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QuickBrake By JOHN CLARKE . . . reduces the risk of rear-end collisions According to crash data for the Sydney region, 26% of crashes are rear-end and almost half (42%) result in injury. The QuickBrake reduces the risk of a rear-end collision by giving a much earlier “brake lights warning” to following drivers than any normal car. It does this by switching on the brake lights even before you have a chance to depress the brake pedal. Q uickBrake senses when you quickly lift your foot off the throttle (accelerator) pedal and then instantly switches on the brake lights, well before you have had a chance to depress the brake pedal. It does this by sensing the difference between normal throttle movements and the quick lift-off when you are about to suddenly brake. The sensing is done by monitoring the voltage from the throttle position sensor (TPS) or a Manifold Absolute Pressure (MAP) sensor. Fast pedal movements will show up as an abrupt voltage change from the sensor. Whenever these fast voltage changes are detected, QuickBrake will switch on the brake lights. Before we continue we should point 54  Silicon Chip out that the QuickBrake will not work if you are in cruise control since the throttle pedal is not in use. And it may not be useful in cars with manual gearboxes since it could be confused by the throttle pedal movements when you are accelerating rapidly and changing the gears with gusto. That’s because there is no difference in the sensor voltage changes between lifting your foot off from the throttle during gear changes to those when you are about to brake suddenly. Finally, if you are coasting on a “trailing throttle”, there will be no signal to the QuickBrake if you suddenly need to apply the brakes. So why do you need QuickBrake? You might think that you can move your foot very quickly between the throttle and brake pedals in a panic stop situation but the reality is very different. It can depend on a whole range of factors: your age, fitness, whether you are alert or sleep-deprived, the shoes you are wearing (thongs, high heels?), the closeness and height difference of the pedals, pedal offset and so on. The reality is that the typical time to move your foot from throttle to brake ranges from 250-750 milliseconds! If you don’t believe those figures, have a look at www.researchgate.net/publication/233039156_Brake_Reaction_ Times_and_Driver_Behavior_Analysis Of course, that time to move your foot is on top of the time it takes to resiliconchip.com.au The parts all mount on a 105.5 x 60mm PCB. This can be clipped into a UB3 utility box and fitted under the dashboard or in the boot. act to the fact that you actually need to apply the brakes! That can be as long as 250-500 milliseconds (provided you are not affected by tiredness, alcohol, fatigue etc). Unfortunately, QuickBrake cannot do anything about your initial reaction time and you need to give yourself a good margin for error by making sure you keep a“3-second gap” from the vehicle ahead. So QuickBrake’s function is to drastically eliminate the time from throttle lift-off to brake light illumination, to give following drivers a much earlier warning that your brakes are about to be applied. How much earlier? QuickBrake’s response time from throttle liftoff is typically only 10 milliseconds and that is mainly the response of the switching relay. So if the typical driver’s pedal response time is 0.5 seconds, then QuickBrake will react 490ms earlier; virtually instantaneously! At a speed of 100km/h that is a distance of almost 14 metres! That gap could be the difference between a sudden stop for the following driver and a serious accident involving major injuries and severe vehicle damage. More to the story So far we have talked about how fast QuickBrake can apply power to the stop lights. But how long does it take the stop lights to come on when they are powered up? And what is the difference in response between LEDs and the 5W filament lamps typically used for the CHMSL (centre high mount stop light) and 21W main brake lights? As most readers would be aware, filament lamps are notoriously slow to light up. Standard 21W filament bulbs can take somewhere between siliconchip.com.au 200-230ms to fully light up after power is applied to them. CHMSLs are faster, due to their smaller filaments, at around 60-80ms to fully light. So to give you a further safety margin, we strongly recommend changing the brake