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Pt.1: By JOHN CLARKE
Voltage Interceptor
For Cars With ECUs
At one time, the sensors in cars with engine management
were regarded as untouchable. But now you can change the
signal response of many of the sensors to improve your car’s
driveability, throttle response, handling and so on. This voltage
interceptor allows you modify and program the response of any
voltage sensor in your car, without prejudicing reliability or
affecting the ECU in any way. Use it for restoring correct air/fuel
ratios after engine modifications, preventing turbo boost cuts or
altering sensor signals for improved driveability.
Main Features
•
•
•
•
•
•
Output follows input plus
adjustment value
Programmed using a pushbutton
controller
Easy installation
Works with sensors with voltage
outputs
Adjustable sensitivity
Smooth transition between
adjacent output points
24 Silicon Chip
M
ODERN CARS have lots of sens
ors to closely monitor the engine
and other systems and they provide
information to the ECU (Engine Control Unit). In turn, the ECU controls
the fuel injectors and ignition timing,
based on this information.
Some of the sensors you can intercept and modify include the airflow
meter, oxygen sensor, accelerometers
(or G force sensors) used in stability
control and active 4-wheel drive sys-
tems and the throttle position sensor
(TPS). For cars with an electric throttle
rather than a throttle cable, modification of the TPS signal can transform
the way the car drives.
For example, you can alter the signal so that there is less pedal travel
required to provide more throttle. This
will make the car behave as though it
has more power.
Alternatively, the signal can be
altered so that more pedal travel is
siliconchip.com.au
required for throttle openings only at
smaller throttle settings. This could
make the car a lot smoother when
moving off and make it safer to drive
in wet and slippery conditions.
VOLTAGE
SIGNAL
FIG.1(a)
Interceptor concept
An interceptor allows the signal
from a sensor to be altered before it
is monitored by the car’s ECU. Fig.1
shows the concept. Fig.1(a) shows a
typical sensor connected to the ECU
while Fig.1(b) shows the interceptor
connected between the sensor and
the ECU.
The voltage interceptor does not
necessarily modify the sensor signal
at all times; when no changes are
required, the signal at the output will
be the same as at the input.
For example, an airflow sensor
output may provide 0.9V at idle and
4.1V at full engine load and high RPM.
The latter reading will normally result
in a very rich mixture. So you might
decide to limit the range of the airflow
output for signals above 2V (say) and
to change the slope.
The result would be fuel mixtures
that are not quite so rich at large throttle openings and your fuel economy
should improve.
We have already given the example
of modifying the output of the throttle
position sensor and there are quite a
few other applications. There is one
proviso though: each sensor to be
modified will require its own interceptor. Want to modify the output of three
engine sensors? You will need three of
these interceptors.
Pusbutton controller
The interceptor needs to be programmed and for this you need a
Pushbutton Controller which is also
described in this article. This controller has nine buttons and a 2-line LCD
panel. It connects to the interceptor via
a cable and a 25-pin connector which
plugs into the PC board. After programming, you disconnect the pushbutton
controller and the interceptor is used
on its own.
The good thing is, regardless of
how many interceptors you decide
to install, you only need one Pushbutton Controller. We’ll talk about
this controller in more detail, later in
this article.
Note that while we have designed
this interceptor for use in automotive
applications, there are many other
siliconchip.com.au
ENGINE
CONTROL
UNIT (ECU)
SENSOR
+12V
MODIFIED
VOLTAGE
SIGNAL
VOLTAGE
SIGNAL
IN
SENSOR
VOLTAGE
INTERCEPTOR
ENGINE
CONTROL
UNIT (ECU)
OUT
HAND
CONTROLLER
FIG.1(b)
Fig.1: the Voltage Interceptor is installed immediately after the sensor and
modifies the sensor’s signal before it is fed to the ECU.
