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A FUEL INJECTOR MONITOR FOR CARS Have you ever wondered how much petrol you use when you accelerate away from the traffic lights? Perhaps you would like to know how your fuel consumption increases as you climb a hill. If you have a fuel injected car, this project is for you. By RICK WALTERS & LEO SIMPSON Back before cars had engine management computers, they often had a vacuum gauge which was supposed to give an indication of fuel economy. Low vacuum readings meant you were using lots of juice while high vacuum meant that you were driving with a light throttle. In practice, a vacuum gauge was often a distraction as it fluctuated wildly each time you depressed the accelerator, as you moved up or down through the gears. Some drivers even 24  Silicon Chip went so far as to cover up the vacuum gauge to avoid its distraction. Now we’re in the 90s and vacuum gauges are decidedly “old hat”. Most modern cars have fuel injection and the drive signal to the injectors can be monitored to provide a very good guide to fuel use. The amount of fuel provided by the injectors is con­trolled by the amount of time they are open. When your car is at idle, the injectors are open only about 5% of the time. During normal driving, the injectors are open between 10% and 20% of the time. And when you are accelerating absolutely flat out, with the engine wound out to 5000 RPM or more and the accelerator fully open – “pedal to the metal” – the injectors will be open for more than 90% of the time. Since the injectors are fed from the fuel rails at essen­tially constant pressure, the fuel used by the motor is directly proportional to the injector opening time. The Fuel Injector Monitor is housed in a compact case, allowing it to be conveniently placed on your car’s dashboard at eye level. The straightline display consists of 20 light emit­ ting diodes (LEDs), 18 green, one orange and one red. The display is semi-logarithmic, with the first 10 LEDs showing 10 steps of 1% from 0-9%, while the second group of LEDs covers from 10% to 100%. The LED display takes the form of a bargraph which shows the average +15V 0.1 10k D1 1N914 LK2a 10k INPUT IC1b LK2b CA3260E LK1b 6 LK1a D2 1N914 16 8 5 2 VR1 10k 7 2 bx 3 cy 13 ay 1 IC1a 3 4 47k D3 1N914 4.7 2.2M IC2 5 4053 cx 12 ax 10 10 4.7k 10k +15V 1 C 9 11 A 10 B c 4 14 a 15 b by 6 7 8 +12V LED9 LED10 GRN GRN A LED1 LED2 LED3 LED4 LED5 LED6 LED7 LED8 GRN GRN GRN GRN GRN GRN GRN GRN A A A A 10   A K  1 K 18 K  A K  17 K 16  A  K 15 K 14  22k A 13 K 12  A K  11 K 10 6 7 4 8 2 K 18  A K  1 9  A K  IC3 LM3914 5 LED11LED12 LED13LED14 LED15 LED16LED17 LED18LED19LED20 GRN GRN GRN GRN GRN GRN GRN GRN YEL RED A A A A A  K 17 K 16  A  K 15 K 14 10  A  K 13 K 12 A  11 K 10 IC4 LM3914 5 3 3 6 7 820  820  8 4 2 9 6.8k 680 ZD1 5.6V 400mW +12V +12V 0.1 33k 7 0V 47k 4 8 IC5 555 6 2 3 5 1 D5 IN914 100 IN D4 1N914 REG1 7815 GND 100 OUT +15V 4.7k 10k 10 B .01 Q1 BC327 C +12V .01 E B A K E A K 100k C VIEWED FROM BELOW I GO FUEL INJECTOR MONITOR Fig.1: the 4053B multiplexer (IC2) enables the LM3914 LED drivers to give a dot and bar display to indicate the average and peak injector duty cycles. The 555 timer (IC2) controls the switching of the 4053 and also steps up the battery voltage to provide for a +15V regulated supply. opening times, combined with a brighter “peak” LED which shows more rapid fluctuations of the injector openings, as can happen, for example, when you blip the throttle. The peak LED is actually a “peak-hold” display which captures the rapid tran­sients and “holds” them so that they can be more easily seen. Unlike some car circuits, installation of the Fuel Injector Monitor is quite straightforward: one lead to ground (chassis) and two leads to the injector leads (one switched and the other battery positive) – more about that later. Circuit details The circuit of Fig.1 consists of five ICs plus a regula­tor, the 20 LEDs and a few other minor components. In most modern cars, all the injector solenoid coils are wired in parallel with one side connected to the battery posi­tive, through the ignition switch. The coils are switched to ground via a transistor when fuel is to be injected. This means that the pulse waveform fed from the injectors to our monitor is a +12V signal going to ground. While most cars have negative-going pulse injector wave­ forms, we have provided for vehicles with the opposite waveform polarity. This is done via two links to allow the selection of either system. The input circuit consists of IC1, a dual opera­ tional August 1995  25 amplifier. IC1b is used as a comparator while IC1a is used as a peak detector. The injector signal is applied via a 10kΩ isolation resis­ tor to diodes D1 and D2. These diodes provide transient protec­tion for the following op amp by clamping any input signal bet­ween ground and +15V (more pre- LED1-LED20 0.1 820W VR1 IC1 CA3260E +12V 4.7k 1 LK1a LK2b LK2a D3 1 D4 .01 .01 0.1 IC5 555 100uF 4.7uF 47k 33k 1 10uF 2.2M 10uF 0V IC2 4053 10uF 10k 680  100k ZD1 100uF LK1b 6.8k 47k D1 820W 10uF D2 INPUT Q1 1 1 10k IC4 LM3914 4.7k IC3 LM3914 10k 22k 10k K 10uF A D5 REG1 7815 Fig.2: install the parts on the board as shown here. The electrolytic capacitors must all lie flat on the board, otherwise it will not fit into the plastic case. Fig.3: this is the full-size etching pattern for the PC board. 