abnormal operating conditions and have the sensor checked and,
if necessary, replaced.
Engine modifications
This unit will also be invaluable if
you are a car modification enthusiast.
It will soon show whether or not the
mixture is too lean during acceleration
or too rich under cruise conditions
and allow you to make adjustments
accordingly.
This can be particularly handy if
you are swapping the ECU chip for
an aftermarket type or if you are experimenting with the fuel injectors.
It’s all too easy to damage an engine if
the mixture is too lean under certain
circumstances.
Oxygen sensor types
In order to monitor the air/fuel ratio, the vehicle must be fitted with an
oxygen sensor. These are fitted to all
vehicles that have fuel injection and
engine management, although most
cars use what is known as a “narrowband” oxygen sensor.
For a detailed explanation on how
oxygen sensors work and a description
of the two basic types, refer to the article “Narrowband & Wideband Oxygen
siliconchip.com.au
Main Features & Specifications
MAIN FEATURES
•
•
•
•
•
•
•
•
•
3-digit LED display plus 7-segment bargraph.
Linear display with 0-5V wideband range or 0-1V S-curve range.
Alternative display switching (A or B readings for wideband values); petrol
or LPG readings for narrowband S-curve.
0V and 5V endpoint value limit adjustments for both A and B displays.
Decimal point positioning.
Display leading zero suppression.
Bargraph can be operated in dot, bar or centred-bar mode for wideband
range. S-curve set-up allows for dot or centred bar styles.
Display dimming with minimum brightness and dimming threshold
adjustments.
Quieting period used for input measurement to ensure accuracy.
SPECIFICATIONS
Power Supply: 6-15V <at> 240mA (full display brightness)
Input Current Loading: less than ±1mA
Digit Update Period: 250ms
Bargraph Update Period: 30ms
Wideband Display Reading Range: 0-999
Narrowband Display Reading Range: 11.8 to 20.6 for unleaded petrol with
the stoichiometric ratio set for 14.7; 12.6 to 21.4 for LPG with stoichiometric
at 15.5. The display shows an “L” for ratios below the lowest value and an “r”
for ratios above the highest value.
Sensors” on page 27 of this issue.
In practice, the oxygen sensor is located in the exhaust system to monitor
the exhaust gas after the fuel has been
burnt in the engine. Basically, the fuel
is mixed with air inside each cylinder
prior to firing. This air/fuel ratio is
varied under the control of the ECU
in order to obtain the desired engine
(and emissions) performance.
Under light engine-load conditions,
the engine is usually run with exactly
the correct proportion of fuel and air
to ensure complete combustion. When
this happens, the air/fuel ratio is said
to be “stoichiometric” and this ratio
is typically 14.7 for unleaded petrol.
Putting it another way, 14.7kg of air is
mixed with each 1kg of the unleaded
fuel to achieve the stoichiometric ratio.
Note, however, that the stoichiometric ratio is different for different fuels
because it depends on the chemical
composition of the fuel and its combustion characteristics. For liquid petroleum gas (LPG), the stoichiometric
+12V
S-CURVE OUTPUT
(SIMULATED
NARROW-BAND
SENSOR SIGNAL)
Rcal
Ip
Vs/Ip
Vs
H–
H+
+12V
WIDEBAND
CONTROLLER
0–5V AIR/FUEL
RATIO SIGNAL INPUT
8.8.8.
WIDEBAND
DISPLAY
WIDEBAND
SENSOR
Fig.1: here’s how the display unit is used with a wideband sensor and its
associated controller. The narrowband S-curve output from the controller is
fed to the engine management computer (see text).
November 2008 59
NARROWBAND
SENSOR
+12V
+12V
+12V
INPUT
NARROWBAND
S-CURVE OUTPUT
HEATER
8.8.8.
WIDEBAND
DISPLAY
SET FOR
S-CURVE
RESISTIVE
SENSOR
Fig.3: here’s how to use the display
unit with a resistive sensor (eg, a
temperature gauge).
Fig.2: the original narrowband sensor fitted to the car can
be used to directly drive the display unit if accuracy isn’t
important. The display must be set to run in S-curve mode.
tion the “lambda” (λ) value and it has
a value of “1” at the stoichiometric
point.
Basically, the Lambda value is simply the actual air/fuel ratio divided by
the stoichiometric ratio. This means
that lean air/fuel ratios have a lambda
greater than 1, while rich air/fuel ratios
have a lambda that’s less than 1.
In practice, air/fuel ratios are a
compromise between driveability,
engine power and the production of
air pollutants. Air pollutants are also
reduced using a catalytic converter.
This converts nitrous oxides to nitrogen and oxygen, carbon monoxide
(CO) to carbon dioxide (CO2) and the
unburnt hydrocarbons into carbon
dioxide and water.
LED1
A
BAR
LED7
NON INVERTED:
INVERTED:
0V
5V
2.5V
2.5V
5V
0V
0V
5V
2.5V
2.5V
5V
0V
LED1
B
CENTRED
BAR LED7
NON INVERTED:
INVERTED:
C
DOT
Oxygen sensor display unit
LED1
As shown in the photos, the SILICHIP Oxygen Sensor Display unit
is housed in a small plastic case. It
features a 3-digit LED display to show
the air/fuel ratio plus a 7-segment bargraph which indicates the signal trend.
Just three leads are used to connect
the unit to you car: one for 12V power,
another for the ground and the third for
the signal. In addition, two more leads
can be wired to switch the unit from
one set of display values to another.
Inside the box are four pushbutton
switches located along the top edge
of the PC board. These are used to
initially set up the way the unit works.
However, they are not normally used
once the various settings have been
made.
Another feature of the unit is automatic display brightness. During
daylight, the displays are driven to
full brightness so that they can be easily seen. By contrast, as the ambient
light dims, the display brightness is
reduced so that they don’t become
CON
LED7
NON INVERTED:
INVERTED:
8.8.8.
