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Items relevant to "Build A Knock Indicator For Leaded-Petrol Engines":
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Knocking can cause
serious damage
to an engine. This
simple circuit warns
you when engine
knock is occurring,
so that you can ease
up and avoid costly
engine damage.
Do you drive an old car? If so, build this . . .
Knock indicator for
leaded-petrol engines
By JOHN CLARKE
D
RIVERS OF OLD CARS are facing
an increasing problem. With
the progressive decrease in the lead
content of super grade petrol, many
older engines are starting to “ping”
(or knock) when called on to deliver
the goods. This pinging effect typically occurs when the engine is under
load (eg, when lugging up a hill), or
during periods of moderate to heavy
acceleration.
Even fairly light engine loads can
cause pinging in severe cases.
The reason for this is that the reduced lead content in super grade
petrol has lowered its octane rating.
And that in turn means that the fuel
is more disposed to pre-detonation,
particularly in high-compression en-
gines. Modern engines designed to run
on lead-free petrol avoid this problem
by running lower compression ratios
than the old leaded engines.
In addition, modern engines use
devices known as knock sensors.
These sensors typically screw into the
engine block and listen for the onset
of knocking. If knocking is detected,
they feed a signal to the engine management system which then retards
Fig.1: block diagram
of the Engine Knock
Indicator. Signals picked
up by the knock sensor
are amplified, filtered
and fed to a rectifier to
derive a DC voltage. This
voltage is then fed to a
LED bargraph display,
which indicates the
knock severity.
72 Silicon Chip
Fig.2: the final circuit diagram. IC1a, IC1b & IC1c are the amplifier and filter
stages, D1 is the rectifier and IC2 is the LED bargraph display driver. IC1d and
Q1 ensure that the circuit only “listens” for engine knock while the coil is firing.
the ignition timing so that knocking
ceases.
On older cars, knocking can sometimes be alleviated by retarding the
static ignition timing and/or by altering the weights in the distributor to
change the centrifugal advance curve.
On some leaded cars, however, the
ignition timing was controlled electronically and could not be altered, so
this is not option. The VK Commodore
is one such example.
Another problem with older cars
is that most are now well past the
100,000km mark and are no longer
carefully maintained. Often, the
ignition system will be in need of
adjustment or the head could do with
a decoke. The build up of carbon deposits on the head of an old engine
can be a major cause of pinging,
because it gets hot and pre-ignites
the fuel.
Stopping an old engine from ping-
ing is usually easier said than done.
Although it’s sometimes possible to
have the engine modified, such modifications are usually expensive and
not regarded as economically viable.
As a result, drivers of older cars either ignore the problem or, if they are
aware of it, drive so that engine knock
is minimised.
More often than not, however, the
problem is one of ignorance. Many
drivers do not know what pinging
is and just com
pletely ignore the
characteristic noise coming from
the engine. Unfortunately, this can
April 1996 73
The LED bargraph display was mounted with its top surface 27mm above the PC
board, so that it would protrude through a matching slot in the lid of the case.
Note that shielded cable is used to connect to the knock sensor.
cause severe engine damage and lead
to costly repairs. Pinging can cause
piston and valve damage, blown head
gaskets, excessive bearing wear and
overheating (which in turn can distort
the head).
In severe cases, holes can even be
burnt through the piston crowns.
Knock indicator
Although it cannot stop an engine
from pinging, this simple Engine
Knock Indicator can warn a driver
when pinging is occurring so that the
appropriate action can be taken. This
can be as simple as easing off on the
accelerator or changing back a gear to
reduce the engine load.
As in modern cars, the circuit
monitors the output of a piezoelectric
knock sensor which is attached to the
engine block. This sensor connects
to a dash-mounted unit that carries
a bargraph display. When pinging
occurs, the bargraph display indicates
the severity of the problem on a scale
of 1-10 (minor to severe).
In addition, the unit sounds a buzzer
74 Silicon Chip
to provide an audible warning when
the bargraph reaches step 6.
This sort of easily understandable
feedback allows the driver to quickly
adjust his driving technique so that
engine knock ceases. So if you own
an old “bomb” and you suspect that
it is pinging, take a close look at this
circuit. It could save you a packet in
engine repairs.