lamps to LEDs. If that seems too hard, you can still benefit by changing the lamps in your car’s CHMSL to LEDs and thereby provide extra warning time for the motorist behind you when braking. There is a drawback to fitting LEDs and that is because your car’s body computer may sense the higher resistance of the LED lamp assembly as an open-circuit filament. We have taken care of this problem in the QuickBrake circuit, as will be described later. Presentation QuickBrake uses a small PCB that can be mounted inside a plastic case. It needs to be connected to a 12V ignition switched supply, the brake lights and to a TPS or MAP (manifold air pressure) sensor. You would need to fit a MAP sensor to the engine’s manifold vacuum connections in an older vehicle, if it does not does not have a throttle position sensor (TPS). Usually, the TPS voltage is high (say, 3-5V) depending on the throttle opening and drops to zero when the throttle is closed. Similarly, the MAP sensor’s voltage will be high when the throttle is wide open and low when the engine is idling or the throttle is closed. Circuit description Fig.1 shows the circuit. It uses two dual op amps (IC1 & IC2) and a 7555 timer (IC3). The circuit is designed to detect the rapid change of voltage from the TPS or MAP sensor and then switch on a relay which is connected in parallel with the car’s brake pedal pressure switch. The QuickBrake relay then stays on for a period of time before it is switched off. The dual op amps are an LMC6482­ AIN (IC1) and an LM358 (IC2) and these run from a +5V supply. The DC voltage from the MAP sensor or TPS is fed via a 1MΩ resistor with 100 nF low-pass filter capacitor to the noninverting input of IC1a. This operates as a unity gain buffer. Its pin 1 output drives a differentiator comprising a 100nF capacitor, 1MΩ trimpot VR1 and a series-connected 100kΩ resistor. The differentiator acts as a highpass filter, letting fast-changing signals through but blocking slowly-changing signals. This is exactly what we want in order to sense the abrupt change as a person lifts off the throttle prior to braking. The differentiator is connected to a 2.5V reference which is derived from the 5V rail with a voltage divider using 1kΩ divider resistors, bypassed with a 100µF capacitor. With no signal passing through the 100nF differentiator capacitor, the output voltage on the VR1 side of the capacitor sits at +2.5V. Depending on how the vehicle is being driven, the MAP or TPS signal will either be steady or decreasing or increasing in voltage. Exactly how much signal passes through the 100nF differentiator capacitor is dependent on the rate of voltage change and the setting of trimpot VR1. VR1 sets the time-constant of the differentiator so high resistance settings for VR1 will mean that the circuit responds to more slowly changing signals from the TPS or MAP sensor. The differentiator output is buffered using op amp IC1b and it provides the high-to-low (H/L) output. IC2a is wired as an inverting amplifier and it inverts the output from IC1b. This provides the low-to-high (L/H) output. Jumper link JP1 then selects the output of IC1b or IC2a. This allows triggering on a falling (H/L) or rising (L/H) input signal. The selected signal is applied to IC2b, a Schmitt trigger stage. IC2b has its inverting input connected to a 2.27V reference derived using 12kΩ and 10kΩ resistors connected across the 5V supply. The non-inverting input is connected to JP1 via a 10kΩ resistor. A 1MΩ resistor connects between the non-inverting input and IC2b’s output. January 2016  55 Parts List 1 double-sided PCB, code 05102161, 105.5 x 60mm 1 UB3 plastic utility box, 130 x 68 x 44mm 1 12V DC DPDT PCB-mount relay (Jaycar SY-4052 [5A], Altronics S4190D [8A], S4270A [8A]) (RELAY1) 1 set of Quick Splice connectors (Jaycar HP-1206 or similar) 3 2-way PCB-mount screw terminals, 5.08mm spacing (CON1,CON3) 2 3-way PCB-mount screw terminals, 5.08mm spacing (CON2,CON3) 1 3-way pin header, 2.54mm pin spacing (JP1) 1 2.54mm jumper shunt (JP1) 2 1MΩ vertical multi-turn trimpots (VR1,VR2) 4 tapped spacers, M3 x 6.3mm 5 M3 x 5mm screws 1 M3 nut Semiconductors 1 LMC6482AIN dual CMOS op amp (IC1) 1 LM358 dual op amp (IC2) 1 7555 CMOS timer (IC3) 1 LM2940CT-5.0 3-terminal 5V low-dropout regulator (REG1) 1 3mm red LED (LED1) 1 BC337 NPN transistor (Q1) 1 BC327 PNP transistor (Q2) 2 1N4004 1A diodes (D1,D2) 2 1N4148 diodes (D3,D4) 1 1N5822 3A Schottky diode (D5, optional - see text) Capacitors 