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Switch between displaying air/fuel ratios for two different fuels ◀ Accurate air/fuel ratio and lambda measurement and display ◀ Wideband and narrowband O2 sensor compatible outputs ◀ Several display options, including wireless via Bluetooth ◀ Optional exhaust pressure correction for readings ◀ Correct sensor heat-up procedure implemented ◀ Compact size, fitting in a 120 x 70mm case ◀ Factory-calibrated oxygen sensor ◀ Part 1 of John Clarke’s WIDEBAND Fuel Mixture Display Measure your engine’s full range of air/fuel ratios using our Display, along with the latest wideband oxygen sensor from Bosch. It even includes Bluetooth, so you can display the lambda value or air:fuel ratio on a computer, smartphone or tablet! W hether you are driving a vehicle that uses a carburettor to mix air and fuel or with fuel injection, being able to monitor that the air/fuel ratios are correct ensures your car engine is running in prime condition. That’s especially important if it is a high-­ performance vehicle that has been heavily tuned since running lean can quickly destroy an engine under load. The air/fuel ratio can be measured in real-time using our Wideband Fuel Mixture Display (called the WFMD from now on). This is invaluable if you are involved in carburettor tuning in older engines or with car modifications. Anything that can affect air or fuel flow or with engine management remapping can cause an engine to run too lean or too rich. Most modern vehicles include at least one oxygen sensor near the engine on the exhaust pipe to monitor 40 Silicon Chip the exhaust gas. Vehicles made since about 2010 will usually have two or more, with at least one to verify that the catalytic converters are doing their job, converting any excess fuel or oxygen to inert gases. The primary oxygen sensor(s) near the engine allow the engine control unit (ECU) to control the air/fuel ratio being burned. Typically, they are narrowband sensors that can only accurately detect a nearly stoichiometric mixture or air/fuel ratio. A stoichiometric mixture is when there is complete fuel combustion and all the oxygen is used up with no fuel left over. The engine control unit (ECU) usually adjusts the amount of fuel injected per volume of air to maintain a mixture close to stoichiometric. This is called ‘closed-loop’ operation; the ECU controls the fuel mixture with feedback from the oxygen sensor. The ECU will Australia's electronics magazine increase the amount of fuel delivered if the exhaust is lean or reduce fuel if it is too rich. During acceleration or cruising, the mixture may go beyond stoichiometric and become rich or lean, beyond the measurement range of the narrowband oxygen sensor. In these cases, the ECU operates in open-loop mode, using predetermined mixture information stored within the ECU. In this case, it is not using the air/fuel ratio as a feedback parameter (at least, not immediately). The narrowband sensor has a very sharp voltage change around the stoichiometric mixture point, rising above 450mV if the mixture becomes rich and falling below 450mV if it becomes lean. To maintain a stoichiometric air/ fuel, the ECU constantly adjusts the mixture from slightly rich to slightly lean and vice versa, as the narrowband siliconchip.com.au SPECIFICATIONS — Supply voltage: 11-15V — Start-up current: 1.6A (~20W), typically dropping to 0.6A (7.5W) when up to temperature — Reading accuracy: typically ±1% plus 1 digit — Lambda measurement range: 0.7 (rich) to 1.84 (lean) — Air/fuel ratio range: 10.29 to 27.05 for petrol (stoichiometric 14.7:1) and 10.85 to 28.52 for LPG (stoichiometric 15.5:1) Status indication: warming up, operational, error via LED flashing Engine start voltage detection threshold: adjustable from 0-15V; 13V typical Heat-up period: typically <10s from cold Heater maximum effective voltage (Veff): 12Veff after initial preheat and 13Veff for <30s Heater over-current protection threshold: 4A Heater drive frequency. 