0V
5V
2.5V
2.5V
5V
0V
Fig.4: this diagram shows the bargraph display options that are available
when the display unit is operating in wideband mode: (a) bar; (b) centred
bar; and (c) 13-step dot. In each case, the bargraph can also operate in
inverted mode.
value is typically 15.5 (ie, 0.8 greater
than for unleaded petrol).
During acceleration, the engine is
commonly run with a rich mixture,
meaning that more fuel is added to
the air compared to that used in the
stoichiometric ratio. As a result, the
air/fuel ratio becomes lower in value.
This rich mixture provides more
power under load at the expense of
fuel economy.
Unburnt hydrocarbons
When the mixture is rich, there is
insufficient oxygen in the air/fuel mix60 Silicon Chip
ture to provide complete combustion.
As a result, unburnt hydrocarbons are
present in the exhaust gas.
Conversely, when the engine is
running in cruise conditions, the fuel
supplied to the engine can be reduced
to produce a “lean” mixture, so that
there is residual oxygen in the exhaust.
This is done to improve fuel economy
and results in an air/fuel ratio that’s
slightly higher than stoichiometric.
Another way of specifying the
air/fuel ratio is to “normalise” the
stoich
iometric value so the ratio is
referenced to 1. We call this normalisa-
siliconchip.com.au
What Type Of Oxygen Sensor To Use
A wideband oxygen sensor also requires the use of a wideband controller
unit, such as this Tech edge WB02 2J1. It provides a 0-5V output which
is fed to the Oxygen Sensor Display unit, plus a simulated narrowband
S-curve output that’s fed to the engine management computer.
V
IRTUALLY ALL CARS come fitted with narrowband oxygen sensors and
if you want to save money and accuracy isn’t important, you can use
the existing sensor with the SILICON CHIP Oxygen Sensor Display. That said,
it’s best to substitute the Bosch LSM11 narrowband oxygen sensor, since the
display unit is calibrated for this sensor in narrowband mode.
Conversely, if you want high accuracy, you must use a wideband
oxygen sensor such as the Bosch LSU 4.2. This must be teamed with a
wideband controller that gives a 0-5V output. Such controllers include the Tech
Edge WB02 2J1 (http://wbo2.com/home/products.htm) and the Innovate
Motosports LC-1 (http://www.innovatemotorsports.com/products.php).
Alternatively, we intend to publish a wideband controller in a future issue of
SILICON CHIP.
At present, there are only a few vehicles such as Audi and VW that have
factory-fitted wideband sensors, so the chances are that you will have to buy
a wideband sensor and fit it. In most cases, all you have to do is remove the
existing narrowband sensor, substitute the wideband sensor and team it with
a wideband controller. The simulated narrowband S-curve output from the
wideband controller is then connected to the vehicle’s engine management
computer. This replaces the signal from the original narrowband sensor and
allows the engine to operate normally – see Fig.1.
The 0-5V output from the wideband controller unit is connected to the
display unit which then provides accurate air/fuel mixture readings.
distracting, particularly at night.
Fig.1 shows how the unit is used
with a wideband sensor and its associated controller. As can be seen,
the 0-5V output from the controller
provides the air/fuel ratio signal for
the Oxygen Sensor Display. In addition, a wideband controller usually
has a simulated S-curve output and
this can be used to replace the signal
from the original narrowband sensor
for the engine management computer.
siliconchip.com.au
By using the 0-5V signal from the
controller, the display unit can be set
up to show the air/fuel ratio over a set
range. For example, it could be set to
show air/fuel ratios between 7.4 and
22.0. These values are set to match
the 0-5V range from the wideband
controller, with the unit responding
in a linear fashion.
That’s not all it can do though. Basically, this unit can be set to display
what ever values you wish. For ex-
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To apply, visit “careers” on
www.dynalite.com.au
or email your resume and cover
letter to hr<at>dynalite.com.au
November 2008 61
REG1 LM2940CT-5
+12V
IN
OUT
220 F
10V
105 C
GND
470nF
+5V
0V
2.2k
14
Vdd
1
AN2
4
MCLR
INPUT
2.2k
3
100nF
16
RA1
15
RA6
18
RA0
17
RA7
AN4
10nF
IC1
PIC16F88-I/P
+5V
RB5
K
A
K
A
K
A
K
A
K
A
6
RB0
8
RB2
7
RB1
10
RB4
13
RB7
22k
2
11
AN3
12
RB6
9
RB3
LDR1
10k
+5V
MODE
S1
6
4
2
1
a
b
c
d
f
C
3
A
a
3
A
a
3
A
a
b
b
g
dp
d
f
b
c
b
g
d
e
d
c
dp
DISP1
LTS542R
5
c
a
8
f
g
fe
fe
c
d
dp
dp
d
g
b
DISP2
LTS542R
c
dp
dp
DISP3
LTS542R
SC
2008
WIDEBAND OXYGEN SENSOR DISPLAY
12
K
A
a
b
c
d
e
f
g
dp
LED4
LED1
LED8
LED6
LED5
LED3
LED2
A
K
LED7
DISP4
10-LED BAR ARRAY
10
LM2940CT
Q4
C
e
g
E
B
Q3
C
a
S4
E
B
Q2
C
e
9 e
f
10 g
5
UP
S3
E
B
Q1
4x 2.2k
7
S2
E
B
8x 100
DOWN
SELECT
ALTERNATIVE
DISPLAY
SWITCH
GND
Q1–Q4: BC327
76
34
5
B
IN
GND
OUT
E
C
Fig.5: the circuit is based on a PIC16F88-I/P microcontroller (IC1). This processes the sensor signal at its AN4 (pin 3)
input and drives three 7-segment LED displays and an 8-LED bargraph in multiplex fashion.
ample, it could be set to show lambda
values from say 0.51 to 1.50 instead.