1 470µF 16V PC electrolytic 4 100µF 16V PC electrolytic 4 10µF 16V PC electrolytic 1 1µF 16V PC electrolytic 3 100nF MKT polyester Resistors (0.25W, 1%) 2 1MΩ 1 4.7kΩ 1 220kΩ 1 1.8kΩ 1 100kΩ 4 1kΩ 1 47kΩ 1 150Ω 1 12kΩ 2 4.7Ω 5W 4 10kΩ With no signal passing through the differentiator, the voltage applied to the non-inverting input via the 10kΩ resistor to IC2b is 2.5V. Since the inverting input is at 2.27V, the output of IC2b will be high, at around +4V. This 56  Silicon Chip output goes low when the signal from JP1 drops below the 2.27V threshold. The associated 1MΩ feedback resistor provides a degree of hysteresis so that IC2b’s output does not oscillate at the threshold voltage. Relay timer lC2b’s output drives the pin 2 trigger input of IC3, a 7555 timer, via a 1kΩ resistor. IC3 is triggered when pin 2 drops below 1/3rd the 5V supply, at +1.67V. When triggered, IC3’s output at pin 3 goes high, turning on transistor Q1 and Relay1. Diode D2 is connected across the relay coil to quench the spike voltages that are generated each time transistor Q1 turns off. Q1 also drives LED1 via a 1.8kΩ resistor to indicate whenever the relay is energised. Before IC3 is triggered, its pin 3 output and its discharge pin (pin 7) are both low. So pin 7 causes the negative side of the 1µF capacitor to be pulled toward 0V via a 150Ω resistor. Whenever IC2b’s output goes low it also turns on transistor Q2, wired as an emitter follower. The transistor keeps the negative side of a 1µF capacitor tied at 0V. This keeps the 1µF capacitor charged while ever IC2b’s output is low. When IC2b’s output goes high, Q2 is off and the 1µF capacitor discharges via trimpot VR2 and the series 1kΩ resistor, so that the negative side of the capacitor rises toward the 5V supply. When the negative side of the 1µF capacitor rises to 2/3rds of the 5V supply (about +3.3V), the threshold voltage for pin 6 is reached. At this point, pin 3 goes low and transistor Q1 and the relay are switched off. IC3’s timing period can be set from around 100ms up to one second, using VR2. Power-up delay The components connected to pin 4 of IC3 are used to provide a powerup delay. When the vehicle ignition is switched on, the Quick Brake circuit is prevented from operating the relay for a short period. The delay components comprise a 470µF capacitor, diode D4, and 47kΩ and 220kΩ resistors. When power is first applied to the circuit, the 470µF capacitor is discharged and so pin 4 is held low. This holds IC3 in reset so its pin 3 cannot go high to drive Q2 and the relay. IC3 becomes operational after about two seconds when the 470µF capacitor charges via the 220kΩ resistor to above +0.7V. The 47kΩ resistor is included to set the maximum charge voltage at 880mV. That’s done so the 470µF capacitor will discharge quickly via diode D4 and the 47kΩ resistor when power is switched off. Power for the circuit comes via the +12V ignition supply. Diode D1 provides reverse polarity protection and an LM2940CT-5.0 automotive regulator (REG1) provides a 5V supply for the circuitry, with the exception of the relay and LED1. Brake light switching As mentioned, Relay1 is used to switch on the stop lights using the normally open relay contacts which are connected in parallel with the brake switch contacts. The normally closed contacts of the relay connect 4.7Ω 5W resistors in parallel with the brake lights, when the brakes are off (and Relay 1 is unenergised). This has been done so that the brake lights can be changed to LED equivalents without causing problems where the car’s body computer monitors the brake lights for blown filaments. (If LEDs were fitted without these extra resistors, the car would display warnings on the instrument panel). We mention these resistors at this point but the fitting of LED brake lights will be covered next month. Fig.1 shows the brake light wiring to connector CON3 for a vehicle where the brake pedal switches are in the positive side of the lamps (ie, high side switching). In this particular case, we are showing the connection for a car which has blown filament monitoring for the main brake lights and also for the CHMSL lamp. This means that the brake pedal switch has three sets of contacts, ie, a 3-pole single-throw (3PST) switch, so that each lamp filament is isolated from the others. So how do we fool the car’s body computer into ignoring the fact that a LED equivalent may be fitted in place of an incandescent lamp in the CHMSL socket? Ideally, we would