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readout * Digital plus bargraph shows * Display air-fuel ratio to install * Easy in a vehicle auto* Display dims at night Monitor engine air-fuel ratios with this: Mixture Display Monitor the air-fuel ratio of your car’s engine in real-time with this Fuel Mixture Display. It boasts both digital and bargraph displays and can be used as a tuning aid, or when making engine modifications or just to indicate when there are problems. Pt.1: By JOHN CLARKE Your car engine’s air-fuel ratio has a considerable bearing on fuel consumption and air pollution. For this reason, modern cars use an exhaust gas oxygen (EGO) sensor mounted in the ex­haust system to continuously monitor air-fuel ratios and generate corresponding output voltages. This information is then fed to the engine management computer (EMC) which continuously adjusts the mixture to provide optimum power and econ54  Silicon Chip omy, consistent with low exhaust emissions. As can be imagined, if the EGO sensor is not functioning correctly, engine performance suffers and this can lead to very high fuel bills. Conversely, a properly functioning sensor en­sures good engine performance and helps keep fuel costs down – something that’s even more important than ever given the recent petrol price hikes. A system that’s in good nick also minimises air pollution. In modern cars, the combustion products from the engine are made safe by a catalytic converter which is mounted in the exhaust system. Combustion byprod­ucts such as carbon monoxide (CO) are converted to carbon dioxide (CO2), unburnt hydrocarbons to CO2 and water (H2O) and nitrous oxide (NO) to nitrogen (N2) under the action of the catalysts within the converter. However, this only works properly if the air-fuel ratio is held within certain limits. And that in turn is dependent on the EGO sensor. An EGO sensor, by the way, does not last indefinite­ly. Depending on the car, it’s usually a good idea to replace it every 50,000 to 80,000km. Stoichiometric ratio A catalytic converter works best when the air-fuel ratio is such that there is just sufficient oxygen to give complete com­bustion. This is called the “stoichiometric” ratio. It varies according to the fuel used but is generally 14.7:1 for unleaded petrol; ie, the air mass must be 14.7 times the fuel mass. For propane (C3H8), the stoichiometric ratio is a little higher at 15.6 and so more air is required compared to unleaded petrol to ensure complete combustion. Note that the EGO sensor does not measure the air-fuel ratio directly. Instead, it monitors the resultant oxygen (O2) content after combustion and provides a voltage output to in­ dicate whether the mixture is lean, stoichiometric or rich. Fuel Mixture Display The concept behind the SILICON CHIP Fuel Mixture Display is quite simple – it monitors the EGO output signal and, after processing, displays the results on a bargraph and on a digital readout. This digital readout can be calibrated to show the air-fuel ratio for either unleaded petrol or propane. For petrol, the unit is calibrated to display air-fuel ratios ranging from 11.8:1 (rich) to 20.6:1 lean, with the stoichiometric point at 14.7. The corresponding range for propane is from 12.7:1 to 21.5:1, with the stoichiometric point at 15.6. Note, however, that the fuel-air ratio doesn’t remain static while you are driving. Instead, it fluctuates in a rapid rich-lean-rich-lean sequence as the engine management computer (EMC) responds to the EGO sensor’s output. Put your foot down for acceleration and you will immediately see that the mixture is enriched to provide more power. Conversely, under a trailing throttle, the air-fuel ratio “leans off” and again this will immediately be indicated on the displays. Under a constant throttle, the air-fuel ratio should quick­ly settle at a fixed value. This point is leaner than the stoichiometric point, to ensure that emissions are kept low. It’s all made possible by the ECU which continuously monitors the EGO output and controls the fuel injectors to maintain the desired ratio. What sort of figures are we talking about? Well, many late-model engines operate with air-fuel ratios approaching 19.0:1 at constant throttle. As well as relying on the EGO sensor, mixture Main Features • • • • • • • • • • • • • Suits vehicles with Zirconia EGO probes Compact size 3-digit LED readout plus 7-LED bargraph display