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Any sensor that outputs a varying voltage
can be used by the Simple Voltage Switch
to turn things on and off . . . intercooler
sprays, boost control solenoids, warning
lights, fans, water injection – you name it!
Design by JOHN CLARKE
Simple Voltage Switch
For Car Sensors
This Simple Voltage Switch can be used anywhere you want a relay to
switch when a voltage reaches a preset level. It has lots of applications
in cars but can be used in any application where you have 12V DC
available. Having switched the relay on, it will then switch if off as the
voltage being monitored drops below the preset level.
I
N CAR APPLICATIONS, many
engine sensors have variable voltage outputs and these can be used for
relay switching. For example, if your
car has an air-flow meter with a voltage output (most cars have), then you
Main Features
• Adjustable switching level
between 0V and 16V at input
• DPDT 5A relay
• Configurable to switch on
rising or falling voltage
• Adjustable hysteresis
• High input impedance – won’t
load down sensors
72 Silicon Chip
can use that as an engine load signal
to switch things on and off.
For example, do you want a warning
when fuel usage is going through the
roof, as it will be when the air-flow
is high? If you use this project, it can
turn on a light and/or sound a buzzer
so you can ease off on the accelerator
or change down a gear, or both.
Or you could use the throttle position sensor directly, to do the same
thing. Or going back to the air-flow
sensor, in a turbocharged engine, you
could use the Simple Voltage Switch
to run a solenoid to close off the turbo
waste from the boost pressure source
whenever engine loads are low.
Or maybe you could use the unit to
control a water spray onto the intercooler. We are sure that you will be
able to think of plenty of nifty ideas.
This project was first presented in
our “Performance Electronics for Cars”
book published a few years ago and we
are re-presenting it this issue to give it
a wider exposure.
It is quite simple in presentation;
just a PC board with a relay and a
handful of other components. You
should be able to assemble it in less
than one hour.
Circuit description
Fig.1 shows the circuit of the Simple
Voltage Switch. It relies on comparator IC1a, which compares the input
voltage to a preset reference level.
The input voltage (VIN) is divided via
two 1MΩ resistors in series which
effectively apply one half of the voltsiliconchip.com.au
Fig.3: the circuit is based on comparators IC1a & IC1b. IC1a compares the input voltage (VIN) to a reference voltage
set by trimpot VR1 and switches its output (pin 1) high or low accordingly. IC1b acts as an inverter, while link LK1
allows the circuit to be set to trigger on either a rising voltage or a falling voltage. The selected comparator output
drives transistor Q1 & the relay.
age to the inverting input, pin 2, of
IC1a. Zener diode ZD2 and the 100nF
capacitor are there to protect against
transient voltages on the input signal.
IC1a’s non-inverting input (pin 3) is
connected to reference trimpot VR1,
via a 10kΩ resistor. When pin 2 is
above pin 3, IC1a’s pin 1 output is low,
ie, close to 0V. When pin 2 is below
pin 3, pin 1 is high, at around +10V.
Hysteresis (positive feedback from
pin 1 to pin 3) has been added to prevent the output from oscillating at the
trigger voltage. This is provided via
trimpot VR2 and diode D3.
This feedback causes the output to
“pull” the voltage at pin 3 either higher
or lower, depending on whether the
output at pin 1 is high or low and also
on the orientation of diode D3. If D3
is installed as shown (ie, anode to pin
3), the voltage on pin 3 will be pulled
lower than the reference voltage set by
VR1 when IC1a’s output (pin 1) goes
low. However, if pin 1 is high, D3 will
be reverse biased and the reference
voltage is unaffected.
siliconchip.com.au
Conversely, if D3 is installed the
other way around (cathode to pin 3),
pin 3 will be pulled higher than the
reference voltage if IC1a’s output goes
high.
In practice, this means that diode
D3 is inserted with its anode towards
pin 3 if you want the Simple Voltage
Switch to trigger on a low to high
(L\H) transition and with its cathode
towards pin 3 if you want it to trigger
on a high to low (H\L) transition.
