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Increase your driving safety with
Quick
Brake
Are you concerned about
the risk of a rear end
collision when driving in
traffic? With QuickBrake,
your brake lights come
on faster than you could
ever apply them, giving
you literally metres more
safety.
Words by Julian Edgar
Design by John Clarke
B
Main Features
• Reduces brake light turn-on time by 200ms
• Works with throttle sensors with 0-5V output
• Responds to rapid reduction in throttle sensor
•
•
•
output
Activates relay to power brake lights
Adjustable timer for brake light on period
Power-up delay to prevent false triggering at ignition
switch-on
10 Silicon Chip
ack in the March 2003 issue, we covered the
advantages of LED brake lights on cars – in
addition to longer life and much lower current
drain, LEDs reach full brightness far faster than filament bulbs. And the quicker that you can indicate to
drivers behind you that you’re braking, the less likely
they are to run into the back of your car. In fact, using
LEDs in your brake lights can provide the following
driver with as much as 200ms earlier warning . . . that’s
5.5 metres at 100 km/h.
But with QuickBrake you can do even better than
this and provide another 200-250ms earlier warning!
By combining LED brake lights with QuickBrake, you
can give at least 400ms earlier warning that you’re
stopping – that’s 11 metres at 100km/h. It’s a brilliant
technique that we’ve not seen anywhere else – even
in new cars.
www.siliconchip.com.au
Fig.1: the circuit monitors the car’s throttle position sensor and if a rapid negative transition occurs, the 7555 is
enabled to briefly activate the relay and the car’s brake lights.
Think about what occurs during an
emergency stop. You’re driving along,
mind dwelling on all things interesting – including the other traffic – when
you suddenly realise the cars ahead
are abruptly stopping. You rapidly lift
off the accelerator and then transfer
that foot to the brake pedal, quickly
jabbing down on it. But “rapidly” and
“quickly” are relative terms – in fact
it takes about a quarter of a second
(250 milliseconds) from the time that
you start to lift off the throttle to the
time the brake pedal is pushed and the
brake lights come on.
But why wait that long before illuminating the brake lights? There’s no
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logical reason – only the engineering
tradition of turning on the brake lights
with a brake pedal switch.
So why not trigger the brake lights
when you rapidly lift your foot off
the throttle? “Oh that won’t work”,
you say. Well, why not? With a little
circuitry, you can sense the speed of
the throttle movement quite easily, just
by tapping into the throttle position
sensor. Then, if you have the circuit
detect a rapid reduction in voltage
from the throttle sensor (as happens
when you’re about to stop in a hurry),
you can use a relay to switch on the
brake lights.
Finally, a timer could be used to
hold the relay on to cover the time
between the throttle closing and the
brake light switch being activated.
This is just what our QuickBrake
circuit does. And it’s just uncanny
watching a car fitted with the project
simulate an emergency stop. The brake
lights come on “soooooo” fast that
you suddenly realise that the pause
between deceleration and braking that
normally occurs is quite clearly able
to be seen, even from outside the car.
QuickBrake can be very handy when
you’re plagued with a “tailgater” too.
If someone is following you much
too closely, just lift off the accelerator quickly and the brake lights will
March 2004 11
Fig.2: this diagram shows where each of the components is placed on the PC board. Also shown are the connections
you need to make when installing QuickBrake in your car. The input signal to QuickBrake is derived from the
throttle position sensor output. The Normally Open and Common contacts of the relay are wired in parallel with the
brake light switch. Ignition-switched power and an earth connection finish the wiring.
come on for a brief period, without
you even having to touch the brake
pedal. Nifty, huh?
need to check this point out, before
you buy the kit!
PC board module
Fig.1 shows the circuit of the
QuickBrake which is based on four
op amps (in IC1 & IC2) and a 7555
timer. In effect, the circuit is designed
to detect the rapid change of voltage
from the throttle position sensor and
then close a relay for a brief time. The
relay switches on the brake lamps for
a pre-determined time. In the meantime, if the driver’s foot hits the brake
pedal, the brake lights will stay on. If
not, the brake lights go out when the
relay drops out.