INPUT
INPUT PROCESSING
(IC1a, IC1b)
GAIN OR ATTENUATION
WITH VR2
OUTPUT PROCESSING
(IC2b, IC3b)
GAIN OR ATTENUATION
WITH VR7
5V
MAXIMUM
+12V
0V
POWER SUPPLY
(REG1, ZD3, ZD4)
SENSITIVITY
(VR6)
ADJUST SIGNAL
(IC2a, IC3a)
OFFSET WITH VR5
MICROCONTROLLER
(IC4)
MINIMUM SET
(VR3)
OUTPUT
+11.4V
+5.6V
+5V
–7.5V
HAND
CONTROLLER
VOLTAGE INTERCEPTOR
Fig.2: how the unit processes the sensor signal. The signal is first amplified
or attenuated and fed to microcontroller IC4. IC4 then digitises the signal
and modifies its response to produce a control voltage that’s used to offset
the output of IC2b.
applications where it is desirable to
convert a DC signal into a smaller,
larger or non-linear voltage.
Interceptor workings
The block diagram of Fig.2 shows
how the Voltage Interceptor processes
the sensor signal. First, the sensor
signal needs to be amplified (gain) or
reduced (attenuation) to bring it within
a range of 0-5V which the microcontroller (IC4) can handle. Yes, inevitably, the Interceptor uses a micro.
The Voltage Interceptor’s microcontroller digitises this gain-changed
signal from the sensor into a “map”
with 256 separate load sites. The
output circuitry (IC2b & IC3b) then
applies a reverse amount of gain or
attenuation to cancel out the gain/
attenuation originally applied in the
input processing stage. This ensures
that the Interceptor’s output has the
same overall voltage range as the sensor itself.
The idea is to present a signal to the
ECU with exactly the same characteristics as the signal from the sensor. That
way, the overall operation of the ECU
is not prejudiced in any way and it acts
exactly as if the sensor was connected
directly to it.
You then have the option of changing the Interceptor’s output response
at each and every load site. The Pushbutton Controller is used to set this
December 2009 25
SC
2009
8
A
K
ZD1–ZD4
20k
10k
2
1k
4
A
K
D1 1N4004
RB2
RB0
PWM
A
K
8
6
9
Vdd
3
AN4
7
RB1
14
D1, D2
Vss
5
10nF
22k
IC1b
IC4
PIC16F88-I/P
AN3
MCLR
REF–
17
RA0
16
RA7
13
RB7
12
RB6
11
RB5
10
RB4
15
RA6
18
AN1
10k
TP4
TP3
1
10 F
+5V
5
6
VR2 100k
IC1: LMC6482AIN
–7.5V
1
+5.6V
10nF
VR3
1k
MINIMUM
VOLTS
4
IC1a
10k
3
2
470k
5V SET
10k
LK1
10k
100
VR5
10k
2.2k
5V MAX
TP2
OFFSET
LOCK
7
VOLTAGE INTERCEPTOR FOR CARS
10nF
VR4
1k
THRESHOLD
100 F
1M
RLY1a
1nF
A
K
2
3
10k
V++
1
A
7
10
10k
100nF
100nF
43k
K
2
3
8
20k
0.5W
A
K
10nF
9.1k
4
IC2a
470
+11.4V
IC2: LMC6482AIN
IC2b
10k
D3, D4: 1N4148
+11.4V
ZD2
15V
1W
5
6
100k
10nF
4
IC3a
8
VR6
50k
SENSITIVITY
10k
10nF
2.2k
K
A
LED1
E
C
B
B
100nF
+5.6V
ZD4
5.6V
1W
V+
B
Q2
BC547
1
IC3b
7
ZD1
16V
1W
E
C1
K
A
B
C
Q1–Q4
A
K
Q4
BC327
C
E
E 100 F
Q3
BC337
C
V++
–7.5V
+5V
IC3: LMC6482AIN
5
6
10nF
10k
OFFSET
MEASURE
TP5
VR7 50k
OUTPUT
SET
D4
ZD3
7.5V
1W
220
OUT
ADJ
IN
LM317T
V+
10 F
120
K
A
RLY1
Q1
BC337
TP1
E
C
–7.5V
5V
ADJUST
OUT
ADJ
100 F
16V
VR1
500
IN
B
100 F
A
K
REG1 LM317T
C2
A
LED1
D3 100 F
K
K
A
1k
1k
D2
1N4004
150
RLY1b
V+
OUT
(CON2b)
OUTPUT
Fig.3: the Voltage Interceptor is based on a PIC16F88-I/P microcontroller (IC4) which has the ability to adjust the output at 256 points. It accepts the incoming
signal from IC1b at its AN4 (pin 3) input and outputs a PWM signal at pin 9. This signal is then filtered and fed to IC2a to produce an offset voltage which is fed
to pin 5 of IC2b via VR6.