122Hz during warm-up and >100Hz during operation Other protection: 5A fuse, heater open-circuit detection Sensor temperature: regulated to 780°C Exhaust pressure correction: up to 900hPa above standard atmospheric pressure of 1013hPa Sensor cell temperature/impedance measurement: AC drive at 1.953kHz and 243μA Sensor cell DC loading: <4.5μA Reference current: 20μA — — — — — — — — — — — — OUTPUTS — MM: 0.7-1.84V corresponding to 0.7-1.84 lambda — MV+: 10.29-27.05V representing air/fuel ratios of 10.29:1 to 27.05:1 for petrol OR 10.85-28.52V representing air/fuel ratios of 10.85:1 to 28.52:1 for LPG — MI: 0.7-1.84 lambda — Narrowband output: simulates the 0.8-1.17 lambda S-curve of the Bosch LSM11 narrowband sensor — Narrowband response time: 100ms time constant — Wideband response time: 100ms for a 5% change in oxygen content — Bluetooth: 9600 baud ASCII serial stream (8-N-1) — Bluetooth display works with Windows, macOS, Linux and Android devices sensor voltage swings above and below 450mV at around 1Hz. Fig.1 shows the typical output from a narrowband oxygen sensor. It has a very sharp response on either side of the stoichiometric point (lambda of 1), ranging from about 300mV up to 600mV. For rich mixtures, it ranges from around 600mV to almost 900mV (lambda up to 0.8), is quite non-linear and varies markedly with temperature. It is similarly non-linear for lean mixtures, ranging from around 300mV down to a few mV (lambda of about 1.15). The overall sensor response follows what is called an S-curve. To learn about lambda, refer to the explanatory panel later in this article. If you haven’t changed anything on your vehicle, there is little reason to worry about the fuel mixture since the ECU takes care of it. But if you have made any changes to improve siliconchip.com.au its performance, especially if you’ve tuned it or added/changed something like a turbo, you need to check that the mixture is OK. Part changes that can affect the mixture include the inlet air filter, throttle body, injectors, manifold absolute pressure (MAP) or mass airflow (MAF) sensors, custom ECU chips, adding a supercharger or turbocharger, catalytic converters, exhaust manifolds, mufflers and resonators, or anything resulting in changes to fuel mixtures and oxygen sensor readings. Note that if your vehicle already has a wideband oxygen sensor, you won’t be able to replace that with this one. The narrowband output on the wideband fuel mixture display unit can only be used if the vehicle has a narrowband oxygen sensor; in that case, the original narrowband sensor can be replaced by the wideband sensor Australia's electronics magazine via the WMFD’s narrowband output. Suppose your vehicle already has a wideband sensor. In that case, you can add the sensor to another bung (threaded hole) in the exhaust near the original sensor to monitor the air/ fuel ratio separately with the WFMD. Alternatively, the sensor can be placed in the tailpipe for temporary use. The Bosch LSU4.9 sensor Our new controller is designed to work with a Bosch LSU4.9 wideband oxygen sensor. This type of sensor (or a similar type from another manufacturer) is used in some late-model cars to measure and control the mixtures over the full range of engine operation. When combined with the WFMD, mixture readings cover the range of air/fuel ratios from lambda values of 0.7 (very rich) to 1.84 (very lean). Our WFMD is housed in a small plastic case, as shown in the accompanying photo. It includes an 8-pin socket for the wideband oxygen sensor connection plus cable glands for the power input leads, pressure sensor leads and the panel meter or a connection to a multimeter. It has an output that simulates a narrowband sensor. This enables the vehicle’s existing narrowband sensor to be replaced with the Bosch LSU4.9 and still provide for normal engine Fig.1: the output of a typical narrowband O2 sensor like the LSM11, known as an ‘S-curve’. The lambda value varies rapidly beyond about 50mV and 800mV on either side of the stoichiometric point (450mV), so it can’t accurately measure very rich or lean mixtures. April 2023  41 Fig.2: in contrast with Fig.1 for a narrowband sensor, the output of a wideband sensor after processing (here from the MM output), is a nice straight line over a wide range of lambda values (lambda is the measured air/fuel ratio divided by stoichiometric ratio). operation by connecting the narrowband signal to the ECU. The simulated narrowband signal is the same as it would receive from the original narrowband sensor, so ECU and engine operation are normal. The narrowband output from the WFMD is as shown in Fig.1. If your engine uses a carburettor or does not have an oxygen sensor, the wideband sensor can be installed in the exhaust pipe near the engine. You can also use the wideband sensor by temporarily installing it into the end of the exhaust pipe. You might want to do this for easy monitoring of different vehicles. More details on this will be given in a later article in this series. Improvements