Back in the March 2003 issue, we covered the advantages of LED
brake lights on cars – in addition to longer life and much lower
drain, LEDs reach full brightness far faster than filament bulbs. And
quicker that you can indicate to drivers behind you that you’re
less likely they are to run into the back of your car. In fact, using
your brake lights can provide the following driver with as much as 200ms
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
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.
Think about what occurs during an emergency stop. You’re
driving along, mind dwelling on all things interesting – including
traffic – when you suddenly realise the cars ahead are abruptly
rapidly lift off the accelerator and then transfer that foot to the
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
you start to lift off the throttle to the time the brake pedal is pushed
brake lights come on.
But why wait that long before illuminating the brake lights?
There’s no logical reason – only the engineering tradition
of turning on the
brake lights with a brake pedal switch.
- 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
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
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
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
time between the throttle closing and the brake light switch being
This is just what our QuickBrake circuit does. And it’s just
uncanny watching a car fitted with the project simulate an emergency
brake lights come on "soooooo" fast that you suddenly realise that the
between deceleration and braking that normally occurs is quite clearly
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
accelerator quickly and the brake lights will come on for a brief
without you even having to touch the brake pedal. Nifty, huh?
PC board module
As shown in the photos, QuickBrake is a small PC board module
measuring 105 x 60mm. It uses the engine management system’s
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.
car does not have engine management or it uses a throttle position
(rather than a potentiometer), QuickBrake cannot be used. You have
warned – you need to check this point out, before you buy the
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
designed to detect the rapid change of voltage from the throttle
and then close a relay for a brief time. The relay switches on the brake
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
relay drops out.
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.
So let’s look at the circuit in more detail. The DC voltage
from the throttle position sensor is fed to a low pass filter consisting
1MΩ resistor and 100nF
capacitor and then to op amp IC1a which is connected as a unity gain
From there, it goes to a differentiator consisting of a 100nF capacitor,
VR1 and a 100kΩ resistor. A
differentiator can be thought of as a high pass filter – it lets
changing signals through but slowly changing signals are blocked.
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
amp IC1b, which is another unity gain buffer, and then via link LK1 to
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
reduction in throttle sensor signal), IC3’s pin 3 immediately goes
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
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,
Diode D2 is connected across the relay coil to quench spike
voltages generated each time transistor Q1 turns off. Q1 also drives
the 1.8kΩ series resistor
and this lights whenever the relay is energised. It is handy when you
setting up the QuickBrake circuit on your car.
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.
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
operating conditions to have stabilised before QuickBrake starts
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
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
when the engine is stopped. This is important so that QuickBrake is
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
REG1, with the exception of the relay and LED1.
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.
All the circuitry of QuickBrake is on a small PC board
measuring 105 x 60mm and coded 05103041. The component overlay diagram
Install the resistors first, checking the values with your
multimeter as you install each one. Use 0.8mm tinned copper wire for the
wire links. Make sure that you insert the polarised components the
around. These parts include the diodes, ICs, LED, transistors, voltage
and electrolytic capacitors.
The relay and the screw terminal strips can be installed
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
diagram, with the adjustment screw closest to IC2 and IC3 respectively.
install the trimpots incorrectly, the initial adjustment instruction
give in the set-up procedure will be wrong.
During assembly, look closely at the photos, Figs.1 & 2 and
the parts list to avoid making mistakes.
Fitting it to your car
Fig.3: check your PC board against
this pattern before installing any parts.
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
time to measure the output of the throttle position sensor. This should
with the engine off (but the ignition on!) by probing the throttle
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
Yes, you can manually open and close the throttle by operating the
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
into the throttle position output wire – you don’t need to
Next, connect ignition-switched +12V and 0V (chassis) to the
QuickBrake. The other connections, to the brake switch, don’t need
to made at
Rotate trimpot VR1 (sensitivity) fully anti-clockwise and VR2
(timer period) fully clockwise – this increases the sensitivity of
QuickBrake to 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
with quick throttle lifts. (Link 2 causes the device to activate with
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.
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
lift off, checking that the relay pulls-in and the LED lights. The relay
click out (and the LED go off) fairly quickly, so then adjust VR2
and again push down and then quickly lift the throttle. This time the
should be longer. Adjust VR1 clockwise until the QuickBrake responds
the throttle is being lifted with ‘real life’ quick
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
pull it out –just wind the adjustment fully in the other
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
come on. Then run these wires to the adjoining "Normally Open" and
connections on the QuickBrake relay connector.
Setting up the QuickBrake is also easy. Normally, you’ll find
that driving on the road actually involves slightly different speeds of
movement than you thought during the static set-up, so the sensitivity
(trimpot VR1) will need to be adjusted accordingly. The length of time
set the timer (VR2) to operate for will depend on how quickly you
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
tucked out of sight.
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
QuickBrake is slowing!
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.
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.
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
These performance applications will be covered in a SILICON CHIP high
performance automotive electronics special.
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 1MW multi-turn top-adjust trimpots (VR1,VR2) (Jaycar RT-4658 or
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)
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