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Items relevant to "A Rev Limiter For Cars":
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By JOHN CLARKE
Don’t blow your
engine – fit this
rev limiter
Do you regularly rev your engine to the
red line on your car’s tacho? Have you
ever missed a gear change and spun
the engine to squillions of revs? Or
have you actually blown your engine
by over-revving it? If so, you need this
rev limiter. It can save thousands of
dollars damage to your engine.
80 Silicon Chip
This versatile rev limiter works by
blocking some of the ignition sparks
when the engine exceeds the preset
limit. It is not a “hard” limiter which
kills the engine RPM by stopping fuel
and all the sparks from the ignition
system. Instead, it blocks about 50%
of the sparks once you exceed the preset rev limit. So instead of suddenly
“running into a wall” your engine runs
out of puff” and it won’t be damaged.
The rev limiter incorporates three
indicator lamps, two (green) to indicate that you’re approaching the RPM
limit and the third (red) to indicate
that “rev limiting” is occurring.
Features
•
•
•
•
•
•
•
Limits engine RPM by ignition
spark reduction
Uses Hall Effect, points, low
voltage signal or reluctor input or ignition coil to measure
RPM
Adjustable limit for RPM
restriction
Two prelimiting warning indicators
One limit warning indicator
Can be used as a gear
change indicator
Single component selection to
suit most engines
As an alternative use, this project
could be employed as a simple gear
change indicator, with or without the
bonus of rev limiting.
If you have a performance engine in
your car and it has a typical 5-speed
manual gearbox, you already know
how easy it is to spin the engine out to
and beyond its red line on the tacho.
The red line is not an arbitrary limit
but is based on a judgment made by
the car manufacturer about risk of
damage to your engine.
Provided you drive below the red
line, your engine should have a long
life, all other things being equal. But
exceed that limit and you risk doing
serious damage and even catastrophic failure, such as putting a con rod
through the side of the block.
The risk of damage to your engine
is much greater if you exceed the red
line when the engine is unloaded, as it
is if you happen to miss a gear change
when accelerating strongly. So if your
car is capable of high performance and
you are keen to push it to the limit at
every opportunity, then you really do
need a rev limiter.
Of course, some modern cars already have very effective rev limiters
built into their engine management
systems but the majority of cars do not
have this very worthwhile protection.
The Rev Limiter comes in two parts.
The Rev Limit Controller is housed in
a small plastic instrument case which
can be mounted on your car’s dash
panel. It has three lights on the front
Fig.1: this block diagram shows
the frequency to voltage converter
and the three comparators of the
Rev Limit Controller. Comparator
3 controls the operation of the
Ignition Switcher board.
Fig.2: the LM2917 frequency-to-voltage converter monitors
the spark rate as a measure of engine RPM.
panel and an on/off switch. Four trimpot adjustments set the sensitivity and
the RPM thresholds for the three indicator lamps and the rev limit itself.
The Rev Limit Controller operates the
Ignition Switcher which is a modified
version of the Engine Immobiliser
circuit which was published in the
December 1998 issue of SILICON CHIP.
The Ignition Switcher operates by
shorting out the engine’s ignition coil
switching transistor (or the ignition
points) about 50% of the time. This
severely restricts engine power and
hence limits the RPM. The Rev Limit
Controller monitors engine RPM and
is connected to the ignition trigger
system which can be Hall effect pickup, reluctor pickup or a low voltage
signal from the engine management
computer to the ignition switching
transistor.
If you have a conventional points
ignition (ie, Kettering not transistor-assisted or CDI), there is a bit of a
problem. The engine speed monitoring will take place at the same point
as the ignition blocking action and
therefore the rev limiting action may
be inconsistent and will tend to give
quite rough engine operation when
limiting is occurring. Mind you, we
assume that there will not be too
many performance engines which
don’t have some sort of high energy
ignition system.
Block diagram
Fig.1 shows the block diagram, embracing both parts of the Rev Limiter.