Dot or bar mode option for bargraph Air-fuel ratio matched for Bosch LSM11 (0258104002) EGO probe Indicates air-fuel ratios from 11.8 to 20.6 for petrol Indicates air-fuel ratios from 12.7 to 21.5 for propane/LPG Fully lean and fully rich air-fuel indication Bargraph display follows the non-linear response of the EGO sensor 13 indication levels in dot mode;, 7 levels in bar mode Fast 220ms update time for bargraph; 440ms for 3-digit display Automatic display dimming 0-1V display for setting up adjustments Note: this device is not suitable for use on cars that run on leaded petrol. If your car doesn’t already have an EGO sensor, you can fit one yourself but the engine must run on unleaded fuel, propane or LPG in order to use the Fuel Mixture Display continuously (leaded petrol poisons the sensor). settings this lean are, in part, made possible by the use of knock sensors. These listen for engine knocking and if it is detected, the ECU retards the ignition timing until the knocking ceases. This allows many engines to run just below the point of knocking, thus significantly boosting fuel economy while cutting emissions. Basic features As shown in the photos, the Fuel Mixture Display is housed in a compact plastic case. Its size and presentation matches that of three previous car projects – the Speed Alarm described in November 1999, the Digital Voltmeter in February 2000 and the Digital Tacho in April 2000. But the similarities don’t end there. As with those earlier designs, this circuit is based on a PIC16F84 micro­controller. This has allowed us to dramatically shrink the parts count and also makes the unit incredibly easy to build. If you look at those earlier circuits, you will notice that they are all quite similar. Most of the hardware modifications involve the input sensor circuitry. The big difference between them lies in the software that’s programmed into the PIC chip. Naturally, we’ve retained the automatic display brightness feature that was built into the previous designs. In bright light, the LED displays are at maximum brilliance so that they can be easily seen. However, as the ambient light falls, the displays automatically dim so that they don’t become distracting. Another feature of the unit is that it’s easy to install. There are just three external connections – two for power and the third to the existing EGO sensor. No EGO sensor? If your car’s engine runs on leaded petrol, it won’t have a factory-fitted EGO sensor. The way around this is to source a sensor from a wrecker and install it in the exhaust manifold yourself. Note, however, that running leaded petrol will soon poison the sensor. As a result, this approach should only be used for tuning purposes, with the sensor then removed and the hole plugged with a bolt of the same thread for everyday running. LED displays OK, let’s see how we have arranged the LED displays to match the output from the EGO sensor. The first thing to realise here is that the output from the EGO sensor is far from linear. Fig.1 shows the output voltage curve from a Bosch zirconium oxide EGO sensor, plotted against air-fuel ratios for both un­leaded petrol and SEPTEMBER 2000  55 LED7 is alight for minimum sensor output. By contrast, the bar mode has only seven threshold points. In this mode, LED4 (the centre LED) covers the central stoichio­metric point for sensor output voltages ranging from 340-650mV (region D). The remaining six LEDs are then used for the rich and lean portions of the display. As the mixture becomes richer, LEDs3-1 progressively come on. Similarly, as the mixture leans off, LEDs5-7 come on. Note that LED4 is on all the time. This makes the bar mode a little unconventional, since it starts from the central LED. However, this approach is perfectly logical. The advantage of the LED bargraph, in either dot or bar mode, is that it can rapidly respond to signal variations from the EGO sensor. This is handy because the EGO sensor output can fluctuate quite rapidly during normal driving. By contrast, the 3-digit readout is set up so that it re­sponds more slowly, so that it can be read. In normal operation, it gives a direct readout of the air-fuel ratio. However, ratios that are less than 11.8:1 or greater than 20.6:1 for petrol are respectively shown as “r” (for rich) or “L” (for lean). Alternatively, the digital readout can be set to show vol­tages ranging from 0.00 through to 1.05V instead