need a 3-pole double-throw relay for Relay 1 and additional 4.7Ω 5W resistors. However, since 3-pole relays are larger and much harder to obtain, we have elected to provide for this possibility by effectively connecting the CHMSL lamp in parallel with the lefthand side brake lamp via a Schottky power diode, D5. siliconchip.com.au siliconchip.com.au January 2016  57 100nF 1M +12V SCHMITT TRIGGER IC2b 1M K QUICKBRAKE A 16V 7 100nF IN TRIG 100nF 5 2 470 µF D4 1N4148 OUT GND 1k 1k 10 µF 1 3 6 7 TIMER OUT DISCH 8 TIME 1k B 150Ω 10 µF +5V A K C 1.8k 1 µF Q2 BC327 VR2 1M E D3 100 µF 1N4148 +2.5V IC1: LMC6482AIN IC3 THR 7555 4 A K DIFFERENTIATOR VR1 1M 100k SENSITIVITY REG1 LM2940CT-5.0 1k 47k 220k 1 100 µF BUFFER 4 IC1a 8 10 µF B A K E C H/L 3 2 4 IC2a 8 K A K 1N4004 A 1N4148 4.7 Ω 5W 4.7 Ω 5W 100 µF 1 IC2: LM358 INVERTER 10k L/H RELAY 1 +12V JP1 +2.5V 10k ONLY NEEDED FOR LED LAMPS Q1 BC337 D5: 1N5822 +5V +12V 7 K D2 1N4004 A λ LED1 BUFFER IC1b 4.7k K A 6 5 10 µF K A A Y C2 C1 X R CON3 H GND K LEFT BRAKE LAMP CENTRE HIGH BRAKE LAMP LED E B C BC327, BC337 GND IN OUT LM2940CT-5.0 GND RIGHT BRAKE LAMP BRAKE PEDAL SWITCHES * D5 MUST BE FITTED WITH REVERSED POLARITY WHEN LAMPS ARE ON ‘HIGH’ (+12V) SIDE (I.E., GROUND SIDE SWITCHING) D5* +12V Fig.1: the QuickBrake circuit. IC1a monitors and buffers the signal from the throttle position sensor and feeds it to a differentiator stage which passes fast-changing signal transitions only. The differentiator’s output is then buffered by IC1b and fed to Schmitt trigger IC2b via JP1 or via inverter stage IC2a and JP1. A rapid negative transition occuring from the throttle position sensor (ie, during a fast throttle lift-off), causes IC2b’s output to briefly go low and this triggers 7555 timer IC3 which is then enabled to briefly activate Relay1 and the car’s brake lights. 20 1 6 SC  GND IGNITION 6 5 2 3 D1 1N4004 100 µF 10k CON1 10k 12k * REQUIRED ONLY FOR THE MAP SENSOR GND* SIG +5V* CON2 +5V CON3 +12V H LEFT BRAKE LAMP R X CENTRE HIGH BRAKE LAMP RIGHT BRAKE LAMP C1 C2 Y Fig.2(a): the wiring set-up when the brake lamps are low side switched and the vehicle checks for blown lamp filaments. GND NB: SEE FIG.3 FOR DIODE D5 ORIENTATION FOR GROUND SWITCHED LAMPS BRAKE PEDAL SWITCHES When the brake lights are on, the forward voltage drop across the Schottky diode will cause only a slight reduction in lamp brightness for an incandescent type and even less at the low current drain of a LED equivalent. So that takes care of isolated switching for the CHMSL lamp but does not provide a resistor to simulate a lamp filament if a LED equivalent is fitted. In that case, it will be necessary to add an additional resistor across the CON3 terminals for the CHMSL lamp (but only if a LED equivalent is fitted – more on this topic next month). So that takes care of the high side switching of brake lamps where blown filament monitoring is a feature of the vehicle. Inevitably though, we have had to provide for other brake light switching combinations such as “low side” switching Other switching combinations are shown in Fig.2. Let’s describe these variations. Fig.2(a) shows the set-up where the brake lights are “low side” switched, ie, in this the contacts of the brake pedal switch are in the negative side of the brake lights and again, we are catering for the situation where the vehicle has monitoring for blown lamp filaments. Finally, Fig.2(b) & Fig.2(c) show the situations for low and high side switching where the brake pedal switch has only one contact and all the CON3 brake lamps are effectively in parallel. In this case, the vehicle cannot monitor for blown lamp filaments. Construction The QuickBrake is built on a PCB coded 05102161 and measuring 105.5 x 60mm. It can be fitted into a UB3 plastic utility box that measures 130 x 68 x 44mm, with the PCB supported by the integral side clips of the box. Alternatively, you can mount the PCB into a different housing on short stand-offs using the four corner mounting holes. Fig.3 shows the component layout for the PCB. The low-wattage resistors can be installed first. Leave the 4.7Ω 5W resistors out for the moment. The respective resistor colour codes are shown in Table 1 but you should also use a digital multimeter to check each resistor before it is installed. The diodes can go in next and these need to be inserted with the correct polarity with the striped end (cathode, K) orientated as shown. Also, be sure to install D5 with its anode orientated correctly