The signal from the ignition pickup
is processed in a frequency-to-voltage
April 1999 81
Fig.3: the Rev Limit Controller uses the LM2917 and three comparators to
control the indicator lamps and the Ignition Switcher board. Once the red-line
limit is reached, the Ignition Switcher cuts out around 8 sparks in every 16,
effectively cutting engine power and preventing a further rise in engine speed.
converter which produces a DC voltage which is proportional to the input
frequency. The frequency-to-voltage
converter is the well-proven LM2917
and its block diagram is shown in
Fig.2.
The output from the frequency-to-voltage converter is fed to three
comparators, one of them inside IC1.
Two of the comparators drive warning
lamps to warn the driver of the onset
of rev limiting while the third comparator actually controls the Ignition
Switcher board.
The Ignition Switcher must be set
so that it only blocks out a nominal 8
sparks in every 16. It switches on at
a rate which is fast enough to reduce
engine power but not produce any
noticeable jerking in the engine which
would be the case if it switched at a
lower rate.
The setting to switch out 8 sparks
in 16 (a duty cycle of 50%) is fairly
critical. If more sparks are switched
out, there is a higher risk of backfire,
while less sparks cut out will mean
82 Silicon Chip
less power reduction and rev limiting
will be less effective.
Circuit description
Fig.3 shows the circuit for the Rev
Limit Controller while Fig.4 shows
the circuit for the Ignition Switcher.
Fig.3 comprises two ICs and a
regulator plus several transistors and
passive components. There are two input circuits, one for a reluctor pickup
and the other for the remaining types
of engine ignition triggers. Only one
of these should be used at any time.
The signal from the ignition points
or Hall effect input is fed to a voltage
divider comprising 22kΩ and 10kΩ resistors bypassed by a .056µF capacitor.
The signal is then AC-coupled via a
1µF capacitor to a 10kΩ resistor and
a 4.7V zener diode to provide signal
clamping. The 1kΩ input resistor is
there to provide a low voltage signal
input point such as the 5V signal from
an engine management computer.
Further filtering is provided at this
point using another .056µF capacitor
before the signal is applied via a 1kΩ
resistor to pin 1 of IC1.
The reluctor input uses a 1µF coupling capacitor to provide isolation
from the trigger circuit used on the engine ignition while a 100pF capacitor
filters out any high-frequency hash.
The signal is then applied to the base
of transistor Q4 via 47kΩ and 220kΩ
resistors and a 470pF speed-up capacitor. The collector of Q4 is normally
low and a negative-going reluc
tor
signal switches off Q4 which then
has its collector pulled high via the
10kΩ resistor. This signal is applied
to pin 1 of IC1.
Following the op amp comparator
within IC1 is a charge pump. This
basically switches charge from the
.033µF capacitor at pin 2 to the 2.2µF
capacitor connected to pins 3 & 4. This
occurs on each comparator detection
of a signal on pin 1. The 10kΩ resistor
and trimpot VR4 at pin 3 discharge
the 2.2µF capacitor to provide a time
constant for the charge pump circuit.
VR4 provides the calibration adjustment for the circuit.
A second comparator within IC1
monitors the voltage at pins 3 & 4.
The inverting input of this internal
comparator (pin 10) connects to
trimpot VR1 which sets the threshold
voltage. The comparator output (pin
8) is an open-collector transistor and
this output drives transistor Q1. When
pin 3 of IC1 goes above pin 10, pin 8
goes low and this turns on transistor
Q1 and Lamp 1 then lights up.
Comparators IC2a & IC2b also monitor the pin 3 output of IC1. IC2a’s
output goes low when its pin 2 goes
higher than the preset voltage from
trimpot VR2 at pin 3. When this happens, transistor Q2 turns on and this
lights Lamp 2.