Alternatively, you can set it up to
display either the air/fuel ratio or the
lambda value at the flick of a switch. In
that case, there are two sets of values
labelled “A” and “B” and you select
between them.
Similarly, for cars that run on both
unleaded petrol and LPG, it’s possible
to switch the unit so that it displays
the correct air/fuel ratio for the selected fuel.
Narrowband sensor
Fig.2 shows how the unit is used
with a narrowband oxygen sensor. In
this case, the display includes a preset response for the standard Bosch
LSM11 narrowband oxygen sensor and
shows the air/fuel ratio for unleaded
petrol from 11.8 to 20.6 (stoichiometric
at 14.7).
For air/fuel ratios below 11.8, the
display shows an “r” for rich while
ratios above 20.6 give an “L” for lean.
Similarly, for LPG, the range is 12.6 to
21.4 (stoichiometric at 15.5), with an
“r” shown for ratios below 12.6 and
an “L” for ratios above 21.4.
62 Silicon Chip
One option here is to have a dot
or a centred bargraph display for the
S-curve narrowband mode. For more
information on this, refer to the panel
titled “Using The Unit With A Narrowband Sensor”.
If the output from the sensor does
not cover the full 0-5V range, then the
values set at the 0V and 5V end points
are obtained by extrapolation. This
involves first drawing a graph similar
to Fig.9 or Fig.10 that shows two points
that correspond to the output from the
sensor and their corresponding values.
The graph is then extended until it
reaches the 0V and 5V points.
The values that are obtained at the
0V and 5V points are the endpoint
values that need to be entered into
the display during the setting up
procedure.
Bargraph display
As indicated previously, the LED
bargraph shows the sensor the voltage
level and is useful for indicating the
voltage trend. Its response to voltage
changes is significantly faster than
that of the digital display which is
deliberately slowed down so that the
values can be easily read.
Fig.4 shows the three bargraph display options that are available in the
wideband operating mode. Note that
although a 10-LED bargraph display
is used, only seven LEDs are used in
these displays.
Fig.4(a) shows the “Bar” display
mode. Here, the number of LEDs lit
increases from one to seven over six
steps in response to a rising sensor
voltage. Alternatively, it can be set up
to increase the number of LEDs lit in
response to a falling sensor voltage (ie,
an inverted display).
The “Centred Bar” mode is displayed in Fig.4(b). In this mode, the
centre bar is always lit (2.5V sensor
output), with the bar then extending
either up or down in response to a
rising or falling sensor voltage. Once
again, an inverted display option is
available.
This option is the most useful when
showing the air/fuel ratio, with the
bars indicating as the mixture moves
into either rich or lean ratios. The
centre bar is the stoichiometric point.
Finally, Fig.1(c) shows the “Dot”
mode option. In this case, there are
siliconchip.com.au
Using It As A General-Purpose Display
B
ecause it’s based
on a microcontroller, this unit can also be used as a
general-purpose display to monitor other sensors (ie, you don’t have to
use it with an oxygen sensor).
Basically, it can display any number ranging from 0-999 in response to any
sensor with a 0-5V output signal. You can set it up so that the display either
increases in value as the sensor output voltage increases or set it so that
the display decreases in response to rising sensor voltages. A decimal point
can also be included and can be positioned after the first or second digit.
If no decimal point is selected, then the display features leading zero blanking.
This means that a value of 007, for example, will be displayed as 7, while a
value of 021 will be displayed as 21. Similarly, if the decimal point is positioned
after the second digit, a value of say 00.2 will be shown as 0.2.
This decimal point selection and zero blanking feature also applies when
displaying air/fuel ratios from a wideband controller.
13 levels, with either one or two LEDs
being lit as the sensor voltage varies.
As with the previous two modes, an
inverted display option is available.
Circuit details
Despite its versatility, the circuit for
the Wideband Oxygen Sensor Display
is really very simple. Fig.5 shows the
details.
As shown, it’s based on a PIC16F88I/P microcontroller (IC1), with most of
the clever stuff hidden inside its firmware program. Apart from that, there
are the three 7-segment LED displays
(DISP1-DISP4), the 10-LED bargraph
display, four driver transistors (Q1Q4), a 3-terminal regulator (REG1) and
a few sundry bits and pieces.
IC1’s monitors the input voltage
from the sensor, processes the data
and drives the LED displays to show
the calculated air/fuel ratio value. Output ports RB0-RB7 drive the display
segment cathodes, while transistors
Q1-Q4 switch the common display anodes, ie, the displays are multiplexed
so that only one display digit is driven
at any given time.
Note that all the segments common
to each display are tied together. For
example, the “a” segment of DISP1
connects to the “a” segments of DISP2
and DISP3. In addition, LED4 within
the LED bargraph (DISP4) also connects to the “a” segments of DISP1DISP3.
These “a” segments are driven via
the RB5 output of IC1 via a 100Ω resistor. As a result, when this output
is low, the “a” segment in one display
will light, depending on which driver
transistor is turned on.
siliconchip.com.au
PNP transistors Q1-Q4 are driven by
ports RA0, RA1, RA6 & RA7 via 2.2kΩ
resistors. For example, transistor Q1
is controlled by RA1 and when this
output is high, Q1 is held off.
Conversely, when RA1 goes low
(0V), Q1’s base is pulled low via its
2.2kΩ resistor and so Q1 turns on. As
a result, any segments within DISP1
that have their cathodes pulled low via
IC1’s RB outputs (and their respective
100Ω resistors) will now light.
Transistors Q2, Q3 and Q4 are driven in a similar manner to Q1 to control
DISP2, DISP3 and the LED bargraph
(DISP4). For example, to light DISP2,
we switch off Q1, set the required segment driver outputs required for the
DISP2 display and then switch on Q2
by taking RA6 low. A similar process
is then used to switch on DISP3 and
DISP4 in turn.