for +12V switched or ground switched brake lamps. Take care when installing the IC sockets (optional) and the ICs. Make sure that their orientation is correct and that the correct IC is inserted in each place. REG1 is installed with its leads bent over at 90° so as to fit into the allocat- +12V H R X CON3 LEFT BRAKE LAMP CENTRE HIGH BRAKE LAMP RIGHT BRAKE LAMP C1 Y Apply power to the +12V and GND terminals of CON1 and check for 5V at CON1 between the +5V & GND terminals. If the voltage is within the range +12V H BRAKE PEDAL SWITCH R X C2 BRAKE PEDAL SWITCH Fig.2(b): the configuration for low side switching where the lamps are wired in parallel & the brake switch has only one contact. 58  Silicon Chip Initial testing C1 C2 GND ed holes in the PCB. The regulator is then secured to the PCB using an M3 x 5mm screw and M3 nut before its leads are soldered. The 3-way pin header for JP1 is installed now with the shorter pin length side inserted into the PCB, leaving the longer pin length for the jumper link. The two 5W resistors can be installed now but these are only required if you intend replacing the brake lamps with LED equivalents. The capacitors can now go in. The electrolytic types must be installed with the polarity shown, with the plus side oriented toward the sign as marked on the PCB. The ceramic and polyester capacitors (MKT) can be installed with either orientation on the PCB. Install transistors Q1 and Q2 next. Make sure that Q1 is a BC337 and Q2, BC327. LED1 must be installed with its anode side (longer lead length) orientated as shown. The LED is normally just used to provide a relay-on indication that is useful when testing, so the LED can be mounted close to the PCB. VR1 and VR2 can go in next. Both are 1MΩ multi-turn top-adjust types and the screw adjustment needs to be orientated as shown. This is so that the slow rate adjustments set by VR1 and longer time periods set by VR2 are achieved with clockwise rotation. The screw terminal blocks are installed with the open wire entry sides facing outwards. The 7-way screw terminal block (CON3) consists of two 2-way and one 3-way blocks which are simply dovetailed together before installing them on the PCB. Finally, complete the PCB assembly by fitting the relay. Y GND LEFT BRAKE LAMP CENTRE HIGH BRAKE LAMP RIGHT BRAKE LAMP Fig.2(c): high side switching with the lamps wired in parallel. The vehicle cannot detect individual blown lamp filaments in Figs.2(b) & 2(c). siliconchip.com.au 10 µF H R +12V SWITCHED X A 100 µF + QUICK BRAKE LIGHTS Q1 BC337 C1 A D2 4004 1.8k C2 GND Y 220k 470 µF + A LED1 *CAPACITOR MUST BE 1 µF: IGNORE PCB MARKING 10k 10k D4 4148 RELAY1 1k GND SWITCHED 1M TIME NC COM NO D3 D5 1N5822 BC327 4.7 Ω 5W IC3 7555 16120150 NC COM NO 4.7k 1 00 nF 05102161 Rev.C C 2016 ST H GIL EKAR B K CIU Q CON3 VR2 1M 100 µF 1 µF* 4148 1k Q2 + + SIG GND 100k JP1 100nF SENSIT 10k 10k 100 µF IC1 LMC6482 + VR1 1M +5V CON2 10 µF + 1M H/L 100 µF + 4.7 Ω 5W 100nF L/H CON1 47k 1k REG1 1k 150Ω 12k 4004 10 µF LM2940 IC2 LM358 D1 + + 10 µF +12V GND of 4.85-5.15V, then this is OK. If the voltage reads 0V, the 12V supply may have been connected with reversed polarity or D1 may have been orientated the wrong way. Before doing any adjustments, trimpots VR1 and VR2 should be wound anticlockwise until a faint click is heard, indicating that the adjustment is set fully anticlockwise. This sets VR1 for maximum sensitivity to sensor voltage change and VR2 for minimum relay on-time. Then place a jumper link in the H/L position. To simulate a throttle position sensor, connect a linear 10kΩ potentiometer to CON2, with the outside terminals connected to GND and +5V and the wiper to the SIG (signal) input. Adjust the 10kΩ potentiometer clockwise and then wind it quickly anticlockwise. The relay should switch on and LED1 should light. You can now check the effect of adjusting VR1 clockwise; this will mean that the 10kΩ potentiometer will need to be rotated more quickly anticlockwise before the relay switches on. VR2 can then be rotated clockwise to set more on-time for the relay. We suggest one second. Fig.3: follow the parts layout diagram to assemble the QuickBrake. Note that the electrolytic capacitor immediately to the left of VR2 must be 1μF in this project (ignore the marking on the PCB). Installation Most modern vehicles will have a TPS and so this sensor can be used as the signal source for the QuickBrake. In this case, only the signal input terminal is used and connected