IC2b operates in a similar manner
to IC2a and has a threshold set by
trimpot VR3. It drives Q3 which lights
Lamp 3 and it also goes low to drive
the Ignition Switcher circuit shown
in Fig.4.
Power for the circuit comes from the
car’s ignition switch, switch S1 and
a 10Ω resistor to a 16V zener diode
which provides protection from any
spike voltages. From there it goes to
a 3-terminal regulator REG1 which
provides a 5V supply for IC1. IC2, the
transistors and the Lamps run from
the +12V rail.
Ignition switcher
As noted above, the Ignition
Switcher circuit in Fig.4 is an adaptation of the Engine Immobiliser circuit
which appeared in the December 1998
issue of SILICON CHIP. This circuit
uses a single 555 timer IC and four
transistors.
Q1 is a high-voltage Darlington tran-
This view shows the assembled Ignition Switcher PC board. It’s virtually
identical to the Engine Immobiliser circuit published in the December 1998
issue of SILICON CHIP.
sistor designed for ignition systems.
It can switch the heavy coil current
and can withstand the voltages that
are produced across the coil (typically
around 250V peak) when the engine
is running normally. The four 75V
zener diodes between the collector
and emitter of Q1 prevent voltages
over 300V from damaging the device.
Normally, the Ignition Switcher
circuit is quiescent (ie, not active)
and transistor Q1 is off. The circuit
is activated by a low signal from the
Rev Limit Controller and this turns
transistor Q4 off.
When this happens, 555 timer (IC1)
is able to oscillate, at a frequency determined by the two 100kΩ resistors
and capacitor C1, connected to pins
2, 6 & 7.
The resultant waveform at pin 3 is
a square wave. Each time pin 3 goes
high it turns on Q3 and this turns on
Q2 and Q1. Each time Q1 turns on,
it effectively shorts out the ignition
points or the main ignition coil driver
transistor (in a transistorised ignition
system). And each time this happens,
no sparks are delivered to the engine.
Spark switching rate
C1 must be selected to suit the
rev limit for your engine. To do this,
you must do a simple calculation, as
follows:
Spark rate = revs x sparks/rev ÷ 60.
Fig.4: based on our previous Engine Immobiliser circuit, the Ignition Switcher shorts out the
main switching transistor in the car’s ignition system, effectively removing 8 out of every 16
sparks, once the red-line limit is reached.
April 1999 83
Table 1: Choosing C1
Spark Rate
up to 250sp/s
250 to 300sp/s
300 to 350sp/s
350 to 420sp/s
420 to 500sp/s
500 to 600sp/s
C1
0.47µF
0.39µF
0.33µF
0.27µF
0.22µF
0.18µF
The figure for revs is the red-line
limit for your car’s engine. The figure
for sparks/rev is the number of firing
strokes per revolution of your engine.
For example, a 4-cylinder (4-stroke)
engine has two firing strokes/revolution, a 6-cylinder has three firing
strokes/revolution and a V8 has four
firing strokes/revolution.
You multiply these two figures and
divide by 60 to get a result in sparks
per second. For example, if you have
a 6-cylinder engine with a 6000 RPM
red-line limit, multiplying 3 by 6000
and dividing by 60 gives a result
of 300 sparks/second. If you have a
4-cylinder with a 8000 RPM limit, the
result is 267 sparks/second and for a
V8 with a 5000 RPM limit, the result
is 333 sparks per second. This should
give you the picture.
The value for C1 can then be chosen
from Table 1.
Note that C1 does not set the rev
limit. This is done by setting trimpots
VR3 & VR4 on the Rev Limit Controller. C1 merely sets the number of
sparks which are blocked out during
the limiting action at the specified
RPM.
Fig.5: use this component layout for the Rev Limit Controller circuit if your
car has a reluctor distributor. Check your etched PC board carefully for defects
before installing any of the parts and make sure that all polarised parts are correctly oriented (transistors, diodes, ICs, electrolytic capacitors, etc).