Basically, the hysteresis is the difference between the switch-on and
switch-off voltages and this is set
using VR2. We need hysteresis in the
circuit otherwise the relay would tend
to switch on and off very rapidly when
Suggested Uses For the Voltage Switch
• Intercooler water spray control (from air-flow meter, throttle position sensor
or oxygen sensor signals)
• Anti-lag turbo wastegate control (operating a wastegate disconnect solenoid
triggered from the air-flow meter signal)
•
•
•
•
Nitrous oxide switching (from throttle position sensor signal)
Intercooler fan control (from air-flow meter signal)
Dashboard monitoring LED (eg, oxygen sensor output signal)
Switching in and out engine management and auto transmission control
modifications (from air-flow meter, throttle position sensor or oxygen
sensor signals)
• Low battery voltage warning and/or disconnect
December 2008 73
Parts List
1 PC board, code 05112081 or
05car061, 106 x 61mm
5 PC-mount 2-way screw terminals, 5mm pin spacing
1 12V PC-mount DPDT 5A relay
(Relay1)
1 3-way header, 2.54mm spacing
1 jumper shunt, 2.54mm spacing
1 1kΩ multi-turn top adjust trimpot (VR1)
1 1MΩ horizontal trimpot (VR2)
Semiconductors
1 LM358 dual op amp (IC1)
1 7808 3-terminal regulator
(REG1)
1 BC337 NPN transistor (Q1)
1 5mm red LED (LED1)
2 16V 1W zener diodes
(ZD1,ZD2)
2 1N4004 1A diodes (D1,D2)
1 1N4148 small signal diode (D3)
Capacitors
2 100μF 16V PC electrolytic
2 10μF 16V PC electrolytic
1 100nF MKT polyester (code
104 or 100n)
Resistors (0.25W, 1%)
2 1MΩ
1 1.8kΩ
1 22kΩ
1 1kΩ
4 10kΩ
1 10Ω
the input voltage is close the to preset
threshold.
IC1b is an inverter and it provides a
signal which is the opposite polarity
to IC1a’s output. It compares IC1a’s
output with the +5.5V set on its noninverting input. When IC1a’s output
goes high, IC1b’s output goes low. And
when IC1a’s output goes low, IC2a’s
output goes high.
Link LK1 provides the option of
driving the relay with a falling (H/L)
input voltage or a rising (L/H) input
voltage, respectively. The output
The Simple Voltage Switch could be used to monitor the oxygen sensor signal,
allowing devices to be turned on or off when the mixture is too rich or too lean.
The unit won’t load down the signal, so it can still be used by the ECU.
selected (either from IC1a or IC1b)
drives transistor Q1 which in turn
drives the relay.
The diode across the relay coil (D2)
is there to quench the reverse voltage
that is generated by the collapsing
magnetic field of the relay coil when
it is switched off. Without the diode,
the relay could generate very high
positive voltages which could blow
the transistor.
Power for the circuit is obtained
from the switched +12V ignition supply. Diode D1 gives reverse connection
protection, while the 10Ω resistor,
100µF capacitor and zener diode ZD1
provide transient protection at the
input of regulator REG1.
The reference circuitry is powered
from the output of REG1 (+8V), while
the remainder of the circuit is powered from the +11.4V rails which are
derived before the regulator.
Construction
While the unit is simple to build,
Resistor Colour Codes
Value
4-Band Code (1%)
5-Band Code (1%)
1MΩ
22kΩ
10kΩ
1.8kΩ
1kΩ
10Ω
brown black green brown
red red orange brown
brown black orange brown
brown grey red brown
brown black red brown
brown black black brown
brown black black yellow brown
red red black red brown
brown black black red brown
brown grey black brown brown
brown black black brown brown
brown black black gold brown
74 Silicon Chip
you need to know one thing about
its eventual application. Will you be
using it to detect a voltage that will
be increasing (L/H) to the preset trip
point or falling (H/L) to the preset trip
point? The unit can be made to work
either way but if you know this before
you assemble it, there will be no need
to make changes when it is ultimately
installed.