Our last O2 sensor controller was published in the June, July & August 2012 issues (siliconchip.au/Series/23). While it used the same sensor and worked well, this new version has some clear improvements. Firstly, this version fits in a more compact box measuring 120 x 70mm compared to 155 x 90mm. That can be important in a car where there often is little room to add new hardware. Secondly, the new version can show lambda and the air:fuel ratio simultaneously, and the air:fuel ratio scaling can be switched between two different fuel types, eg, petrol and LPG. The new version also has the Bluetooth feature lacking in the older one. This revised unit can also deliver a voltage directly proportional to the air:fuel ratio, not just a voltage derived from the lambda. This version can also handle compensation 42 Silicon Chip for higher exhaust pressures, up to 900hPa above 1013hPa rather than just 587hPa. We’ve also switched to using a commonly available automotive pressure sensor. Due to packing more features into an even smaller PCB, this version uses more SMDs than the last one, including a 44-pin micro, compared to the 18-pin DIP chip used in the 2012 design. Display options The WFMD includes several ways to view the air/fuel ratio and lambda. In its most basic form, a multimeter can be used to read off either value. A second option is to use a panel meter that includes both a voltage and current display. The lambda value is shown on the current display, while the voltage display shows the air/ fuel ratio. To do this, the current measurement section of the panel meter is modified to increase the shunt resistance. That’s so that the WFMD only needs to provide milliamps of current instead of amps. A third display method is via a Bluetooth connection, where the air/fuel or lambda value is shown on a phone, tablet or computer screen. This method avoids having any wired connection between the WFMD and the actual readout and would be especially useful if the WFMD needs to be mounted at the rear of the vehicle but monitored from the front. Fig.2 shows the WFMD controller lambda output over the range of air/ fuel ratios from 0.7 to 1.84 lambda. Two lambda outputs are available. The multimeter (MM) output is shown as a voltage on the left Y axis, while the V/A meter output (MI) is shown on the right Y axis as a current. Both these outputs are linear with respect to lambda values from 0.7-1.84. There is another output labelled MV+ for the V/A meter or a multimeter that provides a direct air/fuel ratio to voltage scale, ranging from 10.29V to 27.05V for petrol, when set for a 14.7:1 stoichiometric mixture, or 10.85V to 28.52V when set for a 15.5:1 stoichiometric mixture for LPG. These values can be set to other air/ fuel ratios if desired; you can even switch between two different scaling factors using a jumper shunt or external switch. Effectively, the voltage from the MV+ output is the same as from the MM output but multiplied by the air/ fuel ratio at stoichiometric for your type of fuel so that a voltmeter will give Air/fuel ratio & lambda The air/fuel ratio (or air:fuel ratio) is the ratio of the mass of air to the mass of fuel being burned. Lambda is the ratio of the actual air/fuel ratio to the stoichiometric air/fuel ratio. A stoichiometric mixture is when the air/fuel ratio is such that there is the exact mass of air required to completely burn the exposed mass of fuel. By definition, a stoichiometric mixture has a lambda of 1. For petrol, the stoichiometric air/fuel ratio (lambda of 1) is 14.7:1. This can drop to 13.8:1 when 10% ethanol is added and even further for E85 (85% ethanol), to 9.7:1. The stoichiometric air/fuel ratio is typically 15.5:1 for LPG. These values can differ depending on the exact fuel composition. For petrol, a lambda of 0.7 is equivalent to an air/fuel ratio of 0.7 × 14.7:1 = 10.29:1. Similarly, a lambda of 1.84 is equivalent to an air/fuel ratio of 27.05:1. Lambda is a universal measure of air/fuel mixtures since it is not dependent on the specific fuel. More details on the LSU4.9 wideband sensor Comprehensive data for the LSU4.9 sensor is available in a PDF file at: www.ecotrons.com/files/Bosch_LSU49_Tech_Info.pdf Australia's electronics magazine siliconchip.com.au a direct reading of the air/fuel ratio. Status indication A red status LED inside the controller, seen through the transparent lid, indicates when the controller is heating the sensor to its operating temperature. This occurs each time the controller is switched