of the air-fuel ratio and this is to allow the unit to be calibrated. This is done using two trimpots – one to set the voltage range (span) and the second to set the minimum voltage that can be measured (offset). How the circuit works Fig.1 the above graph shows the output voltage curve from a Bosch zirconium oxide EGO sensor, plotted against air-fuel ratios for both un­leaded petrol and propane. Also shown is the response of the bargraph display as the EGO output varies, for both dot and bar modes. propane. Also shown are the corresponding Lambda (λ) values which are calculated by dividing the air-fuel ratios by the stoichiometric value. This means that a Lambda of 1 is at the stoichiometric point. Fig.1 also shows the corresponding response of the LED bargraph display as the EGO output varies, for both dot and bar modes. First, the dot mode – this has either one or two LEDs lit at any time, giving 13 separate display 56  Silicon Chip points. The stoichiomet­ric point at 600mV is indicated by the central indicator LED4, which lights over the range indicated by “G” on the curve. As the voltage climbs towards the rich end, both LEDs 3 & 4 light, then LED 3 lights on its own, then LEDs 2 & 3 together and so on until only LED 1 is lit at the end of the range (ie, maxi­mum voltage). A similar sequence of events occurs as the mixture leans off, until only Fig.2 shows the circuit for the Fuel Mixture Display. IC1 is the PIC micro­ controller which forms the basis of the circuit. It accepts an input from the EGO sensor via op amp IC2a and drives the LED displays. IC2a functions as an inverting comparator. As shown in Fig.2, the signal from the EGO sensor is applied to its pin 2 input via a filter circuit consisting of a 1MΩ resistor and a 0.1µF capacitor. Note that the resistor is made large to reduce transient loading on the EGO sensor. In fact, the current from the EGO sensor must be at less than 1µA so that its output voltage (and thus the engine performance) isn’t affected. In operation, IC2a compares the SEPTEMBER 2000  57 Fig.2: the PIC microcontroller (IC1) processes the input signal from the EGO sensor (via IC2a) and drives the 7-segment LED displays and the LED bargraph. Q6, D1, D2 & REF1 provide a voltage offset for pin 3 of op amp IC2a, while IC2b & LDR1 automatically vary the display brightness, so that they don’t appear too bright at night. Fig.3: here are the assembly details for the two PC boards. Resistor R1 is installed if you want the bargraph to operate in bar mode and is left out of circuit for dot mode operation. Take care to ensure that you don’t get the transistors mixed up. sensor voltage at pin 2 with a DC voltage at its pin 3 input. This DC voltage is derived by applying a pulse width modulated (PWM) square-wave signal from the RA3 output of IC1 to an RC filter/divider circuit. As a result, pin 1 of IC2a switches low when ever the vol­tage on its pin 2 input is greater than the voltage on pin 3. This signal is then fed via a 3.3kΩ limiting resistor to the RB0 input of IC1. The resistor limits the current flow from IC2a when its output swings high to +12V, while internal clamp diodes at RB0 limit the voltage on this pin to 5.6V (ie, 0.6V above the supply rail). A-D converter Among other things, IC1 performs analog-to-digital (A/D) conversion. This converts the signal on its RB0 input into a digital value which is then used to drive the LED dis­plays. As mentioned above, the output at 58  Silicon Chip RA3 produces a PWM signal and this operates at 1953Hz with a wide-ranging duty cycle. A high output from RA3 is at 5V while a low output is at 0V. VR2, the 180kΩ resistor and the 0.1µF capacitor filter this output to produce a DC voltage, while the 100kΩ and 1kΩ resistors from pin 3 to ground form the bottom of the voltage divider. In practice, VR2 is set so that it divides the RA3 output by 3.9. This means that if the duty cycle is 50% (ie, a square wave) the average at RA3 will be 50% of 5V or 2.5V. As a result, the voltage at pin 3 will be 2.5/3.9V, or 0.64V. This will vary either up or down, according to the duty cycle. The A-D conversion is as follows: initially, the RA3 output at pin 2 of IC1 operates with a 50% duty cycle and this sets the voltage at pin 3 of IC2a to 0.64V. At the same time, an 8-bit register inside IC1 has its most significant bit set high so that its value is 10000000. This 50% duty cycle signal is produced at a