There’s just one proviso here – this
circuit is designed to pick up engine
knock under everyday driving conditions. It will not reliably detect
Main Features
•
LED bargraph shows knock
intensity
•
•
Preset sensitivity control
•
Knock severity depends on
repetition rate and intensity
Audible warning when
bargraph reaches threshold
level
engine knock at very high revs or on
a high-performance engine that makes
a lot of noise. In these situations, the
noise from the engine simply swamps
out the knock frequencies that this
circuit is designed to detect (note:
some modern cars get around this by
using special filtering techniques plus
a second sensor that’s specially tuned
to detect knock at high revs).
What is knock?
Before we take a look at the circuit,
let’s take a closer look at what causes
engine knocking.
In simple terms, knocking is caused
by the irregular burning or explosion
of the fuel-air mixture in the combustion chamber of the engine. The result
is widely varying cylinder pressures
that vibrate the engine components. By
contrast, a correctly burning mixture
within the combustion chamber produces a smooth pressure that causes
a steady increase in the acceleration
of the piston.
When an engine knocks it does so
at a particular frequency and this can
be calculated as follows:
F = 900/πr
where F is the frequency in hertz and
Fig.3a (right): the
parts layout on the PC
board. Make sure that
you don’t get ZD1 and
ZD2 mixed up and
note that they face in
opposite directions to
each other, as do the
ICs. Fig.3b (far right)
shows the full-size
etching pattern.
r is the cylinder radius in metres. For
most cars, this equates to a frequency
somewhere between 800Hz and 5kHz.
In addition, the major knock sounds
become audible from 0-60° after top
dead centre.
Designing an engine knock indicator can be difficult since it must be
able to discriminate between knock
and all the other noises produced by
the mechanical action of the engine.
These noises include those produced
by the valve operation, chain drives,
pumps, camshaft and crankshaft, plus
any other mechanical noise makers
which can mask the knock. One way
to filter out these unwanted sounds is
to only “listen” for knock during the
time that it occurs.
sufficient level and then fed to highpass and low-pass filter stages. These
effectively select only the frequency
band of interest (800Hz to 5kHz).
Following the filters, the signal is
rectified and filtered. It is then fed to
a LED bargraph display. The number
of lit LEDs in the bargraph depends on
the knock intensity and repetition rate.
The audible warning is provided
when LED 6 on the bargraph lights.
This is detected by Q2 and Q3 which
in turn drive a buzzer.
Block diagram
Fig.1 shows a block diagram of the
circuit arrangement. The knock sensor
consists of a piezo element which
is attached to the engine block. The
resulting signal is first amplified to a
TABLE 1: RESISTOR COLOUR CODES
❏
No.
❏ 1
❏ 1
❏ 1
❏ 2
❏ 3
❏ 1
❏ 8
❏ 1
❏ 1
❏ 1
❏ 1
❏ 2
❏ 1
Value
1MΩ
100kΩ
27kΩ
18kΩ
15kΩ
12kΩ
10kΩ
9.1kΩ
6.2kΩ
2.2kΩ
1.2kΩ
1kΩ
10Ω
4-Band Code (1%)
brown black green brown
brown black yellow brown
red violet orange brown
brown grey orange brown
brown green orange brown
brown red orange brown
brown black orange brown
white brown red brown
blue red red brown
red red red brown
brown red red brown
brown black red brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
brown black black orange brown
red violet black red brown
brown grey black red brown
brown green black red brown
brown red black red brown
brown black black red brown
white brown black brown brown
blue red black brown brown
red red black brown brown
brown red black brown brown
brown black black brown brown
brown black black gold brown
April 1996 75
Fig.4: basic detail for a do-it-yourself knock sensor. The
piezo element is scrounged from a crystal earpiece.
The piezo element is removed from the earpiece by
first carefully cutting the housing at the glued joint.
Schmitt trigger stage IC1d monitors
the ignition coil primary to provide a
dwell gate signal for the rectifier/filter
stage. This ensures that the rectifier/filter stage only receives signal from the
low pass filter during the time that the
ignition coil is firing; ie, when there is
a high voltage on the switched side of
the ignition coil primary.
This measure effectively restricts
the “listening” time of the circuit to the
coil firing period, when knock is most
likely to occur. At other times, signals
from the low pass filter are “blocked”,
to prevent false alarms which may be
generated during the remainder of the
ignition cycle.