on, and it takes less than 10 seconds for the operating temperature to be reached. Once the sensor is at operating temperature, this LED flashes rapidly. From that point on, the wideband controller monitors the oxygen sensor signal and feeds a simulated narrowband signal to the ECU. By contrast, the LED flashes more slowly if there is a sensor error. Wideband oxygen sensor operation The wideband sensor operates very differently from a narrowband sensor. In its most basic form, a narrowband sensor has only one wire carrying the sensor output voltage. The common connection is via another wire or the sensor body connection to the chassis (ground). Many narrowband sensors also have an internal heater, and these units will have more wires for the heater element. Still, there are usually at most four wires on a narrowband sensor. By contrast, the wideband sensor has eight connections up to the sensor socket, with six wires connecting from the sensor socket to the controller. This is because the wideband sensor includes a narrowband oxygen sensor, an oxygen ion pump and a heater. The heater and oxygen ion pump need to be controlled, which is where the WFMD is required. Before we describe how a wideband sensor and its associated controller work, it’s necessary to explain the characteristics of a narrowband sensor. Fig.3 shows a cross-section of a typical narrowband sensor. It’s about the same size as a spark plug and is threaded into the exhaust system so the sensor is exposed to exhaust gases. The assembly is protected by a shield that includes slots so exhaust gas can pass into the sensor. The sensor is made from a zirconia ceramic material with a thin layer of porous platinum on both sides. These platinum coatings form electrodes to monitor the voltage produced by the zirconia sensor as the exhaust gas siliconchip.com.au Fig.3: this shows the structure of a typical narrowband sensor. Exhaust gasses coming in contact with the zirconia ceramic sensor generate a voltage between the interior and exterior platinum electrodes that’s related to the concentration of oxygen in the exhaust compared to the outside air. passes through it. The sensor is called a Nernst or fuel cell and produces a voltage when exposed to air/fuel mixtures. The device operates by measuring the difference in oxygen content between the exhaust gas and outside air. The oxygen content of air (about 20.95%) serves as the reference oxygen concentration. A voltage is produced between the electrodes because the zirconia sensor has a high conductivity for oxygen ions at high temperatures. When a narrowband sensor includes a resistive heating element, this heater quickly brings the sensor up to its operating temperature. It thereby allows the ECU to run in closed-loop mode sooner than without the heater. The arrangement of the wideband sensor is shown on the left side of Fig.4. It also includes a narrowband sensor (the sensor cell), but there are major differences in how it is used. Instead of obtaining reference oxygen from the outside air, it uses a pseudo oxygen reference chamber. This chamber obtains oxygen ions from the exhaust gases. When burning a lean mixture, oxygen is available from the unused oxygen in the exhaust gas. When the air/ fuel ratio is rich, oxygen is extracted from gases such as CO2 and H2O (the latter in the form of steam). Oxygen ions are maintained in this chamber by applying a small reference current to the sensor. Australia's electronics magazine The Bluetooth module is on the left, while the microcontroller is in the middle. April 2023  43 Fig.4: a wideband sensor (left) is similar to a narrowband sensor but needs the more complex control electronics shown on the right. Those electronics drive an oxygen ion pump in a negative feedback loop. By measuring the current required to run that ion pump, we can determine the air:fuel ratio of the exhaust gas entering the measurement chamber. A pseudo reference chamber is used to provide an oxygen reference instead of from the outside air because, when using outside air, the reference chamber needs to be constantly replenished with oxygen. The only pathway for the gas is via the sensor leads between the copper wire and the insulation. Any contamination of the sensor leads from oils, tars and fuels can affect the oxygen flow to the sensor. The leads are also susceptible to damage if the sensor lead connections are soldered during wiring maintenance (instead of crimped). Soldering will melt the plastic insulation sufficiently