to the signal wire from the TPS which will normally be connected to the accelerator pedal. In some cases though, it may be located on the inlet manifold butterfly valve. The connections can be found by checking the wiring against a schematic diagram and connecting to the wiper of the TPS potentiometer. Alternatively, This is an early prototype. All external wiring connections are made via the screw-terminal blocks. you could probe the TPS wires to find the one that varies with throttle position. Note that some TPS units will have two potentiometers plus a motor. Use the potentiometer wiper output that varies with throttle pedal position. Once you have identified the correct wire from the TPS, you can connect a wire from it to the QuickBrake PCB using a Quick Splice connector (Jaycar Cat HP-1206; packet of four). Just wrap it around the existing TPS wire and the new wire and simply squeeze it to make a safe connection. If you have an older vehicle, then it will not have a TPS or engine manage- Table 1: Resistor Colour Codes o o o o o o o o o o o o siliconchip.com.au No.   2   1   1   1   1   4   1   1   4   1   2 Value 1MΩ 220kΩ 100kΩ 47kΩ 12kΩ 10kΩ 4.7kΩ 1.8kΩ 1kΩ 150Ω 4.7Ω 4-Band Code (1%) brown black green brown red red yellow brown brown black yellow brown yellow violet orange brown brown red orange brown brown black orange brown yellow violet red brown brown grey red brown brown black red brown brown green brown brown not applicable 5-Band Code (1%) brown black black yellow brown red red black orange brown brown black black orange brown yellow violet black red brown brown red black red brown brown black black red brown yellow violet black brown brown brown grey black brown brown brown black black brown brown brown green black black brown not applicable January 2016  59 QuickBrake Lamp Response Measurements As part of the design work on the QuickBrake circuit, we needed to take a series of measurements to show the times for brake lamps to light in a somewhat non-typical situation. In this case, the vehicle used had an almost ideal throttle and brake pedal set-up, with both pedals being quite close together, no offset to the right and with similar height above the floor (ie, almost co-planar). We then did a lot of practice brake applications and we determined that the quickest anyone could move his or her foot from the throttle to the brake in a simulated emergency was close to 110ms, ie, much faster than the typical times for most drivers, as quoted at the start of this article. A phototransistor was used to monitor the brake lamp brightness. We arranged the phototransistor as an emitter follower so that its voltage rises with increasing light level. The phototransistor was placed away from the brake light at a distance where full brightness of the lights gave maximum positive voltage output and zero for lights off. We found this positioning to be quite critical. If the phototransistor is too close to the brake lamp, the phototransistor output will be at maximum when the lamp is barely glowing. This would give a false indication. By contrast, the phototransistor positioning for LED lamps is not at all critical since their response is extremely fast. ment. In this case, a MAP sensor can be used to connect to the inlet manifold so as to monitor the inlet pressure. Using a MAP sensor for manifold pressure readings is suitable only for petrol engines though, not diesels. The 5V supply provided on the QuickBrake PCB at CON2 can be used to supply the MAP sensor. It is not critical which MAP sensor is used. A secondhand MAP sensor can be obtained from a wreckers’ yard. Holden Commodore MAP sensors are common. Alternatively, you can obtain a new one from suppliers such as: www.cyberspaceautoparts.com.au/contents/en-uk/d3721_ Holden_Map_Sensors.html to a TPS sensor which has an output of about 0V at no throttle and 5V at maximum throttle. For QuickBrake to work, the JP1 position should normally be H/L but L/H should be used if the voltage varies in the opposite direction when the throttle is released. Note that the TPS output will only vary with throttle position when the ignition is on. A MAP sensor will only vary its output with changes in manifold pressure, ie, when the engine is running. TPS & MAP sensors The voltage output from electronic pressure sensors such as a MAP sensor usually decreases with increasing vacuum; typically 0.5V with a complete vacuum and up to about 4.5V at atmospheric pressure. This is