26  Silicon Chip cisely, to between -0.6V and +15.6V). IC1 can accept signals in this range without damage. Our circuit description will apply to cars with a negative-going injector signal (the most common situation) and so links LK1a and LK2a will be installed. Ignore the links LK1b and LK2b which are shown dotted. Hence, the injector signal is applied via a 10kΩ resistor to pin 6 of IC1b. Pin 5 of IC1b is held at ap­proximately +5V via a voltage divider consisting of 10kΩ and 4.7kΩ resistors. Thus, whenever the injector voltage falls below +5V, the output (pin 7) of IC1b will go high. The output of IC1b is fed to trimpot VR1, a 10kΩ pot wired as a variable resistor. VR1, in conjunction with the 4.7kΩ resis­ tor to ground, provides calibration for the circuit. The output of IC1b is used to charge the 220µF capacitor. This becomes the “average” value of the pulse signal and is used to drive the bargraph portion of the LED display. The “average” signal from the 220µF capacitor is fed to pin 3 of IC1a and to pins 5 & 12 of IC2. IC1a and diode D3 function as a peak detector to charge a 4.7µF capacitor to the “peak” value of the voltage appearing at pin 3. The 4.7µF capacitor is slowly discharged by the 2.2MΩ resistor and so it provides the “peak hold” value for the peak DOT on the LED display. So now we have two voltages, the peak and average values of the injector pulse widths which must be shown on the same 20-LED bargraph. How do we do this? It is done by a technique known as multiplexing whereby two values are alternately flashed onto the LEDs, each value being shown for part of the time. This switching of the signals happens very rapidly so that our eyes are not aware of it. IC2, a 4053, does the multiplexing and is described as a triple 2-channel analog multiplexer. It alternately switch­es the bar signal (pins 5 & 12) and the dot signal (pins 3 & 13) to the LED display drivers (IC3 & IC4). IC5 controls the switching of IC2 and serves another purpose – to step up the car’s battery voltage. The vol­ tage step-up is necessary to enable the display drivers to handle the full range of signal voltage from IC1. We’ll explain more about this later. The 555 timer is arranged as an astable oscillator, with a frequency of about 1kHz. Its pulse output waveform All the LEDs are arranged to sit flat along the edge of the PC board but because of the pin layout of the LM3914 drivers, the display reads from right to left. Consequently, the board hangs upside down in the case to make the display read from left to right. is fed to a voltage doubler consisting of diodes D4 & D5 together with two 100µF electrolytic capacitors. The resulting voltage of about +19V is fed to the 7815 regulator which delivers a stable +15V. Multiplex operation We have already referred to multiplex operation so let’s now look at it in more detail. As noted above, we need to display two signals (the “average” and “peak” values) and at the same time we need to switch the display drivers, IC3 & IC4, between dot and bar modes. IC2, the multiplexer, has three internal switches and while these are not shown on the circuit, they can be identified in the following way. Switch A involves pins 11, 12, 13 & 14; switch B involves pins 1, 2, 10 & 15 and switch C involves pins 3, 4, 5 & 9. Pins 9, 10 and 11 control the position of each associated switch; eg, if pin 9 (the C switch control input) is high, pin 4 (c) is connected to pin 3 (cy) while if pin 9 is low, pin 4 is connected to pin 5 (cx). Returning now to IC5, which provides the switching signal, when pin 3 is low, pins 9 & 11 of IC2 switch the “average” signal to the pin 5 inputs of the display drivers IC3 and IC4. At the same time, pins 9 of IC3 & IC4 are pulled low to select the bar mode. Conversely, when pin 3 of IC5 is high, the “dot” signal at pins 3 & 13 of IC2 are switched to pins 5 of IC3 & IC4 which are then switched into the dot mode. Just to reiterate, the bar mode displays the average signal while the