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Items relevant to "A Low Fuel Indicator For Your Car":
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Ever run out of petrol? This simple
project reminds you to fill 'er up
by lighting a 12V lamp when_
the fuel drops below a preset
level.
A low fuel
indicator for your car
By JOHN CLARKE & GREG SWAIN
OST LATE-MODEL cars have a
low-fuel warning light but any
vehicle that's more than about
five years old probably lacks this important feature. This means that you
have to remember to check the fuel
gauge regularly to avoid runn~ng out
of petrol
Well that's the general theory but if
you drive a "gas-guzzler", it's all too
M
easy to get caught. Result - an empty
tank and you're left stranded. Often,
this is only inconvenient but running
out of petrol can also be dangerous.
That's why late-model cars have a
low-fuel warning lamp. It's easy to
neglect a gauge but a carefully positioned warning lamp ("idiot" light?)
is much more visible and consequently harder to ignore.
The Low Fuel Indicator described
here is easy to build and can be installed in most vehicles. It lights a
12V warning lamp when the fuel in
the petrol tank drops below a preset
level. If you've ever run out of petrol
or have come close to doing so, this
project is well worth building. It could
save you quite a few dollars , not to
mention embarrassment and inconvenience.
Types of sensors
Apart from the lamp, there are just three external wiring connections from the
Low Fuel Indicator: to +12V via the ignition switch, to vehicle chassis & to the
fuel sensor lead. The warning lamp lights when the critical point is reached.
38
SILICON CHIP
In most cars, the fuel sensor consists of a float-controlled rheostat
which is wired in series with the fuel
gauge - see Fig, 1. These two components are then connected to a regulated supply rail derived from the battery. As the fuel level varies, the resistance of the rheostat also varies
and this in turn controls the current
through the fuel gauge (ie, the fuel
gauge acts as a milliammeter).
Usually, the higher the float level,
the lower the resistance of the rheostat and the higher the current through
the fuel gauge. Some circuits work in
the reverse sense to this though, so
our clrcuit has been designed to accommodate both situations. All you
have to do is change the location of a
single wire link.
B_e warned, however, that this circuit is not suitable for cars that have
capacitance-type fuel sensors. In this
type of sensor, the fuel acts as a dielectric between two parallel plates
PETROL TANK
METER
SENDER
r---------,
+V FROM
IGNITION
''
I ______
":'
L
_ ___ JI
which form a capacitor. As far as we
know, only the Ford Falcon (model
XD onwards) uses this type of sensor
but you should still check to ensure
your car has a rheostat sensor before
starting this project.
How it works
Let's now take a look at the circuit
of the Low Fuel Indicator - see Fig.2.
It works by monitoring the voltage
across the rheostat in the fuel sensor.
In greater detail, the voltage from
the fuel sensor is first filtered by a
47kQ input resistor and a ZZ0µF electrolytic capacitor which combine to
provide a 10-second response time.
This 10s delay is built in to prevent
false triggering near the preset level
due to fuel sloshing about in the tank.
IC1a, an LM339 comparator, compares the filtered sensor voltage on its
pin 8 input with a reference voltage
on pin 9. This reference voltage is
derived from a regulated 9.1 V rail via
a lOkQ resistor and trimpot VR1. It is
fed via another lOkn resistor to pin 9
of IC la, to pin 7 of IC1 b and to pin 4 of
IC1c.
Note that because the LM339 has
PARTS LIST
Fig.1: in most cars, the fuel
level sensor consists of a
float-controlled rheostat.
This controls the current
through a meter which then
indicates the fuel level.
1 PC board, code 05203931, 61
x46mm
1 12V panel mount indicator
lamp
1 10kQ horizontal trimpot (VR1)
5 PC stakes
1 3mm machine screw and nut
open collector outputs, pin 14 ofIC1a
is fitted with an external 10kQ pullup resistor. Thus, depending on the
fuel level, pin 14 ofICla will be either
high or low. The lMQ feedback resistor provides the stage with a degree of
hysteresis, so that it switches cleanly
at the transition point and eliminates
lamp flicker.
ICla drives comparator stages IClb
and IClc. One of these stages then
drives Darlington transistor Ql, depending on whether link LKl or link
LKZ is installed. If the fuel sensor
voltage increases with decreasing fuel
level, LKl is installed and Ql is controlled by IClb. Conversely, if the sensor voltage falls with decreasing fuel
level, LKZ is installed and Ql is controlled by IClc.
Let's assume initially that the sensor voltage rises with decreasing fuel
level (ie, LKl must be installed). Thus,
when the tank is full , the sensor voltage will be low and pin 14 ofICla will
be high (ie, at +9.lV). Pin 1 of IClb
will thus be low and so Ql and the
warning lamp will both be off.
As the fuel level falls, the voltage
on pin 8 of ICla steadily increases.
+V FROM
I G N I T I O N D - - - - - - - - - - - - - - - - - - - - - - - ---<11--- - - - .
SWITCH
ZD1
9.IV
1W
100 +
18VW+
10k
1M
10k
.,.
