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Increase your driving safety with Quick Brake

Simple project turns your brake lights on faster than you could ever apply them, to reduce rear-end shunts.

Words by Julian Edgar. Design by John Clarke.

Back 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.

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 logical reason – only the engineering tradition of turning on the brake lights with a brake pedal switch.

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

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 come on for a brief period, without you even having to touch the brake pedal. Nifty, huh?

Click for larger image

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 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 need to check this point out, before you buy the kit!

Circuit description

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.

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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 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 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

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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.

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

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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 is shown in Fig.2.

Install the resistors first, checking the values with your multimeter as you 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.

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 photos, Figs.1 & 2 and the parts list to avoid making mistakes.

Fitting it to your car

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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 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 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.)

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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 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.

Set-up

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 QuickBrake is slowing!

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.

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.

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.

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 1MW 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

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