similar 60  Silicon Chip Scope shots All of the accompanying oscilloscope shots show the TPS voltage as the top yellow trace (channel 1). In each case, the voltage falls from about +4V down to about 0.8V when the foot is lifted rapidly from the accelerator pedal. We set the trigger point sensitivity (VR1) for the QuickBrake at mid position, to give a reasonable reference point. The lower blue trace on each shot is the phototransistor output monitoring the brake lamp. There are also small differences for the same lamps when comparing their QuickBrake response to that when just using the brake switch. These differences are due to variations in the time taken to press the brake pedal and also depend on whether the lamp filaments have fully cooled between each test. Scope1 shows the response of the QuickBrake. You can see that the LED (blue trace) comes on as the TPS voltage (yellow trace) drops just below 2V. The response time is about 10ms; the time for the relay to close. Wiring the brake lights The brake light wiring is relatively straightforward. You require a connection across the brake switch contacts, using the C1, C2 and Y terminals on CON3 on the QuickBrake PCB. As noted above, the circuit of Fig.1 shows the wiring where the brake lights are “high side” switched and with blown filament monitoring. Fig.2 shows the other possible set-ups. Scope 2 shows what happens without QuickBrake and shows a time delay of about 120ms between the same 2V threshold for the TPS voltage and the LED actually lighting up. Scope3 shows the QuickBrake response time when switching a 5W filament lamp (although typical CHMSL lamps have a higher rating and hence a longer response time). Here the response time is about 80ms or there­ abouts for reasonable but not full brightness. Full brightness is achieved at about 150ms. Scope4 shows the same 5W lamp response when being switched by the brake pedal alone (ie, QuickBrake out of circuit). Note that the timebase is now 50s/div, so the time from TPS threshold to full brilliance is more than 200ms. Scope5 shows the QuickBrake response with a 21W lamp and is typical for most cars. The timebase is 100ms/ div and the time taken to fully light approaches 350ms. Scope6 shows the 21W lamp response when switched by the brake pedal (ie, QuickBrake out of circuit). Compare this with Scope5. These scope shots certainly demonstrate the effectiveness of the QuickBrake circuit but they also show an even bigger improvement when LED lamp equivalents are fitted. That will be our story for next month. Most constructors will probably elect to install the QuickBrake PCB (in a plastic case) somewhere under the dashboard, giving easy access to the TPS wire and the 12V feed from the ignition switch. Others may find it more convenient to install it in the boot but this will mean running longer wires from the TPS and the +12V feed from the ignition switch. Final set-up VR1 should adjusted so that the relay switches on when the accelerator pedal is released suddenly. At the same time, it should be set so that normal accelerator movements to do not trigger the relay. That means adjusting VR1 clockwise until normal throttle movements are not detected. Trimpot VR2 is set so that the relay stays on long enough for the brake pedal to be pressed before it goes off. This prevents blinking of the stop lamps when the brakes are applied. siliconchip.com.au Scope 1: this scope grab shows the response of the QuickBrake. The LED (blue trace) comes on as the TPS voltage (yellow trace) drops just below 2V. The response time is about 10ms; ie, the time for the relay to close. Scope 2: this shows what happens without the QuickBrake. There is a time delay of about 120ms between the same 2V threshold for the TPS voltage and the LED actually lighting up. Scope 3: the QuickBrake response time when switching a 5W filament lamp. Here the response time is about 80ms for reasonable but not full brightness. Scope 4: the 5W lamp response when being switched by the brake pedal alone. The timebase is 50s/div, so the time from TPS threshold to full brilliance is more than 200ms. Scope 5: this shows the QuickBrake response with a 21W lamp and is typical for most cars. The timebase is 100ms/ div and the time taken to reach full brightness is 350ms. Scope 6: the 21W lamp response when switched by the brake pedal (ie, QuickBrake out of circuit). Compare this with Scope5; the QuickBrake makes a big difference. SC siliconchip.com.au January 2016  61