to seal the wire against oxygen flow. Conversely, for a pseudo reference, oxygen replenishment is not affected by sensor lead contamination since it derives its oxygen from a different source. The pseudo reference chamber needs to be continuously replenished to avoid being depleted of oxygen. That is because any oxygen in the reference chamber will diffuse into the measurement chamber to balance out the partial pressure of oxygen that is higher in the reference chamber, due to Fick’s First Law. Exhaust gas is sampled within a small measurement chamber (that is separate from and much smaller than the volume within the exhaust pipe), enabling a pump cell to move sufficient oxygen ions into or out of this chamber. The pump cell is driven with pump current to maintain a stoichiometric measurement within the sensor cell (the narrowband sensor). If the measured mixture is lean, the sensor cell detects excess oxygen. The pump cell then drives oxygen ions out of the measurement chamber until the sensor cell produces a stoichiometric Fig.5: the ion pump current plotted against lambda. It is not linear, but by storing a copy of this curve, we can easily perform a look-up and do a little interpolation to determine the actual lambda value from the pump current. 44 Silicon Chip Australia's electronics magazine lambda value, as detected by the narrowband sensor. Conversely, if the mixture is rich, oxygen ions are pumped from the surrounding exhaust gas into the measurement chamber gap until the sensor cell again reaches its stoichiometric lambda value. Current is applied to the pump cell in either direction, depending on whether oxygen needs to be pumped into or out of the measurement chamber. The oxygen pump is used to maintain a stoichiometric lambda value within the measurement chamber. So while the narrowband sensor (sensor cell) is used to ‘look for’ a stoichiometric mixture, it doesn’t provide the air/fuel mixture information. Instead, the amount of current applied to the pump cell required to achieve a stoichiometric mixture provides the necessary information to determine the air/fuel ratio accurately. Fig.4 shows how the wideband sensor is controlled. Vs is the output voltage from the oxygen sensor cell, while Ip is the current into or out of the pump cell. Vs is 450mV for a stoichiometric mixture and this is compared against a 450mV reference. If Vs is higher than the 450mV reference, the mixture is deemed rich and the Vs sense comparator (IC4a) output goes high. The controller then adjusts the Ip current to pump oxygen ions into the measurement chamber to produce a stoichiometric measurement. Similarly, if Vs is lower than the 450mV reference, the mixture is deemed lean and the comparator output goes low. As a result, the controller adjusts Ip to pump oxygen out of the measurement chamber. The pump current (Ip) indicates whether the mixture is actually rich siliconchip.com.au Parts List – Wideband Fuel Mixture Display 1 double-sided, plated-through PCB coded 05104231, 103.5 × 63.5mm 1 120 × 70 × 30mm plastic enclosure [Jaycar HB6082] 1 cable gland to suit 3-6.5mm or 4-8mm cables 1 inline 3AG, blade or mini-blade fuse holder (F1) [Altronics S6001, Jaycar SZ2015] 1 5A fast-blow fuse to suit fuse holder (F1) 1 6-way pin header, 2.54mm pitch (CON1; optional; for programming IC1 in-circuit) 3 2-way pin headers, 2.54mm pitch, with jumper shunts (JP1-JP3) 4 M3 × 15mm panhead machine screws and hex nuts 5 50mm lengths of light-duty hookup wire (red, black, yellow, green & light green; for circular connector to PCB) 2 150mm lengths of 7.5A hookup wire (blue and red; for circular connector to PCB) 2 200mm lengths heatshrink tubing (3mm & 5mm diameter) 2 2m lengths of 7.5A hookup wire (red and black; for power connection) Semiconductors 1 PIC16F18877-I/PT 8-bit microcontroller programmed with 0510423A.hex, TQFP-44 (IC1) 1 OPA2171AID dual rail-to-rail op amp, SOIC-8 (IC2) 1 LMC6482AIM or OPA2171AID dual rail-to-rail op amp, SOIC-8 (IC3) 1 LMC6484AIM quad rail-to-rail op amp, SOIC-14 (IC4) 1 LM317T adjustable linear regulator, TO-220 (REG1) 1 LM2940CT-12 low-dropout 12V automotive linear regulator, TO-220 (REG2) 1 STP16NF06L or IPP80N06S4L 60V 60A logic-level N-channel Mosfet, TO-220 (Q1) 2 BC817 NPN transistors, SOT-23 (Q2, Q5) 1 BC807 PNP transistors, SOT-23 (Q3) 1 BC847 NPN transistor, SOT-23 (Q4) 1 1N4004 400V 1A axial diode (D1) 3 1N4148WS 150mA switching diodes, SOD-323 (D2-D4) 5 SS14 40V 1A schottky diodes, DO-214AC (D5-D9) 1 BZV55-C16 ½W zener diode, SOD-80C (ZD1) 1 BZV55-C33 ½W zener diode, SOD-80C (ZD2) 1 BZV55-C15 ½W zener diode, SOD-80C (ZD3) 1 3mm high-brightness red LED (LED1) Capacitors (SMD M2012/0805 or M3216/1206 size) 5 100μF 16V PC radial electrolytic 1 10μF 16V PC radial electrolytic 3 10μF 50V SMD X5R/X7R ceramic or lean. A negative Ip indicates a rich mixture, while a positive Ip current indicates a lean mixture. The amount of current indicates the deviation of the lambda value from 1.0. Fig.5 shows a graph of Ip versus lambda for the wideband sensor. The lean region curve (up to 1.84) was developed from a graph of Ip versus oxygen concentration provided in the Bosch LSU4.9 data and the equation: siliconchip.com.au 5 1μF 50V SMD X5R/X7R ceramic 1 470nF 63V MKT polyester 1 220nF 63V MKT polyester 6 100nF 63V MKT polyester 2 100nF 50V SMD X7R ceramic 1 3.3nF 50V SMD X7R ceramic 1 22pF SMD NP0/C0G ceramic Resistors (SMD 0805 or 1206 size, 1% metal film) 1 1MW 1 15kW 1 330W 2 560kW 8 10kW 1 150W 2 470kW 1 5.1kW 1 120W 4 100kW 1 2.2kW 1 62W 1 62kW 1 1.1kW 2 10W 3 22kW 1 1kW 1 1W (optional; 1 20kW 1 470W for meter display) 0.1W 3W (2512 package) Trimpots (3296W-style multi-turn top adjust) 2 500W (VR1, VR10) 1 1kW (VR3) 9 10kW (VR2, VR4-8, VR11-13) 1 50kW (VR9) Sensor parts (Tech Edge – http://wbo2.com/) 1 LSU4.9 wideband oxygen sensor [Tech Edge 017123] 1 2.6m sensor extension cable [Tech Edge DIY26CBL] 1 8-pin circular panel socket (male) [Tech Edge S8PIN] 1 8-pin circular line plug (female) [Tech Edge P8PIN] 1 6-pin LSU4.9 sensor connector plug [Tech Edge CNK17025] Optional pressure sensor (recommended) 1 diesel particulate filter differential sensor [VW 076906051A or similar] 1 3-way plug or similar for sensor connection [EFI Hardware C03F-0007] 1 3-way cable rated at 1A or more 1 cable gland to suit 3-6.5mm or 4-8mm cables Optional Bluetooth interface 1 HC-05 Bluetooth module [Core Electronics CE00021] 1 4-pin tactile pushbutton switch (S1) [Altronics S1120, Jaycar SP0600] Optional dual meter display 1 dual digital DC voltmeter and ammeter [Core Electronics 018-05-VAM-100V10A-BL] 1 UB5 Jiffy box with mounting flange [Jaycar HB6016] 1 4-way extension cable rated at 1A or more 2 cable glands to suit 3-6.5mm or 4-8mm cables Lambda (λ) = (1 + Oxygen% ÷ 3) ÷ (1 − 4.77 × Oxygen%) For the rich region, a four-step graph provided in the LSU4.9 Bosch data sheet is used with linear interpolation for values between those steps. A function is applied to the lambda value to produce an S-curve response for the simulated narrowband (S-curve) output shown in Fig.1. Ip is sensed by measuring the voltage Australia's electronics magazine across a 62W ±1% resistor (in parallel with Rcal). During the manufacturing of each sensor, it is calibrated at the Bosch factory using a 61.9W ±0.1% resistor from the E96 range. Rcal is trimmed so that the voltage across this resistor, measured against lambda, is the same for each sensor. Rcal can be a value ranging between 30W and 300W, depending on the characteristics of the individual sensor. April 2023  45 The value for Ip shown on the vertical axis of Fig.5 is therefore not the total pump current; Ip is actually the current through the 62W resistor. So while Fig.5 shows Ip varying between -1.85mA and +1.07mA, the actual current could vary from -2.23mA to +1.29mA if Rcal is the maximum value of 300W, -5.67mA to +3.28mA if Rcal is the minimum of 30W or somewhere in between. This current needs to be supplied by the wideband controller circuit. Pump cell control and sensor measurement Fig.6 shows the general arrangement for the pump cell and sensor cell measurement. A filtered pulse-width modulated (PWM) signal from the microcontroller (IC1, PWM5) is applied to buffer stage IC3a. This supplies current to one side of the pump cell via trimpot VR3 to the Rcal resistance (inside the wideband sensor’s socket) and the 62W resistor. The other side of the pump cell connects to a 3.3V supply at Vs/Ip. When the output of IC3a is at 3.3V, there is no current through the pump cell. For positive current through the pump cell, IC3a’s output goes above 3.3V; when IC3a’s output is below 3.3V, the pump cell current is negative. IC3a’s output can swing between 0V and 5V to allow for the current range required for the lambda extremes of measurement (0.7 to 1.84). The pump Fig.6: the general arrangement of the wideband controller. The PWM5 output of the micro is filtered and then buffered by IC3a to provide a controllable ion pump current. Since the other end of the ion pump is held at +3.3V, the pump current can flow in either direction. It’s monitored via IC4d, while IC4a measures the sensor cell voltage. Fig.7: the percentage difference in ion pump current at various exhaust pressure values. The error also depends on the lambda value, with the effect greater for lean mixtures, so the measured exhaust pressure and lambda are considered when correcting this error. 