dot mode displays the peak which is RESISTOR COLOUR CODES ❏ No. ❏  1 ❏  1 ❏  1 ❏  1 ❏  4 ❏  1 ❏  3 ❏  2 ❏  1 Value 2.2MΩ 100kΩ 33kΩ 22kΩ 10kΩ 6.8kΩ 4.7kΩ 820Ω 680Ω 4-Band Code (1%) red red green brown brown black yellow brown orange orange orange brown red red orange brown brown black orange brown blue grey red brown yellow violet red brown grey red brown brown blue grey brown brown 5-Band Code (1%) red red black yellow brown brown black black orange brown orange orange black red brown red red black red brown brown black black red brown blue grey black brown brown yellow violet black brown brown grey red black black brown blue grey black black brown August 1995  27 Fig.4: this fuel injector waveform was taken from a Ford Laser S with a 1.8 litre engine. The duty cycle is under 10% at around 2000 RPM with the car stationary. The lower waveform is taken directly from the injector, while the upper waveform is the output of IC1b at pin 7. always equal to or higher than the average. To make the peak (dot) display brighter than the average, it is turned on for longer than the average and this is arranged by giving the pulse signal from IC5 a duty cycle of more than 50%. Part of the switching function controlled by IC5 is per­formed by transistor Q1 but because IC5 runs from 12V rather than 15V, its output cannot swing to the +15V necessary to ensure that Q1 is turned off. Therefore, zener diode D4 is included to allow Q1 to turn off when IC5’s output is high. IC3 and IC4, the LM3914 dot/ bar display drivers, which accept analog input signals from IC1, have 10 internal compara­tors which drive 10 external LEDs. The input range is determined by one or two resistors. IC3 is set by the 820Ω resistor between Fig.5: taken from a VP Holden Statesman with a 5-litre V8 engine, these injector wave­forms are again at around 2000 RPM and the duty cycle is under 20%. The lower waveform is the fuel injector driving voltage, while the upper waveform is the output of IC1b at pin 7. pins 6 and 7 and ground, to accept 0.125-1.25V and dis­ play 10 output steps from 0-9%. IC4 with its extra resistors accepts 1.265-12.65V for its 10 outputs, from 10% to 100%. Actually, these display steps should not be thought of as being absolutely precise. For example, if the 10% LED is lit, the injector pulse width can only be regarded as being above 10% but less than 20%. Similarly, if the 30% LED is lit, the injector pulse width is above 30% but below 40%. Construction All the components for this project, including the 20 LEDs, are mounted on a small PC board coded 05108951 and measuring 120 x 102mm– see Fig.2. The PC board is mounted in a small plastic case measuring 141mm wide, 36mm high and 110mm deep. The case Fig.6: this is another injector waveform, taken with a Tektronix TDS744A digital oscilloscope from a Ford Laser S at idle. Note the very narrow pulse width. 28  Silicon Chip splits into two sections, upper and lower, with two removable pieces for the front and back sections. The lower section has four integral pillars for the PC board but because of a layout constraint caused by the LM3914 display drivers, the PC board has had to be designed so that the LEDs run from right to left (to minimise the number of links required). To make the display read from left to right as it should, the PC board is mounted on the base of the case and then it is inverted, so that it “hangs from the roof”. Before you begin assembly, carefully check the PC board for broken or shorted tracks, especially between the pads on IC2 and IC4. First, install the six links, diodes and resistors. The capacitors are next. Be sure to lie the electrolytics flat, as the board will not fit into the case if you stand them up. Be sure to bolt the regulator down flat onto the PC board. Lastly, fit the LEDs, ICs, trimpot and transistor. The LEDs should be mount­ed so that they are flush with the front edge of the PC board. We could not obtain a 5mm square orange LED for our proto­type so we fitted a 5mm round one in that position. We used a thin piece of tinted plastic for our front panel and made an adhesive front-panel label with a rectangular cutout for the LEDs. The PC board is mounted to the integral pillars using 6mm spacers and 12mm long self-tapping screws. After you have carefully checked all your assembly work and soldering, you are ready to do an initial power check. If you don’t have a 12V power supply, you could apply power from a 12V car battery or from your car’s cigarette lighter socket. Make sure that you connect the 12V leads the right way around otherwise you will damage the circuit, with IC5 (the 555 timer) the most likely casualty. Just connect the 12V