Circuit details
Refer now to Fig.2 for the full circuit
details. There are two ICs, 10 LEDs,
76 Silicon Chip
This close-up view shows how the piezo element is mounted
on the baseplate. The cover comes from a 16mm pot.
Use shielded cable to make the connections to
the knock sensor before the cover is fitted.
three transistors, a regulator and a few
other minor parts.
IC1 is an LM324 quad op amp
package which performs the signal
processing. IC1a amplifies the signal
generated by the piezo transducer. Its
gain can be varied from one to 201,
as set by 200kΩ trimpot VR1 and the
1kΩ resistor on pin 9. Its frequency response is rolled off below about 600Hz
by the associated 0.27µF capacitor,
while the 120pF capacitor across VR1
restricts the high frequency response.
The output from IC1a appears at pin
8 and is fed to high-pass filter stage
IC1b. This stage rolls off frequencies
below 800Hz, as set by the RC filter
network on the input. The signal is
then fed to 5kHz low-pass filter stage
IC1c. As a result, IC1b & IC1c together
form a bandpass filter which passes
signals only in the range from 800Hz
to 5kHz.
Note that IC1a, IC1b and IC1c are all
biased at about half supply using common 12kΩ and 10kΩ voltage divider
resistors. This bias voltage is filtered
using a 100µF capacitor.
The bandpass filtered signal appears
at pin 1 of IC1c and is rectified and filtered using diode D1 and its associated
1µF capacitor. The charging time is set
by a 1.2kΩ resistor which prevents
transient signals from providing false
indications on the meter.
IC1d and Q1 provide the gating
signal. In operation, the ignition coil
input is fed to a voltage divider network and clamped to 6.8V using zener
diode ZD2. The ignition coil signal is
then fed to pin 6 of IC1d.
Op amp IC1d is wired as an in-
This commercial
knock sensor is
from a Daihatsu
Mira and worked
quite well with the
circuit described
here.
verting Schmitt trigger. This means
that when the ignition coil input is at
ground (ie, when the points close or
the coil switching transistor turns on),
IC1d’s pin 7 output is high. This turns
on transistor Q1 which then shunts
the signal output from IC1c to ground.
Conversely, when the ignition coil
is firing, pin 6 of IC1d is high (+6.8V)
and so pin 7 goes low. Transistor Q1
is now off and so the signal from IC1c
is fed to the rectifier and filter stage.
The output from the rectifier/filter
stage is fed to IC2, a 10-LED dot/
bargraph display driver wired here
in bargraph mode (pin 9 high). This
device provides a linear output for
signals ranging from RLO (ie, approximately half supply) to RHI. In other
words, the voltage between RLO and
RHI sets the full-scale voltage of the
display.
In operation, the REF OUT voltage
(pin 7) sits 1.25V above the voltage at
REF ADJ (and RLO). The voltage on
RHI is then set by an internal 10kΩ
resistor string (to RLO) and the external 15kΩ resistor. As a result, RHI sits
about 0.5V above RLO which means
that the display has a full-scale voltage
of 0.5V.
The 2.2kΩ resistor between pin 7
and ground sets the LED brightness.
Transistors Q2 and Q3 monitor
pin 14 (LED 6) of IC2. When LED 6
lights, pin 14 goes low and Q2 turns
on. This then turns on Q3, which
drives the buzzer to provide an audible warning. D2 protects Q3 from
high back-EMF voltages when the
buzzer turns off.
Power for the circuit is derived
via the ignition switch. The +12V
supply is fed to 3-terminal regulator
REG1 which provides an 8V rail for
the ICs. The buzzer is powered from
the +12V rail at the input of REG1.
ZD1 and the 10Ω resistor protect the
PARTS LIST
1 PC board, code 05302961,
102 x 59mm
1 plastic case, 130 x 67 x 43mm
1 self-adhesive front panel label,
123 x 60mm
1 10-LED bargraph display
(LED1-LED10)
1 12V buzzer
1 200kΩ miniature trimpot (VR1)
1 3mm screw and nut
6 PC stakes
1 large grommet
regulator against high voltage transients which may be present on the
ignition supply.