46 Silicon Chip Australia's electronics magazine cell current (Ip) is monitored using op amp IC4d, which amplifies the voltage across the 62W resistor by 25.45. Its output is fed to the ANA6 analog input of microcontroller IC1. Simultaneously, op amp IC4a amplifies the sensor cell voltage (Vs) by 4.7 times. A 20μA reference current is also applied to the sensor cell at this point. While this is called a reference current, it is not a critical value; the word ‘reference’ indicates that the current is to maintain oxygen ions for the pseudo oxygen reference. The reference current does not flow through the 62W and Rcal resistances, so it does not affect the calibration of the wideband sensor when it comes to accurately measuring the oxygen content in the measurement chamber. Trimpot VR4 provides an offset voltage that is buffered by IC4b so that IC4a’s output is 2.5V when the sensor cell voltage is 450mV. The microcontroller monitors IC4a’s output at its ANA7 input and varies the pump current to maintain a 2.5V reading. This effectively keeps the sensor cell at its stoichiometric point. When the sensor cell is measuring stoichiometric, the Ip value determines the actual lambda value. One complication with Ip is that it depends on exhaust pressure, which is always above atmospheric pressure. Fig.7 shows the change in Ip versus pressure. Our Wideband Oxygen Sensor Controller provides pressure correction for up to 900hPa above standard atmospheric pressure (1013hPa). At 900hPa above atmospheric pressure, the Ip required for a given lambda value is about 15% higher for lean mixtures and 10.5% for rich mixtures. So the microcontroller can correct for this, an air hose connects from the exhaust manifold to a pressure sensor in the WFMD. However, this is optional if you are not overly concerned about the reading error. Note that the exhaust pressure does not affect stoichiometric mixture readings because Ip is zero. Ip also depends on temperature, so any variation in the sensor cell temperature will affect the Ip readings. Fig.8 shows how the sensor cell resistance varies with temperature; the change in Ip with temperature is around 4% per 100°C. There are two ways to ensure the lambda readings remain accurate. One is to correct for the effect siliconchip.com.au of temperature using the graph and the 4% change per 100°C. The better option is to maintain a constant sensor temperature by driving the heater and monitoring the sensor cell resistance. Fig.8: to make accurate measurements, we need to keep the sensor cell at 780°C. As its resistance varies with temperature, we can determine its temperature by measuring that resistance and use feedback via the heating element to maintain it at the correct temperature. Heater element control By maintaining the sensor at 780°C, the lambda versus Ip graph can be followed to determine the required display values without needing temperature compensation. The sensor cell temperature is measured by monitoring the impedance of the sensor cell, which is high at room temperature, falling to 300W at 780°C. The impedance of the sensor cell is measured by applying an AC signal to it, as shown in Fig.9. A 5V peak-topeak (p-p) AC signal from IC1’s RC0 digital output is applied to the sensor cell via a 220nF capacitor and 10kW resistor. The capacitor blocks DC and the resistor forms a voltage divider with the impedance of the sensor cell. When the sensor cell has an impedance of 300W, the voltage across it is 145.6mV peak-to-peak. IC4a has a gain of 4.7, so its output is 684mV peakpeak. The microcontroller measures this signal at its analog input ANA7 and maintains the 300W sensor impedance by varying the heater current. The sensor cell would need to vary by 25°C to produce a 1% variation, equating to about a 100mV shift in the measured voltage at ANA7. Since we maintain the voltage to within much less than that, the resulting lambda error is minimal. Controlling the heater current Fig.10 shows the heater control circuit. Mosfet Q1 is connected in series with the heater element across the 12V supply and driven by a PWM signal from IC1 (PWM6). The heater