So let’s look at the circuit in more
detail. The DC voltage from the throt-
As shown in the photos, QuickBrake
is a small PC board module measuring 105 x 60mm. It uses the engine
management system’s throttle position
sensor output to monitor the movements of the throttle.
In operation, it is designed to work
with throttle position sensors with
an output voltage that varies within
the range of 0-5V. If your car does not
have engine management or it uses a
throttle position switch (rather than
a potentiometer), QuickBrake cannot
be used. You have been warned – you
Circuit description
Fig.3: check your PC board against this pattern before installing any parts.
12 Silicon Chip
tle position sensor is fed to a low pass
filter consisting of a 1MΩ resistor and
100nF capacitor and then to op amp
IC1a which is connected as a unity
gain buffer. From there, it goes to a
differentiator consisting of a 100nF
capacitor, trimpot VR1 and a 100kΩ resistor. A differentiator can be thought
of as a high pass filter – it lets rapidly
changing signals through but slowly
changing signals are blocked. Putting
it another way, if the rate of change
of the signal is greater (ie, faster) than
the differentiator time constant (RC),
the signal will pass through to op
amp IC1b, which is another unity
gain buffer, and then via link LK1 to
IC2b which is connected as a Schmitt
trigger stage.
The output of IC2b connects to pin
2, the trigger input of IC3, a 7555 timer.
When IC2b briefly pulls pin 2 of IC3
low (as it does for a sudden reduction
in throttle sensor signal), IC3’s pin 3
immediately goes high, turning on
transistor Q1 and RELAY1. This turns
on the brake lights.
At the same time, IC2b’s brief
negative pulse turns on transistor Q2
which pulls the negative side of a
100µF capacitor to 0V and this fully
charges this capacitor to 8V. From this
point, the 100µF capacitor discharges
via trimpot VR2 and the series 1kΩ
resistor. This means that the negative
side of the 100µF rises until it gets to
about +5.3V whereupon pin 3 goes
low and transistor Q1 and the relay
are switched off. The timer period of
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When constructed, your circuit board should
look like this. When assembling the PC board,
make sure that you correctly insert the polarised
components; ie, the diodes, ICs, LED, transistors,
voltage regulator and electrolytic capacitors.
IC3 can be set from around
100ms up to 110 seconds, using VR2. In this QuickBrake application, the timer is set to quite a short
period, typically less than 500ms.
Diode D2 is connected across the
relay coil to quench spike voltages
generated each time transistor Q1
turns off. Q1 also drives LED1, via the
1.8kΩ series resistor and this lights
whenever the relay is energised. It is
handy when you are setting up the
QuickBrake circuit on your car.
Power-up delay
Pin 4 of the 7555 (IC3) is used to
provide a power-up delay. When the
car is first started, we don’t want the
QuickBrake responding to any unpredictable changes in signal from the
throttle sensor; we want all circuit
operating conditions to
have stabilised before
QuickBrake starts operating. Therefore pin 4 of IC3
is connected to a network
comprising a 470µF capacitor, diode
D4, and 39kΩ and 220kΩ resistors.
Initially, the 470µF capacitor is discharged and so pin 4 is low, effectively
disabling IC3 so it cannot respond to
any unwanted trigger signals to its
pin 2.
IC3 is enabled (ie, begins to operate)
when the 470µF capacitor charges to
around +0.7V via the 220kΩ pull-up
resistor. This is after about two seconds. The 39kΩ resistor prevents the
470µF capacitor from charging above
1.2V and this allows it to discharge
quickly via diode D4 when power is
removed from circuit (ie, when the
engine is stopped. This is important so
that QuickBrake is properly disabled
if the engine is immediately restarted.
Power for the circuit comes from
the car battery via diode D4 which
gives reverse connection protection.
The 10Ω resistor, 100µF capacitor and
zener diode ZD1 provide transient
protection for REG1, a 7808 8V regulator. All the circuitry is powered from
REG1, with the exception of the relay
and LED1.