This on-off switching of the displays
is done at such a fast rate (around
2kHz) that the displays all appear to
be continuously lit.
Parts List
1 PC board, code 05311081, 80
x 50mm
1 plastic case measuring 83 x 54
x 31mm
1 rectangular piece of red clear
Perspex 48 x 18mm
4 SPDT micro tactile switches
with a 6mm actuator (S1-S4)
1 LDR with 48kΩ light resistance
1 DIP20 IC socket, 0.3-inch
width
1 DIP18 IC socket
1 DIP16 IC socket
1 DIP14 IC socket
1 M3 x 10mm screw
1M3 nut
5 PC stakes
1 2m length of twin shielded wire
Semiconductors
1 PIC16F88-I/P microcontroller
coded with 0531108A.hex
(IC1)
3 13mm common anode LED
displays (DISP1-DISP3)
1 10-LED DIL bargraph (DISP4)
4 BC327 transistors (Q1-Q4)
1 LM2940CT-5, +5V low dropout
regulator (REG1)
Capacitors
1 220μF 10V electrolytic
1 470nF MKT polyester
1 100nF MKT polyester
1 10nF MKT polyester
Resistors (0.25W, 1%)
6 2.2kΩ
1 10kΩ
1 22kΩ
1 5 x 100Ω individual SIL
resistor array
1 3 x 100Ω individual SIL
resistor array
Display dimming
Light dependent resistor LDR1 is
used to sense the ambient light to
control the display dimming. This is
connected in series with a 22kΩ resistor to form a voltage divider across the
+5V rail and its output is fed to IC1’s
AN3 port.
When the ambient light is high, the
LDR has a low resistance and the voltage at the AN3 input is pulled down
close to 0V. Conversely, in low ambient
light, the LDR has a high resistance
and IC1’s AN3 input is pulled close
to the +5V rail via the 22kΩ resistor.
At intermediate light levels, AN3 will
sit somewhere between 0V and +5V.
In operation, IC1 dims the displays
in response to its AN3 voltage. It does
this by limiting the amount of time
that the displays are lit. In bright light,
each display is lit for almost 25% of
the total time but this reduces as the
AN3 voltage rises in response to falling
light levels.
In fact, at very low levels, each display might only be lit for about 2%
of the time.
Pushbutton switches
Switches S1-S4 allow the unit to be
November 2008 63
100nF
3 x 100 SIL ARRAY
2.2k
10nF
DISP1
Q4
Q3
2.2k
DISP2
2.2k
DISP3
ALTERNATIVE
DISPLAY
SWITCH
DISP4
10k
Q2
2.2k
IC1 PIC16F88-I/P
22k
2.2k
LDR1
S4
S3
Q1
18011350
d n a b e di w
S2
S1
220 F
5 x 100 SIL ARRAY
470nF
REG1
2940-5
2.2k
GND
IN
+12V
NOTE: DISP1–DISP4
ALL MOUNTED IN IC
SOCKETS (SEE TEXT)
Fig.6: install the parts on the PC board as shown here.
The alternative display switch is optional (see text).
Take care to ensure that all the parts are installed on
the PC board with the correct orientation. The LED
bargraph is mounted with its bevelled edge at bottom
right (see Fig.6).
Oxygen Sensor Display
DISPLAY CUTOUT
SILICON
CHIP www.siliconchip.com.au
This view shows the PC board before the 7-segment LED
displays and the bargraph are plugged in.
programmed by providing the Mode,
Select, Down & Up functions. These
are connected respectively to the bases
of transistors Q1-Q4, while the other
ends are commoned and connected to
the +5V rail via a 10kΩ resistor. This
commoned end is also connected to
IC1’s AN2 input, which monitors the
switches.
If switches S1-S4 are all open, IC1’s
AN2 input will be held at +5V via the
10kΩ pull-up resistor. However, if a
switch is closed, AN2 is connected to
the base of its corresponding transistor. As a result, the voltage on the AN2
input will drop to about 0.6V below
the +5V rail (ie, to 4.4V) each time
Fig.7: this full-size artwork can be used as a drilling
template for the front panel.
that particular transistor switches on.
In operation, the microcontroller
periodically checks the voltage at its
AN2 input. As a result, it can decide
if a switch has been closed based on
the AN2 voltage and then determine
which switch it is by checking which
transistor is currently switched on.
The optional external Alternative
Display Switch is connected in parallel with switch S4. This switch can
be a dashboard toggle switch so that,
for example, either the air/fuel ratio
or the lambda value can be shown.
Alternatively, it can be a relay contact
that automatically opens or closes depending on the fuel (eg, petrol or LPG).
Note that this switch is not required
if the display only needs to show one
set of values.
Input signal
The signal from the sensor is fed
to the AN4 pin of IC1. IC1 converts
this input voltage into a 10-bit digital
Table 2: Capacitor Codes
Value
470nF
100nF
10nF
μF Code IEC Code EIA Code
0.47μF
470n
474
0.1μF
100n
104
0.01μF 10n
103
Table 1: Resistor Colour Codes
o
o
o
o
o
No.
6
1
1
8
64 Silicon Chip
Value
2.2kΩ
10kΩ
22kΩ
100Ω
4-Band Code (1%)
red red red brown
brown black orange brown
red red orange brown
brown black brown brown
5-Band Code (1%)
red red black brown brown
brown black black red brown
red red black red brown
brown black black black brown
siliconchip.com.au
100 1/4W RESISTORS MOUNTED END-ON
How The Micro Calculates The Values
5V VALUE
22.0 (EXAMPLE)
PC BOARD
ALTERNATIVE TO USING RESISTOR ARRAYS
Fig.8: separate 100W resistors can
be used instead of the two resistor
arrays. Mount them as shown here.
value which is then processed and the
resulting calculation fed to the display.