0V
CON1
+12V
IN
4
10
11
12
13
9
2
3
6
8
5
CON3
DB25
(CON2a)
INPUT
TO HAND CONTROLLER
26 Silicon Chip
siliconchip.com.au
output response during the setting-up
procedure.
A signal of 0V (min) will be at load
site 0 while a 5V signal (max) will be
at load site 255. However, most engine
sensors do not produce a voltage that
goes as low as 0V; the minimum is
usually several hundred millivolts
above 0V.
For example, the minimum might be
320mV DC and the maximum might
only be 4V, ie, a range from 0.32V to 4V.
After input processing, this signal
is amplified to cover a range of 1-5V.
For this signal range, load sites from
0 to 51 will not be available and this
reduces the overall adjustment points
to just 205 compared to the available
256. To improve this, a minimum set
adjustment is included to allow the
lower adjustment points to be used.
Interceptor circuit details
Our first Voltage Interceptor was
published in 2004 in SILICON CHIP’s
Performance Electronics for Cars.
Called the Digital Fuel Adjuster (DFA),
it proved very popular and is still
available as a Jaycar kit (Cat. KC-5385).
This Voltage Interceptor is considerably upgraded, with the ability to
adjust the output at 256 points instead
of the 128 for the DFA. Its circuitry also
uses far less components while offering better performance, greater output
accuracy and extra adjustments. While
the DFA used eight ICs, the Voltage
Interceptor uses only four (three dual
op amps and the micro) and it fits into
a more compact case.
Now let’s have a look at the full circuit in Fig.3. The input signal comes
in at the top lefthand corner of the circuit and is connected to a relay which
initially bypasses the input directly
to the output. This is to prevent the
car’s ECU from recording a fault code
with the Voltage Interceptor, before the
engine is started.
After the relay, the sensor signal is
connected to op amp IC1a. This is set
up as an inverting amplifier with a gain
of -0.47. Its input impedance is 1MΩ,
to provide a minimal load to sensitive
sensors (oxygen sensors, for example),
that can be affected by loading.
IC1b is also an inverting amplifier
and it can be set to provide gain or
attenuation, using trimpot VR2. The
gain range is from -0.2 to -12.2. This
is sufficient to boost a 1V signal to 5V
or when set to provide attenuation, a
15V signal can be reduced to 5V, suitsiliconchip.com.au
Parts List
1 PC board, code 05112091, 105
x 87mm
1 diecast box, 119 x 94 x 34mm,
Jaycar HB-5067 or equivalent
1 TO-220 mini heatsink, 19 x 19 x
9.5mm
1 DB25 female PC-mount connect
or (Altronics P-3250 or equivalent)
4 extension screws, spacers and
nuts for DB25 connector
1 2-way pin header with 2.54mm
spacing
1 jumper shunt to suit header
1 12V DPDT relay (Jaycar SY4059, Altronics S 4150 or
equivalent)
2 2-way screw terminals with
5.04mm spacing
2 3-6.5mm cable glands
3 DIP8 IC sockets
1 DIP18 IC socket
4 M3 x 6mm tapped Nylon
spacers
8 M3 x 4mm screws
1 M3 x 6mm screw
1 M3 nut
6 PC stakes
Semiconductors
1 PIC16F88-I/P microcontroller
programmed with 0511209A.hex
(IC4)
3 LMC6482AIN dual CMOS op
amps (IC1-IC3)
1 LM317T adjustable 3-terminal
regulator (REG1)
2 BC337 NPN transistors (Q1,Q3)
1 BC547 NPN transistor (Q2)
1 BC327 PNP transistor (Q4)
1 16V 1W zener diode (ZD1)
able for the 5V maximum required by
microcontroller IC4.