Table 2: Resistor Colour Codes
No.
2
1
2
2
2
1
8
3
6
5
2
84 Silicon Chip
Value
4.7MΩ
470kΩ
220kΩ
100kΩ
47kΩ
22kΩ
10kΩ
4.7kΩ
2.2kΩ
1kΩ
10Ω
4-Band Code (1%)
yellow violet green brown
yellow violet yellow brown
red red yellow brown
brown black yellow brown
yellow violet orange brown
red red orange brown
brown black orange brown
yellow violet red brown
red red red brown
brown black red brown
brown black black brown
5-Band Code (1%)
yellow violet black yellow brown
yellow violet black orange brown
red red black orange brown
brown black black orange brown
yellow violet black red brown
red red black red brown
brown black black red brown
yellow violet black brown brown
red red black brown brown
brown black black brown brown
brown black black gold brown
Table 3: Capacitor Codes
Value
IEC Code EIA Code
1µF 1u 105
0.1µF
100n 104
.056µF 56n 563
.033µF 33n 333
470pF 471 470
100pF 101 100
Power for the Ignition Switcher is
taken from switch S1 in the Rev Limiter circuit. Diode D2 isolates the circuit
and a 0.1µF capacitor decouples the
supply to transistors Q2 & Q3. IC1 is
protected from voltage transients by
the 10Ω resistor in series with the
supply and the 16V zener diode ZD1.
The 100µF capacitor decouples the
supply rails.
Construction
The Rev Limit Controller is built on
a PC board measuring 117 x 102 mm
and coded 05304991. This board fits
into a plastic case measuring 140 x
111 x 35mm and we have designed a
label measuring 133 x 27mm for the
front panel.
The Ignition Switcher is built onto a
PC board measuring 106 x 60mm and
coded 05412981. This board can be fitted into a small plastic case measuring
82 x 53 x 30mm or merely fitted with
a sleeve of heatshrink tubing.
Fig.5 shows how the Rev Limit
Controller board is wired for a distributor with reluctor pickup. Fig.6
shows how it should be wired if you
have Hall Effect, points input or low
voltage signal from an engine management computer. Make sure you
use the correct overlay diagram when
assembling this PC board. Fig.7 shows
the component overlay for the Ignition
Switcher board and remember that
you need to consult Table 1 to pick
the value for C1.
You can begin construction by
checking the PC boards for shorts
between tracks and possible breaks
and undrilled holes. Fix any problems
before inserting any components.
Then insert and solder all the links
as shown on the overlay diagrams.
Insert and solder in the resistors,
using Table 2 as guide to the resistor
colour codes. You can also use a digital multimeter to measure each one.
Fig.6: if your car has does not have a reluctor distributor (ie, uses points, Hall
Effect pickup, etc) use this layout to wire up the Rev Limit Controller. Lamps 1 &
2 should be green, while Lamp 3 is red.
Fig.7: this is the layout for the Ignition Switcher board. Note that the zener
diodes (ZD1-ZD5) must all be oriented correctly, otherwise the circuit won’t
work. In particular, note that ZD5 faces in the opposite direction to ZD4. The
assembled board should be enclosed in a plastic case or heatshrink tubing and
mounted under the dashboard.
April 1999 85
The Rev Limit Controller board is mounted inside a standard plastic case (140 x
111 x 35mm). Use automotive hookup wire for all external connections.
Take care with the orientation of the
ICs when you are installing them.
Next, solder in all the diodes, including the zeners, and take care with
their orientation. The transistors can
t
Shop soiled bu
!
HALF PRICE
be installed next and be sure to place
the correct type in each position. Then
insert the capacitors and note that the
electrolytic capacitors must have the
correct polarity. Table 3 shows the
codes which will be shown on the
MKT types.
REG1 is mounted horizontally, with
its metal face towards the PC board.
Bend the leads to insert them into
the holes allo
cated before securing
the regulator with a screw and nut.