Construction
The prototype Engine Knock Sensor was built on a PC board coded
05302961 and measuring 102 x 59mm.
This board clips neatly into a standard
plastic case (130 x 67 x 43mm).
Fig.3a shows the parts layout on the
board. Before starting the assembly,
check the board carefully for any defects in the etching pattern. This done,
install PC stakes at the six external
wiring points, then install the links
and resistors.
Table 1 shows the resistor colour
code but it is also a good idea to check
each value on a digital multimeter,
as some colours can be difficult to
decipher.
The diodes and zener diodes can go
in next. Note that ZD1 and ZD2 face in
opposite directions and that they have
different values, so be careful not to
mix them up. Similarly, note that D1
is a 1N4148, while D2 is a more rugged
1N4004 type.
Take care when installing the ICs, as
they also face in opposite directions
(pin 1 is adjacent to a notch or dot in
the body of the IC – see Fig.3). Once
the ICs are in, the capacitors and transistors can be installed. Note that Q2
is a BC558 PNP type, while the others
are BC338 NPN types.
The 3-terminal regulator (REG1)
is mounted with its metal tab flat
against the PC board and is secured
with a screw and nut. Bend its leads
through 90°, so that they pass through
their designated holes. This done, fit
trimpot VR1 to the board.
The LED bargraph array must be
installed with its anode (A) adjacent to
the 1MΩ resistor – see Fig.3. It should
be mounted so that the top surface of
Sensor
1 crystal earpiece, DSE Cat.
C-2765
1 cheap TO-3 transistor or
equivalent baseplate (to make
sensor)
1 16mm pot (for sensor cover)
1 solder lug
1 3mm screw and nut
Semiconductors
1 LM324 quad op amp (IC1)
1 LM3914 10-LED bargraph
driver (IC2)
2 BC338 NPN transistors
(Q1,Q3)
1 BC558 PNP transistor (Q2)
1 7808 regulator (REG1)
1 16V 1W zener diode (ZD1)
1 6.8V 1W zener diode (ZD2)
1 1N4148 signal diode (D1)
1 1N4004 diode (D2)
Capacitors
2 100µF 16VW PC electrolytic
2 10µF 16VW PC electrolytic
1 1µF 16VW PC electrolytic
1 0.27µF MKT polyester
3 .015µF MKT polyester
1 .0047µF MKT polyester
1 .0015µF MKT polyester
1 .0012µF MKT polyester
1 120pF ceramic
Resistors (0.25W 1%)
1 1MΩ
1 9.1kΩ
1 100kΩ
1 6.2kΩ
1 27kΩ
1 2.2kΩ
2 18kΩ
1 1.2kΩ
3 15kΩ
2 1kΩ
1 12kΩ
1 10Ω
8 10kΩ
Miscellaneous
Automotive hook-up wire, shielded
cable, tinned copper wire, heat
shrink tubing, bullet terminals,
solder, etc.
April 1996 77
1 2 3 4 5 6 7 8 9 10
MINOR
SEVERE
ENGINE KNOCK
INDICATOR
Fig.5: this full-size artwork can be used as a template when making the notch for
the LED bargraph display.
the display is 27mm above the board,
so that it will later fit into a matching
slot cut into the lid of the case.
Once completed, the PC board can
be installed inside the case and flying
leads connected to the power supply,
ignition coil, buzzer and knock sensor
wiring points.
These leads pass through a grommeted hole drilled in one end of the
case. The slot in the front panel for
the bargraph display is made by first
attaching the label and then using this
as a drilling template to give a rough
knockout. The slot can then be carefully filed to shape.
Knock sensor
The easiest way of obtaining a knock
sensor is to scrounge a commercial
unit from a wrecking yard. The commercial knock sensor shown in one of
the photos is from a Daihatsu and this
worked quite well with the circuit.
Alternatively, you can make your
own knock sensor. We made ours
using a piezo transducer taken from
an earpiece. This was mounted on a
TO-3 transistor baseplate and clamped
in position using the rear enclosure
from a 16mm pot. If you don’t have
a transistor baseplate, or don’t want
to destroy a perfectly good transistor,
you can make up your own baseplate
using 3mm steel or brass.
Fig.4 shows the details of our
home-made sensor. The pot cover is
secured by soldering its lugs to the
TO-3 baseplate.