Op amp IC2b is an inverting amplifier with a gain of -1. A 5V signal from
IC1b would produce a -5V level from
IC2b’s output. IC2b’s output can be
level-shifted by trimpot VR6 but we
will give more detail about this later.
IC3b is an inverting amplifier with a
gain that can be varied from between
-0.142 when the wiper of VR7 is set
toward the 10kΩ resistor and -3 when
VR7 is set towards the 20kΩ resistor.
This adjustment has sufficient range
to set the maximum output anywhere
from 1V up to 12V.
1 15V 1W zener diode (ZD2)
1 7.5V 1W zener diode (ZD3)
1 5.6V 1W zener diode (ZD4)
2 1N4004 diodes (D1,D2)
2 1N4148 diodes (D3,D4)
1 3mm red LED (LED1)
Capacitors
5 100µF 16V PC electrolytic
2 10µF 16V PC
3 100nF MKT polyester
7 10nF MKT polyester
1 1nF MKT polyester
Resistors (0.25W, 1%)
1 1MΩ
3 1kΩ
1 470kΩ
1 470Ω 0.5W
1 100kΩ
1 220Ω
1 43kΩ
1 150Ω
1 22kΩ
1 120Ω
2 20kΩ
1 100Ω
10 10kΩ
1 10Ω 1W
1 9.1kΩ
7 0Ω links
2 2.2kΩ
Trimpots
1 500Ω multi-turn top adjust trimpot (Bourns 3296 type) (code
500) (VR1)
2 1kΩ multi-turn top adjust trimpot
(Bourns 3296 type) (code 102)
(VR3,VR4)
1 10kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code
103) (VR5)
2 50kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code
503) (VR6,VR7)
1 100kΩ multi-turn top adjust trimpot (Bourns 3296 type) (code
104) (VR2)
IC4 is the PIC16F88-I/P microcontroller. The signal from IC1b is fed to
its AN4 input (pin 3) and is converted
into an 8-bit digital value with 256
possible levels.
There are two voltage references for
this analog-to-digital conversion: the
5V REF+ and an adjustable voltage,
REF-. The 256 conversion levels are
measured between these two references. When REF- is set to 0V, the
conversion range is 0-5V and the 256
levels are about 19.5mV apart.
REF- is adjustable using trimpot VR3
which is connected to pin 1 of IC4
and bypassed using a 10µF capacitor.
December 2009 27
Pushbutton Controller
Parts List
1 PC board, code 05104073,115
x 65mm
1 front panel label
1 plastic case, 120 x 70 x 30mm
with clear lid (Jaycar HB-6082
or equivalent)
1 LCD module (Jaycar QP-5515
or backlit QP-5516)
5 white click-action switches
(S1, S2, S5, S7, S9)
4 black click action switches
(S3, S4, S6, S8)
1 SPST micro tactile switch with
0.7mm actuator (S10)
1 4017 decade counter (IC1)
1 DIL 14-way pin header
1 DB25 PC-mount right-angle
socket
1 1.8m DB25-pin male to DB25pin male RS-232 connecting
lead (all pins connected) (Jaycar WC-7502 or equivalent)
4 M3 x 12mm tapped plastic
spacers
4 M3 x 6mm CSK screws
2 M3 x 6mm screws
2 M3 x 12mm plastic screws
2 2.5mm thick plastic washers
1 100mm length of 0.7mm tinned
copper wire or 2 x 0Ω resistors
1 10µF 16V PC electrolytic
capacitor
2 10kΩ 0.25W 1 % resistors
1 7-way, 8-way or 9-way 330Ω
terminating resistor array (8-10
leads). Note: six resistors are
used in the circuit and one end
of each resistor connects to the
pin 1 common
1 10kΩ horizontal trimpot (code
103) (VR1)
REF- allows us to optimise the analogto-digital conversion of input voltages
that do not go as low as 0V.
For example, a sensor may have a
range from 0.8V to 4V. Gain adjustment
for IC1b would be set so the AN4 input
receives 1V to 5V from the 0.8V to 4V
sensor signal. With REF- set to 1V then,
a 1V signal will be converted to a digital value of 0 instead of 51 (as noted
above). The 5V would be converted
to 255 and so the full 0-255 range of
digital sites would be available.