FROM FUEL
SENSOR
2.2k
FUEL
LEVEL
SET
VRI 10k
47k
220 +
18VW+
47k
CHASSIS~
12V
LAMP
SW MAX
Semiconductors
1 LM339 quad comparator (IC1)
1 BO679 NPN Darlington
transistor (01)
1 9.1 V 1W zener diode (ZD1)
Capacitors
1 220µF 16VW PC electrolytic
1 100µF 16VW PG electrolytic
Resistors (0.25W 1%)
1 1Mn
2 47kn
310kW
1 2.2kn
1 150!1
1 10Q 5W
Miscellaneous
Automotive hook-up wire,
automotive connectors.
When the fuel drops below the preset
level, pin 14 ofICla switches low and
pin 1 of1C1b is pulled high by a z.zkn
pull-up resistor. This high turns on
Ql and lights the warning lamp. The
lamp then remains lit until the fuel
tank is filled above the preset level (or
until the ignition is switched off).
Now let's consider the circuit action where the sensor voltage falls
Fig.2: the circuit uses IC1a to
compare the voltage from the fuel
sensor with a reference voltage
set by VR1. When the fuel in the
tank drops below the critical
level, pin 14 ofIC1a changes
state. This then switches on Q1 &
the lamp, either via IC1b & LK1
or via IC1c & LK2.
Q1
B0879
.,.
.,.
PLASTIC
SIDE
rn
E
B
C
LOW FUEL INDICATOR
.,.
FEBRUARY
1993
39
FROM F
SENSO
0
c.n
ro
CHAS
0
w
....w
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Fig.3: install LKl if the sensor voltage rises as the
fuel level drops. Alternatively, install LK2 if the
sensor voltage falls with decreasing fuel level.
Fig.4: check your PC board for defects against
this full size etching pattern before mounting
any of the parts.
with decreasing fuel level (ie, LKZ
05203931 and measures 61 x 46mm.
must be installed). In this case, pin 14
Fig.3 shows the assembly details.
of IC la is normally low and thus pin 2 Before starting construction, you first
of IClc is also low.
have to test the sensor to discover
If the fuel level now drops below whether link LKl or link LKZ should
the preset level, pin 14 switches high be installed. To do this, locate the
and so pin 2 of IClc is pulled high by gauge terminal that connects to the
the 2.ZkQ pull-up resistor. This then fuel sensor and short it to chassis via
turns on Ql via link LKZ and lights a l0Q 5W resistor with the ignition
the lamp as before.
on. If the fuel gauge reading goes up,
The level at which the circuit trig- . install LKl. Conversely, if the reading
gers is set by adjusting VRl. Normally, goes down, install LKZ.
this would be set so that the lamp
The remaining parts can be installed
comes on just before the fuel gauge in any order but make sure that ICl,
ZDl and the two electrolytic capacineedle dips into the red danger zone.
Power for the circuit is derived from tors are correctly oriented. We used
the car's battery via the ignition PC stakes at the five external wiring
switch. This +12V (nominal) supply points but these can be considered
powers the indicator lamp directly to
optional.
ensure maximum brightness when Ql
Ql should be mounted flat against
turns on.
the PC board with its plastic side facZDl and its parallel lO0µF filter ing upwards. Bend its leads at rightcapacitor provide the regulated 9.1 V angles so that they pass through the
supply rail. This supplies ICl and the holes in the PC board before securing
voltage divider for the reference volt- it using a screw and nut. Trim away
age, and ensures that the circuit is any excess lead lengths after solderunaffected by fluctuations in the bat- ing.
tery voltage. ZDl also protects ICl by
clipping any voltage spikes that may Installation
Because the PC board is so small, it
be generated by the electrical system.
can be easily wrapped in foam insulaConstruction
tion and hidden under the dashboard.
All the parts for the Low Fuel Indi- The warning lamp can be mounted
cator (except the lamp) are mounted directly on the dashboard and conon a small PC board. This is coded nected to the PC board via flying leads.
Alternatively, the board could be fitted inside a small plastic case, along
with the warning lamp, and the entire
assembly then mounted on top of the
dashboard.
The remaining connections are to
+12V via the ignition switch, to chassis, and to the fuel sensor lead. If you
have trouble identifying the sensor
lead, check the colour code in the
manual or check the lead at the fuel
sensor.
Use automotive connectors to interface to the vehicle wiring and solder the other ends of the leads to the
PC stakes on the circuit board.
Calibration
The best way to calibrate the unit is
to wait until the fuel gauge reaches
the critical level and then adjust VRl
so that the device just triggers. This
critical level will normally be just
above the red zone.
If want to calibrate the unit immediately, disconnect the fuel sensor lead
at the gauge and connect a lO0Q
potentiometer between the gauge and
chassis. Adjust this potentiometer until the gauge indicates the desired critical fuel level, then adjust VRl on the
PC board until the lamp just comes
on.
That's it. Now there's no excuse for
ever running out of petrol.
SC
RESISTOR COLOUR CODES
0
0
0
0
0
0
40
No.
Value
_4-Band Code (1%)
5-Band Code (1%)
1
2
1MQ
47kQ
10kQ
2.2kQ
150Q
brown black green brown
yellow violet orange brown
brown black orange brown
red red red brown
brown green brown brown
brown black black yellow brown
yellow violet black red brown
brown black black red brown
red red black brown brown
brown green black black brown
3
1
1
SILICON CHIP
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