A 2.2kΩ current-limiting resistor
and internal clamping diodes inside
IC1 protect the AN4 input should the
input voltage go above the +5V supply
or below the 0V rail. The associated
10nF capacitor filters any voltage
spikes at the input.
A feature of unit is that it switches
off all the displays for a short period
before measuring the input voltage.
This minimises any voltage drops that
could occur due to supply current
in the ground wiring if the displays
were lit and ensures accurate measurements.
Timing for IC1 comes from an internal oscillator running at 4MHz. This
has an accuracy of about 2% which is
close enough for this application, as
the timing is not critical.
Power supply
Power is derived from the vehicle’s
fused ignition supply. This +12V rail
is fed to a low-dropout LM2940CT-5
+5V regulator. These regulators are
designed for automotive applications and are protected against line
transients and reverse supply voltage
(if the supply is reversed, the output
remains at 0V and no damage occurs).
A 470nF capacitor decouples the
supply for the regulator input, while
a 220μF capacitor filters the +5V output. This output capacitor supplies
the transient current required for the
displays and also prevents the regulator from becoming unstable.
In addition, the supply rail to IC1
is decoupled using a 100nF capacitor
at pin 14. The 2.2kΩ resistor between
IC1’s MCLR-bar input (pin 4) and the
+5V rail provides the power-on reset
signal for IC1.
Construction
This unit is easy to assemble, with
all parts installed on a double-sided PC
siliconchip.com.au
WHEN THE 5V VALUE
IS GREATER THAN THE
0V VALUE:
SPAN
x 2.5
((22.0 – 7.4)
) + 7.4 = 14.7
5V
7.4 (EXAMPLE)
0V VALUE
0V
2.5V
5V
Fig.9: this graph shows how IC1 calculates the display values when the 5V
endpoint value is greater than the 0V endpoint value. This example uses
7.4 and 22.0 for the 0V and 5V endpoint values respectively, giving a 2.5V
sensor output value of 14.7 (ie, stoichiometric for unleaded petrol).
0V VALUE
22.0 (EXAMPLE)
WHEN THE 5V VALUE
IS LESS THAN THE
0V VALUE:
(22.0 – 7.4) x (5V–2.5V)
+ 7.4 = 14.7
5V
(
SPAN
7.4 (EXAMPLE)
5V VALUE
0V
)
2.5V
5V
Fig.10: the equation is slightly different when the 0V endpoint value is
greater than the 5V endpoint value. In this example, 22.0 has been used for
the low endpoint value, while 7.4 has been used for the high endpoint value.
To set the values for the display, you only need to set the endpoint values at
0V and at 5V. The internal microcontroller then processes the input signal and
calculates the correct vales for display.
For example, if the 0V endpoint value is 7.4 and the 5V endpoint value is 22.0, a
2.5V input will give a display reading of 14.7 for the air/fuel ratio. This is calculated
by first subtracting the low endpoint value from the high endpoint value to get the
span value (in this case, 22 - 7.4 = 14.6). This span value is then multiplied by
the input voltage, divided by the 5V range and finally added to the low endpoint
value (7.4 in our example). Fig.9 shows this in graphical form.
If the unit is set up so that the 0V endpoint value is higher in value than the 5V
endpoint value, then the calculation is different (see Fig.10). In this case, the 5V
endpoint value is subtracted from the 0V endpoint value to get the span value.
This value is then multiplied by the difference between the input voltage and 5V,
after which the result is divided by 5V and added to the 5V endpoint value. Fig.10
shows the equation for endpoint values of 22 and 7.4.
Note that in both cases, the 5V value assumes that the reference voltage used
in the Oxygen Sensor Display is exactly 5V. However, the reference voltage from
the regulator that’s used could be anywhere from 4.85-5.15V so there is an adjustment to compensate for this.
If the reference voltage is below 5V, then the Oxygen Sensor Display will not
show readings for input voltages that are higher than this reference. Conversely,
if the reference is above 5V, then the unit will show readings for input voltages
only up to the +5V. By compensating for this reference voltage, the correct value
will be shown on the display.
In practice, the regulator used for the reference is trimmed during manufacture
and its output will probably be very close to +5V.
November 2008 65
Changing The Wideband Display Settings
T
HE FOUR PUSHBUTTON switches
inside the case are for Mode (S1),
Select (S2), Down (S3) & Up (S4).
Pressing the Mode switch initiates
the Settings mode. Pressing it again
then returns the display to the normal
Run mode so that it shows the values
in response to the input voltage.
Once in the Settings mode, you can
alter the way the display operates. You
can set how the dimming works, set
the regulator voltage, alter the “A” or
“B” values selection and alter the 0V
& 5V endpoint values for each selection. In addition, you can change the
bargraph display from dot to bar or to
a centred bar.
The bargraph is used to indicate
which setting is selected. In this mode,
the lower LED (LED8) is always lit – see
Fig.11 (note: LED8 is never lit in the
normal run mode).
The remaining LEDs on the bargraph
show which setting has been selected
(see Fig.11). Note that there are 10LEDs on the bargraph but only the
middle eight (designated LEDs1-8) are
used. You cycle through the settings
by pressing the Select switch (S2).
Minimum Display Brightness: when LED7
is lit, the setting is for the minimum
display brightness that occurs when
the LDR is in complete darkness. This
value is initially set at “14”, as shown
on the display.
When adjusting this value, it’s
necessary to cover the LDR so that
it does not receive any ambient light
either from below or above its surface.
A black film canister is ideal for this
and the value is adjusted using the
Up & Down switches to set the desired
minimum brightness.
The absolute minimum brightness is
reached at 0 but typical settings would
range from 10-30.