Each of those 256 site values can be
changed by the Voltage Interceptor.
28 Silicon Chip
Specifications
Voltage input range: 0-15V maximum
Voltage output: typically set for 0-1V or 0-5V but can cover any range up to 12V
Minimum input voltage adjustment: from 0-2.5V
Output adjustment: ±127 steps
Adjustment range: from 0V through to the full output range
Adjustment resolution: 39mV for a ±5V adjustment range at maximum sensitivity
(finer resolution is available by sacrificing adjustment range)
Input adjustment points: 0-255 corresponding to 19.5mV steps for a 0-5V input
Output adjustment change response: 512µs response plus 10ms to alter to
within 10% of the new value
Display update time: 250ms
The output changes are manipulated
by a control signal from pin 9 of the
micro. This is a pulse width modulated
(PWM) signal running at 7.843kHz
which varies its duty cycle. The PWM
signal is then filtered using a 100kΩ
resistor and a 100nF capacitor to produce an average DC voltage.
The resulting output is 0V for 0%
duty cycle and +5V for 100% duty
cycle. A duty cycle of 50% gives +2.5V
and this is the midpoint which results
in no change in the sensor signal from
input to output.
In essence, the resulting control
voltage from the micro is used to
“offset” the output of op amp IC2b.
The offset voltage either adds to or
subtracts from the DC voltage that otherwise would have been delivered by
IC2b. But first, the DC control voltage
from the micro has to be level-shifted
by op amps IC2a & IC3a.
As stated, at 50% duty cycle, the
voltage at pin 3 of IC2a is 2.5V. IC2a
amplifies this by 1.91 due to its 10kΩ
and 9.1kΩ feedback resistors. With
nothing else happening, IC2a’s output
would be +4.775, or approximately
+4.78V. However, IC3a provides an
offset voltage for pin 2 of IC2a so that
its output actually sits at 0V. This
works as follows.
Trimpot VR5 feeds a voltage to pin 3
of IC3a which amplifies it by a factor of
2. In practice, VR5 is set to feed 2.63V
to pin 3 of IC3a and so the output at pin
1 is +5.25V. IC2a then amplifies this
by -0.91 (9.1kΩ/10kΩ) to give -4.78V
and this will exactly cancel the +4.78V
which would have been there from the
input to pin 3 (of IC2a). Hence, the
output from IC2a is set to 0V.
Then with the normal duty cycle
variation from pin 9 of IC4, IC2a’s
output will swing above and below
0V by ±4.78V. However, there is a
further adjustment of the signal from
IC2a which is fed to Sensitivity trimpot
VR6; it sets the overall level of voltage
applied to IC2b.
When the wiper of VR6 is at 0V,
there is no change in output from the
Voltage Interceptor with PWM changes. When set so the wiper is towards
the 43kΩ resistor, the maximum offset
shift for the Voltage Interceptor output
is available.
Power supply
Power for the circuit comes from
the car’s 12V supply which can rise
to about +14.4V when the battery is
being charged. Diode D1 is included to
protect the circuit from reverse supply
connection. Following this diode we
derive the 5.6V supply using zener
diode ZD4 and a 470Ω series resistor. A
nominal +11.4V supply is provided via
a 100Ω resistor and 15V zener diode
ZD2. The zener diode is included to
suppress transient voltages which are
present on 12V car supplies. The 11.4V
rail supplies op amp IC3.
An LM317T 3-pin adjustable voltage regulator (REG1) is used to derive
the main +5V rail. The incoming 12V
supply is fed to REG1 via a 10Ω resistor while 16V zener diode ZD1 clamps
any voltage transients, with further
filtering provided by a 100µF capacitor.