frequency of 1953Hz for about 8ms, after which the comparator output level (pin 1 of IC2a) is monitored by the RB0 input. Pin 1 of IC2a will be low if the sensor voltage at pin 2 is above 0.64V and high if it is less than this value. If the sensor voltage is less than 0.64V, the pulse width modulation (PWM) output at RA3 is reduced to a 25% duty cycle to produce an average of 1.25V and thus 0.32V on pin 3 of IC2a. The internal register is now set to 01000000. Conversely, if the sensor voltage is above 0.64V, corre­sponding to a low comparator output, the RA3 output is increased to a 75% duty cycle to provide an average of 3.75V. The register is thus set to 11000000, with the most significant bit indicating the 2.5V 50% duty cycle and the next bit indicating the 1.25V 25% duty cycle. Adding the two bits gives us the 3.75V (75%) value which, after division by 3.9, gives 0.96V on the pin 3 input of IC2a. The comparator level is now checked again after about 8ms. The microcontroller then adds or subtracts a 12.5% duty cycle value (0.625V at RA3 or 0.16V at pin 3 of IC2a) and this is then compared with the input voltage again. If the sensor voltage is higher than the PWM waveform, the internal register is now set to X1100000 (where X = 1 or 0 as determined by the first operation). Conversely, if the sensor voltage is lower than the PWM voltage, the register is set at X0100000. This entire process is repeated for eight cycles, the microcontroller adding or subtracting progressively smaller voltages to pin 3 of IC2a. At each step, successively lower bits in the register are set to either 1 or 0 to obtain an 8-bit A-D conversion. The A-D conversion has a resolution of 5mV (0.005V) at the least significant bit. There are also 256 possible values for the 8-bit register, ranging from 00000000 (0) to 11111111 (255). However, in practice we are limited to a range from about 19 to 231. That’s because the software requires a certain amount of time to process the results in IC1 and produce the next waveform at the RA3 output. As a result, the measurement range is from about 95.5mV to 1.16V. However, by applying a slightly negative offset voltage to the pin 3 input, we can effectively cancel out the 95.5mV minimum so that it can be set at 0V. This then allows the comparator to measure from 0V to 1.16V - 95.5mV; ie from 0-1.06V. Following the A/D conversion, the 8-bit register value is converted to the value required for the display using a lookup table. In practice, separate tables are used for the air-fuel ratio display and the two modes for the bargraph. These lookup tables can be easily modified if required. Negative offset voltage The negative offset voltage applied to pin 3 of IC2a is derived using voltage reference REF1, diodes D1 & D2 and transis­tor Q6. Let’s see how this works. In operation, Q6 is driven by the RA4 output of IC1 which incidentally, also drives switching transistor Q1 which controls the LED bargraph. When RA4 is low, Q6 is off and so capacitor C1 (10µF) charges to the +12V supply via a 1kΩ resistor and diode D1. Conversely, when RA4 subsequently goes high, Q6 turns on and connects the positive terminal of C1 to ground. As a result, the negative terminal of C1 goes to -12V and this charges capacitor C2 via diode D2. Table 1: Capacitor Codes    Value IEC Code EIA Code 0.1µF   100n   104 15pF   15p   15 The display board (top) carries the three 7-segment LED displays, the bargraph and the LDR. It plugs into the microcontroller board above, thus eliminating wiring connections between the two. Table 2: Resistor Colour Codes  No.   1   1   1   1   1   2   2   2   1   4   8   1 Value 1MΩ 180kΩ 100kΩ 12kΩ 10kΩ 3.3kΩ 1.8kΩ 1kΩ 1kΩ 680Ω 150Ω 10Ω 4-Band Code (1%) brown black green brown brown grey yellow brown brown black yellow brown brown red orange brown brown black orange brown orange orange red brown brown grey red brown brown black red brown brown black red brown blue grey brown brown brown green brown brown brown black black brown 5-Band Code (1%) brown black black yellow brown brown grey black orange brown brown black black orange brown brown red black red brown brown black black red brown orange orange black brown brown brown grey black brown brown brown black black brown brown brown black black brown brown blue grey black black brown brown green black black brown brown black black gold brown SEPTEMBER 2000  59 resistor is present, then that input will