Dimming Threshold: pressing the Settings
switch again brings up the Dimming
Threshold setting, with LED6 lit. This is
initially set at 200 and determines the
ambient light level below which dimming
begins. Increasing the value means that
dimming begins at a higher ambient
light level, while decreasing the value
sets the dimming to begin at a lower
light level.
Regulator Voltage: the next setting is
for the Regulator Voltage (LED5 lit).
This value is initially set at 5.00V and
is normally adjusted (using the Up &
Down switches) to agree with the actual
regulator output voltage, as measured
between its OUT and GND terminals.
A Or B Display: LED4 indicates the A or B
Display selection. Here, you can select
between the “A” and “B” display values.
If “A” is selected, then the normal Run
mode will show the “A values and the
“B” value can be shown by pressing S4
(Up) or by using the external alternative
display switch.
Alternatively, if the “B” values are
selected, the display will show these
in Run mode and the “A” values will be
shown if S4 is press (or the external
switch is toggled).
Display Format: the Display Format is next
in the sequence (LED3 lit). In this case,
the digital display will show A.AA, AA.A
or AAA for the “A” selection. You can
select the decimal point position using
the Up or Down switches. Similarly, if
the “B” values have been selected, the
display will show b.bb, bb.b or bbb.
0V Display Value: pressing S1 again to
light LED2 selects the 0V Display Value.
This is the value that’s displayed in Run
mode when the input is at 0V and it can
be set to any value from 0-999. Note
that this value will be for the “A” display
if this was previously selected in the “A
Or B Display” option. Alternatively, this
value will be for the “B” display if this
was previously selected in the display
option.
Note that where the “A” and “B”
displays both need to be set, it will be
necessary to temporarily change the
display option from “A” to “B” or from “B”
to “A” and also set the required Display
Format before adjusting the endpoint
value to suit the alternate display.
5V Display Value: this setting is indicated
when LED1 is lit. Again, you can set this
to any value from 0-999 and the same
comments as above apply to setting
values for both “A” & “B” displays.
It’s important to note here that the
0V and 5V values must match the
output from the wideband controller.
This means that if you set the wideband
board with plated-through holes. This
board is coded 05311081 (80 x 50mm)
and is housed in a small plastic case
measuring 83 x 54 x 31mm.
Begin by checking the board for any
defects and by checking the hole sizes
for the major parts. Check also that the
PC board is cut and shaped to size so
that it clips into the integral side slots
in the case.
Fig.6 shows the parts layout. Install
the resistors first, taking care to place
each in its correct position. Table 1
shows the colour code values but you
should also use a digital multimeter to
check each resistor before installing
it. Note that the 100Ω resistors are
in single in-line (SIL) resistor arrays.
However, you can also use standard
0.25W resistors here and these can be
installed by mounting them end-on as
shown in Fig.8.
Next, install the PC stakes. These
are installed from the underside of the
PC board at the three external wiring
positions (the external wiring connects
to the rear of the board).
Transistors Q1-Q4 can go in next and
these must be installed so that their
tops are no higher than 12mm above
the PC board. Follow them with the
four switches (S1-S4). These switches
can only go in with the correct orientation so if the holes don’t line up,
simply rotate them by 90°
REG1 is next on the list. This device
mounts horizontally on the PC board,
with its leads cranked down through
90° so that they pass through their corresponding holes. Secure its tab to the
board using an M3 x 6mm screw and
nut before soldering its leads.
Once it’s in, install the capacitors.
Note that 220μF electrolytic adjacent
to REG1 is installed with its leads bent
through 90°. Its body lies horizontally
across the regulator’s leads as shown
in the photo.
66 Silicon Chip
Mounting the displays
The 7-segment LED displays and the
10-LED bargraph are raised up off the
PC board using IC sockets.
The sockets for the 7-segment dissiliconchip.com.au
LED1
(ALL LIT)
DOT, BAR OR
CENTRED BAR
LED8
LED1
LED8
SETTINGS INDICATOR
(LEDS1–7 INDIVIDUALLY LIT
ACCORDING TO SELECTION)
5V DISPLAY VALUE
0V DISPLAY VALUE
DISPLAY FORMAT
A OR B DISPLAY
REGULATOR VOLTAGE
DIMMING THRESHOLD
MIN DISPLAY BRIGHTNESS
SETTINGS INDICATOR
Fig.11: this diagram shows the
bargraph setting indications for the
default wideband operating mode.
controller to deliver air/fuel ratios over
a range of 7.4 to 22.0, then the display
should also be set to these values.
If you want to have the stoichiometric
value in the middle of the scale (so that
the bargraph display is centred), then the
sum of the 0V endpoint value and the 5V
endpoint value must be twice the stoichiometric value. So if the stoichiometric
air/fuel ratio is 14.7, the 0V endpoint
value and the 5V endpoint value must
add up to 29.4 – eg, you could use 7.4
and 22.0 as the endpoints.
If you intend to display the lambda
value, then the minimum and maximum
values must add up to 2 (eg, 0.52 and
1.48 could be used but other values
could be used instead).
Bargraph Display Option: the final selection brings up the Bargraph Display
Option and in this case all eight LEDs
are lit. Again, the options are selected
plays are made using a 16-pin DIP
socket and a 14-pin DIP socket. These
are cut into strips of two 8-pin and
two 7-pin SIL sockets using a small
hacksaw. One 8-pin and one 7-pin
strip is then installed along the top
edge of the display positions, while
the remaining 8-pin and 7-pin strips
are installed along the bottom edge (ie,
the sockets form two 15-pin strips).
Once these SIL strips are in, install a
20-pin DIP socket for the LED bargraph
and an 18-pin DIP socket for IC1. Be
sure to orientate the socket for IC1
with its notched end towards the top
(ie, towards the 2.2kΩ resistor). Don’t
plug the displays or IC1 in yet, though
Finally, install the LDR (either way
siliconchip.com.au
using the Up & Down switches and are
as follows: (1) dot (shown as doT on the
display); (2) bar (shown as bAr on the
display); and (3) centred bar (shown as
bCn). Note that the “T” in the doT lettering has the lefthand side of its cross
piece located over the “o”.