Similarly, transistor Q1 on the Igni-
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Traffic Lights Simulator & Points Controller); Level Crossing Detector; Sound & Lights
For Level Crossings; Diesel Sound Simulator.
Our stocks of this book are now limited. All we have left are newsagents’ returns which
means that they may be slightly shop-soiled or have minor cover blemishes.
SPECIAL CLEARANCE PRICE: $3.95 + $3 P&P (Aust. & NZ)
Order by phoning (02) 9979 5644 & quoting your credit card number; or fax the details to (02) 9979 6503; or mail your
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86 Silicon Chip
tion Switcher PC board is mounted
horizontally and with a heatsink
sandwiched between the device and
the PC board. Use a screw and nut to
secure this assembly in place.
Finally, insert the PC stakes and
trimpots.
Case
The front panel of the case requires
holes for trimpot access, power switch
S1 and the indicator lamps. Use the
front panel label as a guide to the
positioning of the holes. You will
also need to drill the holes in the rear
panel for the grommets. Fit the front
panel label in position and cut the
holes out with a sharp hobby knife.
The Rev Limit Controller board and
front panel can be placed in the case
and secured with four self-tapping
screws into the integral standoffs in
the base of the case.
Attach S1 and the lamp bezels in
position and connect hookup wire
from the +12V and GND terminals
on the main PC board and pass these
through the grommet. Similarly connect up wires to the reluctor or coil or
low voltage input which are required
to connect to the ignition trigger output on the engine and to the input of
the Ignition Switcher.
Complete the wiring to switch S1
and to the lamp terminals.
Testing
Starting with the Rev Limit Controller, apply 12V between the +12V and
GND terminals on the main PC board.
This done, check that the output of
regulator REG1 is at +5V.
You will now need to apply some
voltage to pins 3 of IC1 using a 10kΩ
resistor between this pin and the 5V
supply. Now adjust VR1 and check
that the light comes on. Similarly,
check Lamp 2 with VR2 and Lamp 3
with VR3. Check that pin 7 of IC2b
goes low when Lamp 3 is lit.
Note that you can check operation
using a signal generator. Apply signal
to the points input or reluctor terminal
and adjust the output frequency to
monitor operation of the lamps.
Installation
The Rev Limit Controller can
be installed into the vehicle using
automotive connectors to make the
connection to the +12V ignition
supply. Use automotive wire for this
connection. The ground connection
Parts List
Rev Limit Controller
1 PC board, code 05304991, 117
x 102mm
1 plastic case, 140 x 111 x 35mm
1 front panel label, 133 x 27mm
1 SPDT toggle switch (S1)
2 green 12V indicator lamps and
bezels (Lamp 1, Lamp 2)
1 red 12V indicator lamp and
bezel (Lamp 3)
1 M3 screw and nut
4 self-tapping screws
2 small rubber grommets
1 100mm length of 0.8mm tinned
copper wire
1 1m length of twin figure-8
medium duty wire
1 1m length of red medium duty
hookup wire
1 1m length of black medium duty
hookup wire
1 1m length of green medium duty
hookup wire
1 1m length of yellow medium
duty hookup wire
14 PC stakes
3 10kΩ vertical trimpots (VR1VR3)
1 200kΩ vertical trimpot (VR4)
Semiconductors
1 LM2917 frequency-to-voltage
converter (IC1)
1 LM358 dual op amp (IC2)
1 7805 5V 3-terminal regulator
(REG1)
3 BC327 PNP transistors (Q1-Q3)
1 BC337 NPN transistor (Q4)
1 16V 1W zener diode (ZD1)
1 4.7V 1W zener diode (ZD2)
Capacitors
1 100µF 16VW PC electrolytic
3 10µF 16VW PC electrolytic
1 2.2µF 16VW PC electrolytic
1 1µF MKT polyester
1 0.1µF MKT polyester
2 .056µF MKT polyester
can be made to the chassis with an
eyelet and self-tapping screw.