be pulled low. The software reads the inputs to determine whether they are high or low and sets the display mode accordingly. It then resets RB1, RB3 & RB4 as outputs so that data can be presented to the LED displays. Display dimming These two photos show how the two boards are married together, with the pin headers on the display board plugging directly into the 7-way sockets on the microcontroller board – see also Fig.4. This process is repeated at a rapid rate, so that about -6V is maintained on the negative terminal of C2. This voltage is ap­plied via a 3.3kΩ resistor to REF1 which produces a fixed -2.49V and this in turn is divided down by a 12kΩ resistor, VR3 and the 1kΩ resistor to ground to give the required 95.5mV negative offset. LED displays The 7-segment display data from IC1 appears at outputs RB1-RB7, as does the data for the LED bargraph. These outputs direct­ly drive the LED display segments and the bargraph LEDs via 150Ω current limiting resistors. As shown, the corresponding display segments are all tied together. In addition, the cathodes of the seven LEDs are each tied to a display segment. In operation, only one 7-segment LED display or the bargraph is on at any instant but because they are rapidly switched in sequence, they appear to be continuously lit. This technique is called “multi­ plexing” and it involves individually switching outputs RA0-RA4 low and then high again in sequence to control 60  Silicon Chip switching transistors Q1-Q4. Q1-Q4 in turn control the LED displays. For example, when RA0 is switched low, Q4 turns on and applies power to the common anode connection of DISP3. Any low outputs on RB1-RB7 will therefore light the respective segments in the display. RA0 is then switched high again and RA1 is switched low to drive Q3 and DISP2, after which it’s the turn of RA2 and RA1. Display modes Resistors R1, R2 and R3 are used to select the various display mode options. When installed, they respectively tie the RB1, RB3 and RB4 lines low. R1 determines whether the bargraph operates in bar or dot mode; R2 sets the display mode to volts or air-fuel ratio; and R3 determines whether the air-fuel ratio is for propane or unleaded petrol. Each time power is applied to the circuit, the software sets RB1, RB3 & RB4 as inputs with internal pullup resistors. Each pullup resistor will hold its corresponding input pin high if there is no external resistor to ground. Conversely, if an external IC2b is used to control the display brightness. This op amp is connected as a voltage follower and drives buffer transistor Q5 which in turn controls the voltage on the emitters of the display driver transistors, Q1-Q4. When the ambient light is high, LDR1 is low resistance and so the voltage on pin 5 of IC2b is close to +5V. This means that the voltage on Q4’s emitter will also be close to +5V and so the displays operate at full brightness. As the ambient light falls, the LDR’s resistance increases and so the voltage at pin 5 of IC2b falls. As a result, Q5’s emitter voltage also falls and so the displays are driven at reduced brightness. At low light levels the LDR’s resistance is very high and the voltage on pin 5 is determined by VR1. This trimpot sets the minimum brightness level. Clock signals Clock signals for IC1 are provided by an internal oscilla­tor circuit and this operates in conjunction with 4MHz crystal X1, between pins 15 & 16. Also included in the clock circuit are two 15pF capacitors. These ensure correct loading so that the oscillator starts reliably. In operation, the 4MHz crystal frequency is divided down internally to produce separate clock signals for the microcon­troller operation and for the display multiplexing. Power Power for the circuit is derived from the vehicle’s +12V ignition supply. This is fed in via a 10Ω resistor which, togeth­er with the 47µF and 0.1µF capacitors, provides decoupling. Zener diode ZD1 is included for transient protection – it limits any spike voltages to 16V and also protects against reverse supply connections. The decoupled supply rail is fed to REG1 to derive a +5V rail. This is then filtered and used to power IC1 and the LED displays. IC2 and the voltage offset circuit are powered directly from the decoupled +12V ignition supply. Software OK, that completes the circuit description. In reality, the hardware only forms half the picture. The other half is locked up inside the microprocessor