Op amp supplies
We have specified three LM6482AIN
dual op amps for this project. These
have a very low input offset voltage
of 110µV (typical), an extremely high
input impedance of more than 10 Tersiliconchip.com.au
DB25
SOCKET
+5V
5
14
2
3
15
10k
10k
1
16
Vdd
CP0
CP1
IC1
4017B
MR
13
12
O5-9
8
Vss
O0 O1 O2 O3 O4 O5 O6 O7 O8 O9
3
2
4
7
5
6
10 1
9
11
S1
S3
S2
S5
S4
S7
S6
VR1
10k
10 mF
3
S9
S8
4
S10
6
9
14
13
6
12
8
11
10
10
11
9
12
8
4
6 x 330 Ω*
DB7
LCD
DISPLAY
MODULE
DB6
DB5
DB4
DB3
DB2
5
SWITCH FUNCTIONS
S1 RIGHT
S6
S2 UP
S7
LEFT
S3 STEP RIGHT
S8
STEP LEFT
S9
DOWN
S4 STEP DOWN
S5 VIEW/RUN
PUSHBUTTON CONTROLLER
R/W
2
STEP UP
* USES 7 x 330 Ω RESISTOR ARRAY
SC
RS
EN
DB1
7 DB0
13
2007
CONTRAST
S10 RESET
Fig.4: the circuit for the Pushbutton Controller is quite simple. It uses 10 switches, an LCD module, a 4017 counter
(IC1), a DB25 socket, a 10m
mF capacitor and a few resistors. Trimpot VR1 sets the display contrast.
aohms (>10TΩ), a 4pA input bias current, an output swing to within 10mV
of the supply rails and a wide common
mode input range which includes the
supply rails. In other word, this is a
pretty special op amp package.
Furthermore, the three op amp
packages have different supply voltage
requirements. For IC1a, the output is
expected to swing from 0V and negative by no more than -7.05V. IC1b is
required to swing from 0V and positive
up to 5.00V. Hence, IC1 uses the +5.6V
and -7.5V supply rails.
For IC2a, the output will swing over
a maximum of ±4.78V, as noted above.
IC2b will typically swing from 0V to
-5V. Hence, IC2 uses the main +5V rail
and the same -7.5V rail as used by IC1.
IC3a’s output is usually fixed at
close to 5V, as noted above, while
IC3b’s output is designed to swing
from a typical 12V maximum down to
0V. Supply for this is 0V for the negative supply and a nominal 11.4V for
the positive supply. This 11.4V supply
will vary with the car battery voltage
and must be over 13V (as it normally
siliconchip.com.au
This is the view inside the completed Voltage Interceptor
unit. It’s build on a single PC board and is housed in a sturdy
diecast metal case. The construction details will be in Pt.2 next month.
December 2009 29
Using The Pushbutton Controller
As already noted, the controller has a 2-line 16-character Liquid Crystal Display
(LCD) and nine pushbutton switches to do the programming. In fact, it is the same
controller that was used in the Programmable Ignition System from the March, April
& May 2007 issues of SILICON CHIP and it is available as a Jaycar kit, Cat KC-5386.
On the top line, the display shows OUTPUT then the output value and either (dV)
or LOCK. The OUTPUT refers to the up or down adjustment made and is 0 when there
is no change in the output compared to the input. Values can be altered in number
by up to ±127. The (dV) is the delta Voltage and is an abbreviation for the change in
voltage made to the output.
If the word LOCK is displayed instead, it means that a jumper link has been installed
preventing any adjustment to the output settings using the push button switches.
On the lower line of the display, it shows INPUT and then a number (from 1-255)
and then either /RUN/ or (VIEW). The INPUT number refers to the way the input value
has been divided up into 256 sections from minimum through to maximum and shows
the particular input value load site and its corresponding output value (on the top line).
The /RUN/ display shows input load sites in real time as they follow any input
voltage variation.
A (VIEW) display does not show the current input value that is connected to the
Voltage Interceptor but the input value selected by the pushbutton switches. This display allows the whole input/output map to be viewed by scrolling through each value.
The display is changed between /RUN/ and (VIEW) using the Run/View switch.
Pressing this switch toggles between the two alternatives. Up and Down switches are
used to change the output value for each input value. A fast up and fast down switch
is also included to increment the values in steps of 4 instead of steps of 1.
Scroll left and scroll right buttons provide for changing the input value when set for
the view display. These switches do not operate for the run display.
A reset button is included and must be accessed using a small probe that inserts
into a hole in the front panel. Pressing this switch for 4s resets all output adjustments
back to 0. The display shows RESET on the top line when this reset is successful.
will be) for the output to reach 12V.