Attach the case in a position convenient to the driver and secure it with
suitable brackets. This done, connect
up the signal input from either the
points, Hall effect, low voltage or reluctor outputs. Now start the engine
1 .033µF MKT polyester
1 470pF ceramic
1 100pF ceramic
Resistors (0.25W, 1%)
2 4.7MΩ
6 10kΩ
1 470kΩ
6 2.2kΩ
2 220kΩ
3 1kΩ
2 47kΩ
1 10Ω
1 22kΩ 0.5W
Miscellaneous
Hookup wire, solder, etc.
Ignition Switcher
1 PC board, code 05412981, 106
x 60mm
4 PC stakes
1 mini heatsink 19 x 19 x 9.5mm
1 M3 x 9mm screw
1 M3 nut
Semiconductors
1 555 timer (IC1)
1 MJH10012, BU941P power
Darlington transistor (Q1)
1 BC327 PNP transistor (Q2)
2 BC337 NPN transistors (Q3,
Q4)
1 16V 1W zener diode (ZD1)
4 75V 3W zener diodes (ZD2ZD5)
1 1N4148, 1N914 signal diode
(D1)
1 1N4004 1A diode (D2)
Capacitors
1 100µF 16VW PC electrolytic
1 0.1µF MKT polyester
1 C1 (see text)
Resistors (0.25W, 1%)
2 100kΩ
2 1kΩ
2 10kΩ
1 82Ω 5W
3 4.7kΩ
1 10Ω
Miscellaneous
Automotive wire, automotive connectors, solder, etc.
and set VR4 to its mid-setting.
To adjust the three trimpots (ie,
VR1, VR2 and VR3), the engine should
be under load. In practice, this means
you need to drive the car along a quiet
(no traffic) street in low gear while a
passenger does the adjustments.
Adjust VR1 so that Lamp 1 lights
April 1999 87
WARNING!
The external leads from the Rev limit Controller pass through two rubber
grommets on the rear panel of the case.
This engine rev limiter
blocks out ignition sparks
and should only be used as a
final protection against engine
damage. It should not be used
to limit engine RPM each time
it is wound out at every gear
change.
The reason for this is that
at limiting there is the risk of
backfire as the exhaust will
contain an explosive mixture
of unburnt fuel. In addition,
the unburnt fuel adds to air
pollution.
about 1000 RPM below the red line.
This done, adjust VR2 so that Lamp
2 lights about 600 RPM below the red
line. Finally, adjust VR3 so that Lamp
3 lights at the red line.
If you have to wind trimpots VR1VR3 fully clockwise in order to turn
on their respective lamps, wind VR4
slightly clockwise. Alternatively, if
these adjustments are too sensitive,
wind VR4 slightly anticlockwise.
Note: this adjustment procedure
is no longer recommended. See page
107 of the October 2007 issue for in
formation on how to adjust the unit
using a signal generator.
Connecting the boards
Fig.8: actual size artwork for the Rev Limit Controller PC board.
Fig.9: actual size artwork for the Ignition Switcher board.
88 Silicon Chip
You can now attach the Ignition
Switcher board to the Rev Limit Controller circuit to test for correct limiting action. The boards can be wired up
using automotive wire, following the
diagrams of Fig.7 and Fig.5 or Fig.6.
We used light duty wires for all
wiring except for the wires to the
ignition coil and ground. Be sure to
ground the Ignition Switcher to a suitable chassis point using an eyelet and
self-tapping screw. This is to allow the
heavy current flow through Q1, when
it is disabling the ignition.
The Ignition Switcher board must
be insulated from the chassis by enclosing it in a plastic case or sleeving
it with heatshrink tubing.
Now test the operation of the limiting action on the engine. The engine
should lose power when limiting is
taking place but we must caution
against driving in this condition for
anything more than a few seconds,
SC
because of the risk of backfire.
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