which performs all the complicated stuff under software control. Do you REALLY want to know how the software works? Do you? We won’t go into the details here because we don’t have space. If you must know, then you’ll find the source code posted on our website. Construction You really don’t need to concern yourself with the software to build this circuit. Instead, you simply buy the programmed PIC chip and install it like any other IC. Fig.3 shows the assembly details. This mainly involves building two PC boards – a microcontroller board coded 05109001 and a display PC board coded 05109002. Once assembled, these two board are stacked together in piggyback fashion using pin headers and cut down IC sockets. This technique eliminates inter-board wiring since the connections are automatically made via the pin headers. Before starting assembly, check both boards for shorts between tracks, open circuits and undrilled holes. Note particu­larly that two holes are required in the display PC board to provide screwdriver access to trimpots VR1 and VR2 on the proces­sor board. These holes are located just below DISP3 and to the left of VR3. The microcontroller board can be assembled first. Begin by installing the wire links, then install the resistors. Table 2 lists the resistor colour codes, although it’s a good idea to also check them using a digital multimeter. Note that the seven 150Ω resistors are mounted end-on as shown. Trimpots VR1 & VR2 can go in next, followed by a socket to accept IC1 (taking care with its orientation). IC2 is soldered directly to the board – install this now, followed by zener diode ZD1 and transistors Q2-Q5. Be very careful here, because Q5 is the odd man out. It’s an NPN BC337 type, whereas Q2-Q3 are all PNP The whole assembly fits neatly into the smallest available plastic utility box and matches several previous car projects based on PIC microcontrollers. LDR1 should be mounted so that its face is about 3mm above the LED displays. BC327s. Mix them up and you’ve got problems. Regulator REG1 must be mounted as shown, with its metal tab flat against the PC board and with its leads bent at rightangles so that they pass through the PC board holes. Make sure that the hole in its metal tab lines up with the matching hole in the board, as this has to later accept a mounting screw. The capacitors can now be installed. Note that the two electrolytic capacitors are mounted horizontally, across the regulator’s leads; ie, their leads should be bent at right angles before they are installed. Note also that these capacitors are polarised, so be sure to mount them with the polarity shown. Crystal X1 mounts horizontally on the PC board but can go in either way around. It is secured by soldering a short length of tinned copper wire to one end of its metal case and to a PC pad immediately to the right of Q3. The three 7-way in-line sockets are made by cutting two 14-pin IC sockets into inline strips. Use a sharp knife or a fine-toothed hacksaw for this job and clean up the rough edges with a file before installing them on the PC board. Finally, install three PC stakes at the external wiring positions (sensor, +12V & GND). Once they’re in, trim these stakes on the parts side of the board so that they cannot short against the display board later on. Display board assembly As before, install the wire links and resistors first but only install R1 if you want the bargraph to operate in bar mode. This done, install the three 7-segment LED displays with their decimal points at bottom right. The LED bargraph can also be in­stalled at this stage – it mounts with its pin 1 (indicated by the bevelled edge) towards transistor Q1. The remaining parts can now all be installed, noting that D1 and D2 face in opposite directions. The two 10µF capacitors are mounted flat against the PC board, while LDR 1 should be in­stalled so that its top face is about 3mm above the displays. The three 7-way pin headers are installed from the copper side of the PC board, with their leads just protruding above the top surface. You will need a fine-tipped iron to solder them in. It will also be necessary to slide the plastic spacers along the pins to allow room for soldering, after which the spacers can be pushed back down again. That’s all we have space for this month. Next month, we will complete the assembly and describe how the unit is installed and calibrated. SC SEPTEMBER 2000  61