supply at first, without connecting the input lead from the injectors. All the LEDs should flash once and then the peak LED moves slowly from right to left. Now connect the injector input lead to 0V and most, if not all, LEDs should come on and stay on. If that checks out OK, you can move to the next step which is calibration. be slightly less, at around 13.8V. This latter lead is the one we’re looking for and is the one which we will make the permanent connection to. Now remove the pin from the other injector lead. To make a permanent connection, again the easiest method is to use a pin. This time, push the pin right through the centre of the injector lead and bend it over and twist the ends together. This way, the integrity of the injector lead itself is preserved. Now solder a lead to the pin while making sure that you don’t damage the injector lead insulation. (Perhaps you might like to practice soldering to a sample pin before you do the actual job on your car!) Having made the connection, carefully wrap it with insula­ tion tape. Having done that, the most convenient place to pick up +12V to power the circuit is from the other injector lead, so repeat the pin soldering to the other injector lead. Now anchor the two leads running away from the injector harness with a plastic cable tie to a convenient point on the engine so that vibration is unlikely to dislodge them. You will need to pass the two leads through the firewall into the passenger compartment. You will then need to make a connection to chassis for the 0V lead. It would also be prudent to install an in-line 1A fuse in the +12V line from the injector harness. Now make your connections to the Fuel Injector Monitor and turn on the ignition. With the engine stopped, all LEDs should be alight. When the engine is started, the LEDs will light up to about 60% or higher and then gradually drop back to the normal idle value of around 5% or 6% as the engine warms up. Calibration This will be the easiest calibration you have ever done. With the input lead connected to the 0V terminal, carefully adjust the trimpot until the red LED just comes on. You will need to wind the trimpot anticlockwise initially and then clockwise until the red LED just comes on. This calibrates the unit to correctly display an injector opening of 100%. Installation The trickiest part of the installation is to identify which of the two injector leads to make the connection to. Unless you have a wiring diagram for your car, you will need to make a voltage measurement on the two leads while the engine is running. In practice, the easiest way to make a temporary connec­tion to your injector leads is to push a pin right through the centre of each of the wires. Now start the car and let it idle for a couple of minutes to let the battery voltage stabilise. Now measure the voltage between each injector lead and chassis. One injector lead will be at the same voltage as the battery (eg, 14.4V) while the other injector lead will Fault finding If you have a problem, the first thing to check is the +15V rail. There should be about +19V into the 7815 regulator and +15V at its output. If the input voltage is 0V to the 7815, then IC5 is FUEL INJECTOR MONITOR 0 1 2 3 4 5 6 7 8 9 10 20 30 40 50 60 70 80 90 100 Fig.7: this is the full-size front panel artwork. PARTS LIST 1 PC board, code 05108951, 120 x 102mm 1 plastic case, 141 x 36 x 110mm 1 front-panel label, 132 x 28mm 1 10kΩ horizontal trimpot (VR1) 1 3mm x 8mm roundhead screw 1 3mm nut 1 3mm SP washer 4 6mm spacers 4 12mm self-tapping screws Semiconductors 1 RCA CA 3620E dual op amp (IC1) 1 4053B triple 2-channel analog multiplexer (IC2) 2 LM3914 dot/bar display drivers (IC3,IC4) 1 555 timer (IC5) 1 7815 15V regulator (REG1) 1 BC327 transistor (Q1) 5 1N914 diodes (D1-D3, D5, D6) 1 5.6V 400mW zener diode (ZD1) 18 LTL9234A 5mm square green LEDs or equivalent (LED1-18) 1 5mm square or round orange LED (LED19) 1 LTL4223A 5mm square LED or equivalent (LED20) Capacitors 2 100µF 25VW PC electrolytic 5 10µF 50VW PC electrolytic 1 4.7µF 50VW PC electrolytic 2 0.1µF MKT polyester 2 .01µF MKT polyester Resistors (0.25W 1%) 1 2.2MΩ 4 10kΩ 1 100kΩ 1 6.8kΩ 4 4.7kΩ 1 33kΩ 2 820Ω 1 22kΩ 1 680Ω not oscillating. Check the component values and soldering around this IC. With the input connected to 0V (as explained in the cali­ bration procedure), pin 7 of IC1 should measure around +14.5V. When the injector input is not connected, pin 7 should be near 0V. If your monitor reads 100% at idle and falls as you accel­ e rate, it means your injector signal is the wrong polarity. Remove links LK1a and LK2a and replace them in positions SC LK1b and LK2b. August 1995  29