The default setting for the bargraph
display is to have the LEDs progressing
upwards with increasing sensor output
voltage. Conversely, if you want them to
progress upwards with a falling sensor
voltage, then it’s just a matter of selecting the inverse, as follows.
To invert the “A” curve selection,
press S2 at power up and the display
will show the current selection. Initially,
this will show “A.ni”(A not inverted)
and this indicates that the A bargraph
is not inverted. If S2 is now held
pressed for four seconds, the display
will change to show “A. i” (A inverted)
to indicate that the bargraph operation
is now inverted.
You simply release the switch when
the required selection is displayed.
Holding the switch down will cause the
display to cycle between the inverted
and non-inverted options.
Similarly, to set the “B” bargraph
sense, S3 is pressed when power is
applied. This will initially indicate “b.ni”
(B bargraph not inverted) but can be
changed to “b.i” (B bargraph inverted)
by holding the switch down for four
seconds.
It’s easy to check the current selection
by pressing S2 or S3 at power up. No
changes will occur unless the switch is
held for more than four seconds and the
display changes to the next option.
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AC/DC Volts 600V
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around) so that its top surface is 15mm
above the top of the PC board.
Kit Contains
Testing
●
Now for the smoke test but first go
over the board carefully and check for
incorrect component placement and
for missed or shorted solder joints.
Next, with IC1 out of its socket,
apply power to the +12V and GND
terminals and check that 5V is present
between pins 14 & 5 of IC1’s socket. If
this is correct, switch off and install
IC1 and the displays. DISP1, DISP2
and DISP3 mount with the decimal
points to bottom right, while DISP4
(the LED bargraph) mounts with its
chamfered edge at bottom right (note:
Diode Test
Analog Bar Graph
Backlight
Min/Max/Avg
Display Hold
Auto/Manual Range
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Sydney (02) 9704 9000
November 2008 67
Using The Unit With Narrowband Sensors
LED1
RICH
LED7
LEAN
Enabling the S-curve response
BAR
A
O 2 SENSOR OUTPUT VOLTAGE (mV)
W
HEN USED WITH narrowband sensors, this unit will display air/fuel ratios
that are calibrated to the S-curve output of a
Bosch LSM11 narrowband oxygen sensor.
Note, however, that this may not be accurate
for other oxygen sensors.
In the case of the LSM11, it shows air/fuel
ratios for unleaded petrol from 11.8 to 20.6,
with the stoichiometric ratio set for 14.7. For
air/fuel ratios below 11.8 the unit will show an
“r” for rich, while for ratios above 20.6 the unit
shows an “L” for lean.
For LPG, the range is from 12.6 to 21.4 with
stoichiometric at 15.5. The unit displays an “r”
(rich) for ratios below 12.6 and an “L” (lean)
for ratios above 21.4.
For narrowband sensors, the bargraph options are as shown in Fig.12; ie, either a centred bar mode or a 13-level dot mode. These
13 different levels are achieved by lighting
either one or two LEDs at a time.
For the bar mode, the centre LED is always
lit and is the only LED that is lit at stoichiometric. The bar then progresses upwards from the
middle LED for richer mixtures or below the
middle LED for leaner mixtures.
BARGRAPH DISPLAY MODE
DOT
1000
A
B
B
C
D
E
800
C
F
G
600
D
H
400
I
200
E
J
0
LAMBDA ()
AIR/FUEL RATIO
(UNLEADED PETROL)
LPG
K
L
M
F
G
0.8
0.9
1.0
1.1
1.2
1.3
11.8
12.4
13.2
13.9
14.7
15.5
16.2
17.1
17.6
18.6
19.0
20.2
RICH
A
A
B
C
B
D
LEAN
CENTRED BAR MODE
C
E
D
E
DOT MODE
F
G
H
F
G
I
J
K
L
M
LED1
RICH
LED7
LEAN
Fig.12: two bargraph options are available when the unit is set to
operate in narrowband mode – either centred bar mode or a 13-step dot
mode. The S-curve graph at top indicates which bargraph LEDs light in
response to the various sensor output voltages.
the chamfer is quite subtle). IC1 goes
in with its notched end towards the
top.
When power is now reapplied you
should be greeted with a display on
the 7-segment digits and the bargraph.
If not, check the orientation of IC1. If
that’s correct, check that transistors
Q1-Q4 are BC327 PNP types.
Final assembly
As mentioned above, the PC board
is designed to simply clip into the
specified plastic case. A 48 x 18mm
cut-out is made in the lid of the box
for the displays and this cut-out is
68 Silicon Chip
Enabling the narrowband S-curve response is easy: just press and hold the
Mode switch as power is applied.
The display will then indicate the current display mode setting. This can be either the Linear (wideband) mode, the S-curve unleaded
mode or the S-curve LPG mode. If the switch
is released before four seconds the current
display mode will not be altered. Conversely,
if the switch is held down, the mode will cycle
from one to the other at a nominal 4-second
fitted with a red Perspex filter. In addition, a hole is drilled in the lid for
the LDR, so that it is exposed to the
ambient light.
A hole at the rear of the box allows
the wiring to exit from the case.
The front-panel artwork shown in
Fig.7 can be used as a template for
cutting and drilling the holes. It can
either be scanned or downloaded
from the SILICON CHIP website and
temporarily affixed to the lid using
double-sided tape.
The cut-out for the LED displays can
be made by drilling a series of holes
inside the inside perimeter of the cut-
out and then knocking out the centre
piece. The cut-out is then carefully
filed to a smooth finish.