The low to high (L/H) voltage condition will be the most common, as
in our example of switching an intercooler spray when the air-flow signal
rises above a particular point, say 4V.
Below 4V, the spray is off and above
4V, the spray comes on.
So ideally, you need to know which
configuration you want before starting
assembly. That way, you will know
how to set the position of the link
on the board and the orientation of
diode D3. On the other hand, if you
do build and later decide to change
the application, it is a simple matter
of changing the link setting and the
orientation of D3.
So for a rising voltage detection,
the moveable link LK1 is placed in
the “L/H” position, as shown in the
component overlay diagram of Fig.2.
Then diode D3 is orientated so that its
cathode band is closest to the top of
the board. For the opposite condition,
detection of a falling voltage, the link
is moved to its “H/L” position and the
diode’s orientation is reversed.
When assembling the PC board we
siliconchip.com.au
D1 1N4004
+
100 µF
10 µF
1k
+
L/H
LK1H/L
L/ H
Q1
+
CN
NC
}
}
RELAY
CONTACTS
1 6 0ra c 5 0 1
CN
22k
C
COM
A
The placement of the link and the
orientation of diode D3 (both circled
here) will depend on whether you
want to activate the switch on a rising
voltage or a falling voltage. As shown
here, the unit is configured to trigger
on a rising voltage, which is the most
common requirement. To trigger on a
falling voltage, reverse the orientation
of diode D3 and move the link to the
H/L position.
ON
NO
100 µF
H/L
K
10k
1M
100nF
ZD2
niV
Vin
COM
NC
1
10 µF
1M
K
1M
NO
RELAY
CONTACTS 2
A
LED1
IC1
LM358
+
1N
4148
ZD1 A
K
10k
K
10k
GND
D3*
MAX
10k
REG1
7808
VR2
1.8k
MIN
K
D2
1N4004
10Ω
+12V
A
VR1 1k
C ON
H CTI WS E GATL OV
K
1N4148
A
RELAY 1
A
BC337
* REVERSE D3 IF LK1
IS IN 'H/L' POSITION
Fig.1: install the parts on the PC board as shown on this layout diagram.
Don’t forget to reverse D3 if LK1 is placed in the H/L position, ie, if you
want the unit to trigger on a falling voltage instead of a rising voltage.
suggest that you start with the resistors
and diodes and then progress to the
larger components. Carefully check
each component value before you install it and make sure that you insert
the polarised components (diodes,
IC, LED, transistor, voltage regulator
and electrolytic capacitors) with the
correct polarity.
Testing it
Test the kit at your workbench (or
kitchen table) to make sure that it is
working, as it should. Do not be tempted to install it straight into your car
or other application before you know
that it is definitely working properly.
You will need a 12V battery or DC
power supply and a variable voltage,
to simulate the sensor output that the
unit will be monitoring. The easiest
way to do this is as is shown in the
photo on page 76 – it’s just a matter
of connecting a pot (eg, 10kΩ or more)
across the supply, to give a 0-12V
variable voltage at the wiper terminal.
Connect the DC supply and a potentiometer, as shown in the photo.
Now rotate the potentiometer back
to forth over its full range. At some
point as you are rotating the potentiometer, the relay should click and
LED1 should turn on or off. Rotating
the potentiometer back the other way
should again make the relay click and
switch LED1 back off.
siliconchip.com.au
Fig.2: here is a typical set-up. The Simple Voltage Switch is connected
to an ignition-switched +12V supply rail and to chassis, while the signal
input is wired to the air-flow meter’s output signal. One of the relay’s
normally open (NO) connections is also connected to the ignitionswitched +12V rail, while the adjacent common terminal is connected to
an intercooler water spray pump. The other side of the pump is earthed.
When the engine load exceeds a preset level, the water spray will be
triggered into action.
This view shows the fully-assembled
PC board. Make sure that you install
the polarised components the correct
way around.
December 2008 75
An easy way to bench test the Simple Voltage Switch is to temporarily wire a 10kΩ (or higher) pot across the power
supply to provide a variable signal voltage. An adjustable 0-12V will be available on the centre terminal of the pot.