Construction
All the circuitry of QuickBrake is
on a small PC board measuring 105
x 60mm and coded 05103041. The
component overlay diagram is shown
in Fig.2.
Install the resistors first, checking
the values with your multimeter as you
Table 1: Resistor Colour Codes
o
o
o
o
o
o
o
o
o
o
o
No.
2
1
1
1
1
5
1
4
1
1
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Value
1MΩ
220kΩ
100kΩ
39kΩ
11kΩ
10kΩ
1.8kΩ
1kΩ
150Ω
10Ω
4-Band Code (1%)
brown black green brown
red red yellow brown
brown black yellow brown
orange white orange brown
brown brown orange brown
brown black orange brown
brown grey red brown
brown black red brown
brown green brown brown
brown black black brown
5-Band Code (1%)
brown black black yellow brown
red red black orange brown
brown black black orange brown
orange white black red brown
brown brown black red brown
brown black black red brown
brown grey black brown brown
brown black black brown brown
brown green black black brown
brown black black gold brown
March 2004 13
Parts List
1 PC board, code 05103041,
105 x 60mm
5 PC-mount 2-way screw terminals with 5mm pin spacing
1 12V PC-mount DPDT 5A relay
1 3-way header with 2.54mm
spacing
1 jumper shunt with 2.54mm
spacing
1 50mm length of 0.8mm tinned
copper wire
2 1MΩ multi-turn top-adjust
trimpots (VR1,VR2) (Jaycar
RT-4658 or similar)
Semiconductors
2 LM358 dual op amps (IC1,IC2)
1 7555 CMOS 555 timer (IC3)
1 7808 3-terminal regulator
(REG1)
1 BC337 NPN transistor (Q1)
1 BC327 PNP transistor (Q2)
1 5mm red LED (LED1)
2 16V 1W zener diodes
(ZD1,ZD2))
2 1N4004 1A diodes (D1,D2)
2 1N914 diodes (D3,D4)
Capacitors
1 470µF 16V electrolytic
5 100µF 16V PC electrolytic
4 10µF 16V PC electrolytic
3 100nF MKT polyester
Resistors (0.25W, 1%)
2 1MΩ
5 10kΩ
1 220kΩ
1 1.8kΩ
1 100kΩ
4 1kΩ
1 39kΩ
1 150Ω
1 11kΩ
1 10Ω
install each one. Use 0.8mm tinned
copper wire for the two wire links.
Make sure that you insert the polarised
components the correct way around.
These parts include the diodes, ICs,
LED, transistors, voltage regulator and
electrolytic capacitors.
QuickBrake monitors the output of the throttle position sensor (circled). When
it detects that the driver is lifting off the throttle very quickly, the relay trips,
illuminating the brake lights. A built-in timer then covers the period before the
brakes are actually applied.
Manual Gearboxes?
QuickBrake may not be suitable for
use in manual cars because it may
not be able to distinguish between
throttle lifts for emergency stops
and those used during rapid acceleration through the gears. On
the other hand, if you normally
drive your manual car in a leisurely
manner, it may not have problems.
The relay and the screw terminal
strips can be installed last.
Note that there is a trap in the installation of the two trimpots. They
can go in either way but they must be
installed as shown in the diagram, with
the adjustment screw closest to IC2 and
IC3 respectively. If you install the trimpots incorrectly, the initial adjustment
instruction that we give in the set-up
procedure will be wrong.
During assembly, look closely at the
Unwanted Flashing
If the QuickBrake is set correctly and a competent driver is at the wheel, the
brake lights should trigger no more frequently than normal. This is because
the project should be calibrated so that it detects only very fast throttle lifts
– the sort that are usually immediately followed by an application of the
brakes. However, poor drivers who use very jerky on/off throttle movements
will cause the brake lights to come on more than usual. Keep in mind that
any brake light illumination will still indicate deceleration.