The hole for the LDR should be
drilled to 5mm, as should the exit hole
in the back of the case. This exit hole
should be positioned near the bottom
edge of the case, so that it will directly
line up with the PC stakes on the back
of the board. Alternatively, you can
drill the hole to 9.5mm and fit it with
a 6mm ID rubber grommet.
Making the connections
We used twin-shielded wire for
the power and input connections but
siliconchip.com.au
rate.You simply release the switch when
the required display mode is shown.
It’s also easy to tell which mode the
unit is in. The display will show “Lin.” for
the linear mode (or wideband mode),
while the two S-curve modes are shown
as S.UL (S-curve unleaded) and S.LP
(S-curve LPG).
Pressing the Mode switch after
power-up has been applied initiates the
Settings mode. As before, this allows
you to alter the way the display operates. You can adjust how the dimming
works, set the regulator voltage, alter the
unleaded or LPG selection and change
the bargraph display from dot mode to
centred bar mode.
As in wideband mode, the bargraph
LEDs are again used to indicate which
setting has been selected. These settings are somewhat different for the narrowband S-curve modes but are altered
in exactly the same manner.
Fig.13 shows the details. As before,
only eight LEDs in the 10-LED bargraph
are used and the lower LED (LED8)
is always lit in the settings mode. The
remaining LEDs on the bargraph show
which setting has been selected and
you can cycle through these settings
by pressing switch S2.
Minimum Display Brightness : when LED7
lit, the setting is for the minimum display
brightness that occurs when the LDR
is in complete darkness. This value is
initially set at “14”, as shown on the
display.
When adjusting this value, it’s necessary to cover the LDR so that it does
not receive any ambient light either
from below or above its surface. A black
film canister is ideal for this and the
value is adjusted using the Up & Down
automotive wire could also be used.
Connect the +12V lead to the fusebox
in the car so that the Oxygen Sensor
display is powered only when the
ignition is on (ie, be sure to connect to
the fused side). The ground connection
should preferably connect to the same
ground as the wideband controller.
For narrowband use, connect the
ground to the same ground as the
sensor. The input lead for the Oxygen
Sensor Display is connected either to
the 0-5V output from the wideband
controller or (if you are saving money)
to a narrowband sensor signal.
Fit a cable tie around the leads on
siliconchip.com.au
LED1
(ALL LIT)
DOT OR
CENTRED BAR
LED8
LED1
LED8
SETTINGS INDICATOR
(LEDS4–7 INDIVIDUALLY LIT
ACCORDING TO SELECTION)
UNLEADED OR LPG DISPLAY
REGULATOR VOLTAGE
DIMMING THRESHOLD
MIN DISPLAY BRIGHTNESS
SETTINGS INDICATOR
Fig.13: the setting indications for
the narrowband mode. This mode is
initiated by pressing and holding the
Mode switch as power is applied.
switches to set the desired minimum
brightness.
The absolute minimum brightness is
reached at 0 but typical settings would
range from 10-30.
Dimming Threshold: pressing the Settings
switch again brings up the Dimming
Threshold setting, with LED6 lit. This is
initially set at 200 and determines the
ambient light level below which dimming
begins. Increasing the value means that
dimming begins at a higher ambient
light level, while decreasing the value
sets the dimming to begin at a lower
light level.
Regulator Voltage: the next setting is
for the Regulator Voltage (LED5 lit).
This value is initially set at 5.00V and
is normally adjusted (using the Up &
Down switches) to agree with the actual
regulator output voltage, as measured
between its OUT and GND terminals.
The regulator voltage adjustment can
the inside of the box, to prevent them
being pulled out of the hole.
Setting up
For use with a wideband controller,
the unit is set up as described in the
panel titled “Changing The Wideband
Display Settings”.
Note that commercial wideband
controllers can have either fixed or
adjustable endpoint values. The adjustable versions have their endpoints
set by connecting them to a computer.
Note also that the endpoint values
programmed into the display unit
must match those of the wideband
also be used to alter the unit’s response
to the oxygen sensor’s output. For example, setting the regulator voltage to a
value that’s higher than the actual regulator voltage results in the unit displaying its full range of air/fuel values over
a reduced voltage range. It effectively
lowers the rich end of the S-curve, so
that rich readings are indicated at lower
oxygen sensor voltages.
Similarly, setting the regulator voltage value lower than the real regulator
voltage increases the voltage range.
This raises the rich end of the S-curve
and rich readings are shown at higher
oxygen sensor voltages.
Basically, this adjustment can be
used to provide a more accurate air/
fuel reading for the particular oxygen
sensor used.
Unleaded Or LPG Display: LED4 indicates
the Unleaded Or LPG Display setting.
This can be toggled using either the Up
or Down switch between S.UL (for unleaded petrol) or S.LP (for LPG). When
normal mode is resumed, the display
will then show the air/fuel ratio values
for the selected fuel.
As before, the unit can be set up
for both unleaded petrol and LPG and
the display reading toggled using an
external switch wired across S4. When
this switch is open, the default air/fuel
readings (as selected in the preceding
paragraph) are displayed.
Bargraph Display Options: finally, S1 is
pressed again to bring up the bargraph
display options (all 8-LEDs are lit).
Again, these are selected using the Up
or Down switch and you can choose
either the centred bar mode (shown as
bCn on the display or the 13-step dot
mode (shown as doT).
controller. This ensures that the unit
is correctly calibrated and gives accurate air/fuel ratio readings.
Switching between the “A” and “B”
display values (eg, between air/fuel
ratio and lambda values or between
unleaded petrol and LPG air/fuel ratios)
can be achieved by wiring an external
switch (or NO relay contacts) in parallel
with switch S4 (see Fig.6).
Note that the connections on the
relay contacts or switch must be solely
for this purpose. If you need to switch
a fuel valve or anything else at the
same time, use a double-pole relay
SC
or switch.
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