current is monitored via a 0.1W series resistor; the voltage across this resistor is lowpass filtered by a 22kW resistor and 10μF capacitor and fed to the microcontroller’s AND6 analog input. If the heater is disconnected or goes open-circuit, the lack of current will be detected and the WFMD will shut down. Similarly, if the heater current becomes excessive, the controller will switch off Q1 and the heater. Heating the sensor from a cold start requires a special procedure with a slow increase of heater power. This eliminates moisture buildup in the sensor and prevents thermal shock, siliconchip.com.au Fig.9: the sensor cell impedance is measured by superimposing a small AC signal on the DC sensor cell voltage with a fixed source impedance. The lower the cell’s impedance, the more heavily this AC signal will be attenuated. SC6721 Kit ($120 + postage) Includes the PCB and all the parts that mount directly on it; the microcontroller comes preprogrammed (the Bluetooth module is also included). You need to separately purchase the oxygen sensor, case, wiring, fuse holder, off-board connectors (including those for the O2 sensor) and optional parts like the pressure sensor and LED display. Fig.10: the average heater voltage is controlled by applying a PWM signal to the gate of a Mosfet to switch the heating element on and off rapidly. The current it draws passes through a 0.1W shunt resistor and the resulting voltage is fed to the micro via a low-pass filter to get an average voltage. Australia's electronics magazine April 2023  47 Fig.11: This simple divide-by-three circuit changes the battery voltage of 10-15V into a 3.3-5V range that’s suitable for measurement by 5V-powered microcontroller IC1. The Windows/Mac/Linux software (above) and Android App (below) both show the AFR and Lambda values so you can just read off whichever one suits you. The stoichiometric setting for the AFR reading is set with a trimpot on the main unit. Why is there no iOS App? We tried to create an iOS App similar to our Android App using both Processing and the MIT App Inventor. However, there seems to be an underlying limitation in iOS when it comes to handling Bluetooth serial streams. The problem is that iOS does not seem to support the Bluetooth SPP (serial port profile) that the HC-05 Bluetooth module uses. See: https ://developer.apple.com/ forums/thread/95083 The WFMD might work with an iOS device over Bluetooth if you can find a Bluetooth module similar to the HC-05 that uses a different Bluetooth protocol supported by iOS. We have found modules with the model designation AT-09 or HM-10 to be widely available with claimed iOS support and they appear to be pincompatible with the HC-05. However, it is unclear what that really means. If we can make them work with iOS devices, we will provide an update in one of the upcoming articles in this series. 48 Silicon Chip which could damage the ceramic sensor. The sensor is not heated until the engine starts so that exhaust flow can blow any condensation out of the sensor. A preheat period then begins with an effective 2V applied to the heating element for two seconds. The heater voltage then increases to an effective 7.2V and ramps up by 73.3mV every 187.5ms. This is equivalent to 0.39V/s, just under the maximum 0.4V/s rate specified by Bosch. The effective heater voltage is based on the battery voltage and the duty cycle of the PWM waveform. So the battery voltage is monitored to calculate the required duty cycle to achieve the desired average voltage. The battery voltage is also monitored to detect when the engine starts and stops. When the engine starts and the alternator begins charging the battery, its voltage rises above the resting level. In practice, the battery voltage varies from around 12.5V with the engine off to more than 14V with the engine running when the battery is charged. The battery voltage is measured using a voltage divider comprising 20kW and 10kW resistors, shown in Fig.11. While the sensor cell is heated, the impedance of the sensor cell is constantly monitored and as soon as it reaches 300W, the preheat is complete, and power to the heater is controlled to maintain this value. The pump cell control circuit then starts to operate. Next month There isn’t enough room to fit the full circuit diagram and remaining description in this issue, so we’ll have all those details next month. The construction, wiring, set-up and calibration details will also follow. SC Australia's electronics magazine siliconchip.com.au