14 Silicon Chip
photos, Figs.1 & 2 and the parts list to
avoid making mistakes.
Fitting it to your car
As mentioned earlier, before you
buy the kit you need to check if your
car has a throttle position sensor (not
a throttle switch!). Now is the time to
measure the output of the throttle position sensor. This should be done with
the engine off (but the ignition on!) by
probing the throttle position sensor.
With one multimeter probe earthed
(connected to chassis), you should be
able to find a wire coming from the
connector that has a voltage on it that
varies within the 0-5V range as you
manually open and close the throttle.
Yes, you can manually open and close
the throttle by operating the mechanism on the side of the throttle body.
Once you have confirmed that the
varying signal voltage is present, make
a connection to this wire – ether at the
ECU itself or under the bonnet – and
run it to the QuickBrake signal input.
(Note that you simply tap into the
throttle position output wire – you
don’t need to cut it.)
Next, connect ignition-switched
+12V and 0V (chassis) to the QuickBrake. The other connections, to the
brake switch, don’t need to made at
this stage.
Rotate trimpot VR1 (sensitivity)
fully anti-clockwise and VR2 (timer
period) fully clockwise – this increases
the sensitivity of the QuickBrake to
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Other Uses For The Circuit
QuickBrake is just one of many applications for the
basic module described here. In other applications,
the module can be configured (via link LK2) to trigger
on quick throttle presses (rather than throttle lifts). In
this form, it can be used to sense when the car is
being driven hard.
These performance applications will be covered
in a SILICON CHIP high performance automotive
electronics special.
throttle changes and reduces the timer’s ‘on’ time to a
minimum (note: both these pots are multi-turn so they
don’t have a distinct end ‘stop’). Place the link in the
Link 1 position to configure the QuickBrake to activate
with quick throttle lifts. (Link 2 causes the device to
activate with quick throttle pushes.)
Turn on the ignition but don’t start the car. Wait five
seconds (to allow for the ignition-on reset pause), press
the throttle and then quickly lift off, checking that the
relay pulls-in and the LED lights. The relay should click
out (and the LED go off) fairly quickly, so then adjust
VR2 anticlockwise and again push down and then
quickly lift the throttle. This time the ‘on’ time should
be longer. Adjust VR1 clockwise until the QuickBrake
responds only when the throttle is being lifted with ‘real
life’ quick movements.
Note that if you find the relay clicks off after 10 seconds or so, then it is likely that trimpot VR2 is installed
the wrong way around. Don’t pull it out –just wind the
adjustment fully in the other direction.
Once the QuickBrake module is working correctly,
make the brake switch connections. These are straightforward – connect wires to both sides of the brake pedal
switch and check that when you join the wires, the
brakelights come on. Then run these wires to the adjoining “Normally Open” and “Common” connections on
the QuickBrake relay connector.
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Setup
Setting up the QuickBrake is also easy. Normally, you’ll
find that driving on the road actually involves slightly
different speeds of throttle movement than you thought
during the static set-up, so the sensitivity control (trimpot
VR1) will need to be adjusted accordingly. The length
of time that you set the timer (VR2) to operate for will
depend on how quickly you typically move your foot
from the throttle to the brake pedal. It’s best to set the
time so that it just covers this period.
The PC board fits straight into a 130 x 68 x 42mm jiffy
box, so when the system is working correctly, the board
can be inserted into the box and tucked out of sight.
Conclusion
If you’re often worried about how closely others follow
you at highway speeds, this project is for you. We know
we’ve already said it, but it’s uncanny how quickly the
brake lights come on when a car equipped with QuickSC
Brake is slowing!
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H S ILICON C HIP logo
printed in gold-coloured
lettering on spine & cover
H Buy five and get them
postage free!
Available only in Australia. Buy five & get
them postage free!
Just fill in the handy order form in this issue; or fax
(02) 9979 6503; or ring (02) 9979 5644 & quote your
credit card number.
Silicon Chip Publications, PO Box 139, Collaroy 2097
March 2004 15
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