Here, the blue wire connects this variable voltage to the signal input of the Simple Voltage Switch. Connect the +12V
and earth terminals to the red and black wires respectively and you can easily test the operation of the device.
Using a multimeter, measure the
voltage at the signal input (ie, connect
the positive probe of the multimeter to
the signal wire and the negative probe
to earth) and measure the voltage at
which the unit is activating the relay.
For example, with the unit arranged to
read rising voltages, as you gradually
raise the input voltage the unit might
turn on at 6.00V.
Now very slowly reduce the voltage
to see at what point the relay turns off.
You might find that the latter voltage
is 5.7V, meaning that the hysteresis
(the difference between the switchon and switch-off voltages) is 0.3V.
Rotate VR2, the hysteresis pot, to make
sure that the hysteresis changes. For
example, with a switch-on voltage of
5.00V the switch-off voltage might
now be only 4.96V; but a hysteresis
of just 0.04V is making it too critical!
As you rotate VR2 clockwise, the
hysteresis will increase. Note that
changing the hysteresis will not
change the trip point, allowing the
two to be set individually.
Next, you can test VR1, which sets
the trip point. As you turn VR1 clockwise, the trip voltage will increase.
VR1 is a multi-turn trimpot, so that
the trip point can be set very precisely.
Note that you can keep on turning this
type of trimpot endlessly and never
reach a clear “stop”.
Installation
Fitting the unit to a car is straightforward. You will need to provide an
ignition-switched +12V supply, earth
(chassis) and the connection to the
76 Silicon Chip
sensor signal you want to monitor. For
example, if you are triggering the unit
from the air-flow meter, you’ll need
to use the workshop manual and a
multimeter to find this wire. You will
need to confirm that it has a voltage on
it that rises with engine load and you
will need to drive the car to do this.
The device to be switched by the relay will be connected to the Normally
Open and Common relay contacts.
Fig.2 shows these connections.
Note that because a double-pole,
double-throw (DPDT) relay has been
used, another independent circuit
can be switched simultaneously. This
other circuit can even turn off the second device as the first is switched on.
If you want to simply monitor a
voltage such as that from the oxygen
sensor, you can delete the relay and
mount the LED on the dashboard. In
this way, the LED will come on when
the fuel mixture is rich, flash when the
mixture is oscillating in closed loop
mode, and turn off when the mixture
is lean.
Setting it up
There are two ways of setting up the
Simple Voltage Switch:
(1) Measure the sensor voltage and
then set up the unit on the bench to
operate at this voltage. This will probably avoid any need for fine-tuning
in the car.
(2) Do the complete set-up on the car
itself.
If you are using an oxygen sensor to
trip the unit, then the first way is better.
For example, if you want the unit to
trip when the sensor signal rises above
0.6V, then set it up on the bench to do
this. When you subsequently install
the unit in the car, you will only need
to make a small adjustment to VR1.
However, if you want to turn on a
device when monitoring the air-flow
meter, it’s best to do it on the car, because the air-flow meter signal varies
over a much wider range.
When setting up, set the hysteresis
pot to its minimum setting (ie, fully
anticlockwise) and then adjust the trip
point until the unit triggers when you
want it to. If the relay tends to chatter
around the trip point, rotate VR2 clockwise to increase the hysteresis. When
it is tripping at the correct voltage,
check how long the device continues
to operate as the voltage again drops
(assuming the unit is set to trip on a
rising voltage).
For example, if you are using the
unit to trip an intercooler water spray
on the basis of air-flow output, does the
spray go off fairly quickly as the load
again drops? In some applications, the
hysteresis setting will be critical while
in other applications it won’t matter
much at all.
In most cases, once the unit has been
set up, it won’t need to be altered.
The PC board fits into a standard
130 x 68 x 42mm jiffy box, so when
the system is working correctly the
board can be fitted into the box and
installed under the dash or wherever
SC
it is convenient.
Footnote: a kit for this project is available
from Jaycar Electronics, Cat. KC5377.
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