So how do we generate the -7.5V
rail? This task is performed by the micro, as well as doing its main function
in providing the main Interceptor offset function. It delivers a 975Hz square
wave output from its RB2 output at pin
8. This drives a charge pump circuit
comprising transistors Q2, Q3 & Q4,
diodes D3 & D4 and capacitors C1 & C2.
Transistor Q2 and the 2.2kΩ resistor
act as a level shifter, converting the 5V
square wave from RB2 to a 12V square
wave. Complementary transistors Q3
& Q4 buffer this 12V square wave and
drive the charge pump. When the
square wave is high at about 12V, Q3
switches on and charges C1 to almost
12V via diode D3.
When the 12V square wave is at
0V, transistor Q4 switches on and the
positive side of C1 is pulled down to
0V. The negative side of C1 is therefore
close to -12V and this charges capacitor C2 (negatively) via the now conducting diode D4. The process repeats
with C1 charging and delivering its
charge to C2. This provides the nega30 Silicon Chip
tive voltage supply that is regulated to
7.5V using zener diode ZD3 and the
220Ω series resistor.
Relay operation
As already noted, relay RLY1 bypasses the Interceptor circuitry before
the car’s engine is started, to avoid the
ECU recording a fault condition. The
relay is switched on when IC4’s RB1
output goes high and drives transistor
Q1 via a 1kΩ resistor. Q1 then drives
the relay coil and indicator LED1.
Diode D2 clamps the reverse voltage
from the coil when it is switched off
to prevent damage to Q1.
The relay is switched on at a preset
supply voltage. This can be either as
soon as a 12V supply is connected to
the Voltage Interceptor or at a higher
voltage. The higher voltage threshold
prevents the relay from switching until
the car engine has started, after which
the alternator increases the supply rail,
ie, as the battery is charged.
The relay switching voltage is set
using trimpot VR4. Its wiper is monitored by the AN3 input (pin 2) of IC4
and the setting can be measured at test
point TP4. The voltage is scaled so
that 1.3V at TP4 gives a 13V threshold.
Similarly, for a 12V threshold, TP4
would be set to 1.2V with VR4.
The battery supply voltage is measured at the AN1 input (pin 18) of IC4.
The 12V input is divided down by
a factor of three using the 20kΩ and
10kΩ resistors and so the AN1 input
will be at 5V for a 15V supply.
Pushbutton controller
The 25-pin socket shown on the
lefthand side of the circuit (Fig.3) is for
connection to the Pushbutton Controller. A jumper link (Lock: LK1) at the
RB0 input to IC4 prevents the pushbutton controller from making any
changes to the output. In this LOCK
state, the input and output values can
be viewed but not altered.
The circuit for the Pushbutton Controller is shown in Fig.4. It comprises
an LCD module, a 4017 decade counter
(lC1), a DB25 socket and several pushbutton switches. This unit connects to
the 25-pin connector in the main Voltage Interceptor circuit via a standard
DB25 RS232 cable.
Signals from the microcontroller in
the Voltage Interceptor drive both the
LCD module and counter IC1. IC1 has
10 outputs and each output independently goes high in sequence as it is
clocked at its clock input (pin 14). A
high at the reset (MR, pin 15) sets the
“0” output at pin 3 high.
Each output connects to a switch.
When a switch is closed, it pulls pin 9
of the DB25 socket high whenever its
corresponding output on IC1 is high.
This allows the microcontroller in the
Voltage Interceptor to recognise which
switch is closed.
The LCD is driven using data lines
DB7-DB4. The display readings are
entered via the data lines and are
controlled via the EN and RS (Enable
and Register Select) inputs.
Note that the data lines and the
EN and RS lines are all connected to
ground via 330Ω resistors. These resistors terminate the signals correctly to
prevent false data at the LCD from the
long DB25 cable interconnection. The
resistors also tie all inputs low when
the DB25 cable is not connected
Finally, trimpot VRl is used to adjust
the display contrast.
That’s all for this month. Next
month, we’ll give the construction
details and describe how it’s used. SC
siliconchip.com.au
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