The transistor package is modified
by first cutting the cap off the baseplate using a hacksaw. The two leads
are then removed by breaking them
78 Silicon Chip
Fig.6: basic scheme for connecting
multiple coils to the ignition
input. An extra diode should be
added for each additional coil.
off with pliers and the baseplate filed
to a smooth finish. Warning – transistors can use dangerous materials
inside. Use rubber gloves during this
process and a facemask and goggles
when cutting and filing the baseplate.
Wash both the transistor baseplate and
your hands after the work has been
completed.
Next, one of the transistor mounting holes is enlarged to accept the
mounting bolt (the prototype sensor
was mounted on the edge of the rocker
cover using an existing bolt into the
head).
The piezo element is removed from
the earpiece by first carefully cutting
around the outside of the housing at
the glued joint. This done, carefully
prise the element from the plastic
housing using a knife. You should
leave the wire attached to the top of
the element intact and remove the wire
from the larger lower plate.
The piezo element is now centred
on the baseplate (larger plate down)
and secured using the pot cover – see
Fig.4. Be sure to pass the lead under
the pot enclosure and protect it with
heatshrink tubing before soldering the tangs of the pot cover to
the baseplate.
Finally, bolt a solder lug to
one of the baseplate mounting
holes and connect a suitable
length of shielded cable to the
transducer, so that is can be
wired back to the circuit board.
We used heatshrink tubing to
help secure the wiring.
Testing
To test the circuit, first apply
power and check that pin 4 of
IC1 and pin 3 of IC3 are at 8V.
If this is correct, switch off and
connect the knock sensor wire to
the sensor input on the PC board.
You should also connect the case
of the sensor to the GND terminal (via
the shielded cable braid).
Next, short the base and emitter
terminals of Q1 using a clip lead, set
VR1 fully clockwise and apply power.
If you now lightly tap the knock sensor
with a screwdriver, the LEDs in the
bargraph display should light. Adjust
VR1, so that the display just reaches
the 10th LED each time the sensor is
tapped.
Assuming everything is operating
correctly, remove the short between
the base and emitter of Q1.
Installation
Be sure to install this unit in a
professional manner. The display
should be mounted where it can be
easily seen by the driver, while the
buzzer can be either mounted inside
the case (drill a few holes to let the
sound out) or installed under the
dashboard.
The GND connection can be made
via an eyelet lug screwed to the chassis, while the +12V ignition supply rail
should be derived from the fusebox
using automotive connectors. Make
sure that this rail is fused and only
goes to +12V when the ignition is
switched on.
In most cases, the only wires passing
through the firewall will be to the ignition coil and to the piezo sensor. Be
sure to connect the ignition coil lead
to the switched side of the coil (ie, to
the negative terminal). Do not connect
to the coil lead to the EHT terminal.
If your car uses multiple-coil ignition, use the circuit shown in Fig.6 to
make the connections (add an extra
diode for each extra coil).
The PC board clips into a standard plastic case and the leads brought out
through a grommeted hole. These leads go to the negative side of the ignition
coil, to the power supply (+12V & ground), to the buzzer and to the sensor.
Important: the ignition coil lead
will have up to 500V on it when the
coil is firing and so must be well insulated from the chassis. It would also
be wise to insulate the ignition coil
terminal on the PC board to prevent
accidental contact.
The piezo sensor is best mounted
on the engine block using an existing
bolt. As a second preference, it can be
attached to the head. As mentioned
above, we secured our sensor using
one of the rocker cover securing bolts.
Once the unit has been installed,
start the engine and adjust VR1 so that
the display is just off for all engine
revs while the car is in neutral. This
effectively provides maximum sensitivity for knock signals without also
detecting normal engine noise.
Finally, the unit can be tested by
deliberately provoking engine knock
on the road (don’t overdo this though).
This can be done by lugging up a steep
hill in a higher gear than normal. If
the unit fails to respond to knocking
or is overly sensitive, then it’s simply
a matter of slightly adjusting VR1 for
the correct response.
Now you will always be warned
when engine knock is occurring,
regardless of how loud your kids are
screaming or how far your sound sysSC
tem is cranked up.
20 Electronic Projects For Cars
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April 1996 79
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