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Subscribe to APRIL 2023 ISSN 1030-2662 04 9 771030 266001 $1150* NZ $1290 INC GST INC GST WIDEBAND Fuel Mixture DISPLAY Silicon chirp Your own pet crick et Australia’s top electronics 500 cl as s d am pl ifier magazine use two inexpensive pre-b uilt modules to make the Silicon Chip is one of the best DIY electronics magazines in the world. Each month is filled with a variety of projects that you can build yourself, along with features on a wide range of topics from in-depth electronics articles to general tech overviews. W A T T Published in Silicon Chip If you have an active subscription you receive 10% OFF orders from our Online Shop (siliconchip.com.au/Shop/)* Rest of World New Zealand Australia * does not include the cost of postage Length Print Combined Online 6 months $65 $75 $50 1 year $120 $140 $95 2 years $230 $265 $185 6 months $80 $90 1 year $145 $165 2 years $275 $310 6 months $100 $110 1 year $195 $215 2 years $380 $415 All prices are in Australian dollars (AUD). 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To start your subscription go to siliconchip.com.au/Shop/Subscribe ▶ Factory-calibrated oxygen sensor ▶ Compact size, fitting in a 120 x 70mm case ▶ Correct sensor heat-up procedure implemented ▶ Optional exhaust pressure correction for readings ▶ Several display options, including wireless via Bluetooth ▶ Wideband and narrowband O2 sensor compatible outputs ▶ Accurate air/fuel ratio and lambda measurement and display ▶ Switch between displaying air/fuel ratios for two different fuels Part 2 of John Clarke’s WIDEBAND Fuel Mixture Display Our new WFMD (for short) uses a Bosch LSU4.9 wideband sensor to show a running engine’s live air:fuel ratio and/or lambda. It displays both on an LED panel display or another device via Bluetooth, and it can be permanently installed in a vehicle or temporarily inserted into the exhaust pipe for tuning. This second article in the series mainly covers the circuit details. L ast month in the first article on the new WFMD, we went into quite a bit of detail on how a wideband oxygen sensor works and how this particular circuit functions. However, we ran out of space in that issue, so we still needed to show the complete circuit diagram and explain how it works in detail. Due to the size of the circuit and its description, we will have to end it there, so the third and final article next month will cover the construction, testing, calibration and operation of the WFMD. Circuit description Fig.12 shows the entire circuit. It’s based on a PIC16F18877-I/PT microcontroller (IC1) in a 44-pin TQFP SMD package, running with an internal 32MHz clock oscillator. siliconchip.com.au The remainder of the circuit includes a pressure sensor (connections at upper left), Mosfet Q1 (for the sensor heater), some op amps and a few other components. Each op amp is a rail-to-rail type, meaning that the input and output pins can swing to within a few millivolts of the supply rails. They run from different supplies, so some can swing over 0-5V, some -3V to +12V and some 0-33V. We use the input and output pins on microcontroller IC1 in a few different ways. Its digital outputs can produce either a low (0V) or a high (5V) voltage. That allows us to switch LEDs or transistors on or off, or control anything that requires a digital signal. With the digital inputs, for example, we can detect if a jumper is connected to ground or left open with an internal pullup current to 5V from the micro. Australia's electronics magazine We can also set a pin to monitor a voltage ranging from 0V to 5V, with IC1 converting the voltage to a 10-bit digital value ranging from 0 to 1023. This is called an analog (AN) input. For example, ANC4 is the analog input on portC, bit 4, located at pin 42. Some digital outputs can be used for pulse width modulation (PWM), producing a fixed-frequency rectangular wave with a varying duty cycle. The duty cycle is the proportion of time the output is high and can vary from 0% through to 100%. When zero, the output is always low. At 50%, the waveform is square with equal periods at 0V and 5V. At 100% duty, the output sits at 5V. The PWM signal can be used directly to drive a component such as a Mosfet, or the waveform can be lowpass filtered to produce a varying DC May 2023  73 Fig.12: the full circuit uses microcontroller IC1, several CMOS op amps (IC2-IC4) and a Mosfet (Q1) to control the heater in the oxygen sensor, plus a pressure sensor. The microcontroller and op amps monitor and control the wideband oxygen sensor and provide the narrowband output, air/fuel ratio voltage and lambda outputs for monitoring using a multimeter, V/A panel meter or via Bluetooth. 74 Silicon Chip Australia's electronics magazine siliconchip.com.au siliconchip.com.au Australia's electronics magazine May 2023  75 voltage. The filtering converts a digital value to an analog voltage, provided the filter rolls off the AC signal amplitude well below the PWM frequency. In our circuit, PWM outputs are labelled from PWM0 to PWM6. PWM0 to PWM5 produce 31.25kHz waveforms, while PWM6 runs at around 122Hz. Driving the oxygen sensor Trimpot VR2 across the 5V rail provides the 3.3V reference voltage, which is buffered by op amp IC4c. This op amp drives one side of the pump cell, at the Vs/Ip connection, via a 150W resistor which isolates the op amp output to ensure stability. The Vs/Ip voltage is measured at the ANA4 input of the microcontroller to ensure that the pump current can be set to zero by applying the same voltage (from the PWM5 output) to pump drive buffer stage IC3a. IC3a is driven from the PWM5 output of IC1 (pin 27) via a 10kW resistor and 100nF filter capacitor to produce a steady DC voltage. The duty cycle of the 31.25kHz PWM signal is varied from 0-100% to produce a DC voltage ranging over 0-5V. IC1’s PWM2 and PWM1 outputs (pins 35 & 36) provide the external wideband and narrowband voltage outputs, respectively, again using PWM control. The narrowband output from PWM1 is filtered with a 1MW resistor and 100nF capacitor before being buffered by op amp IC2b. The filter components give a relatively slow response to PWM duty cycle changes, like a narrowband sensor. The 100kW resistor in series with buffer IC2b gives a high output impedance to simulate a narrowband sensor. For the air/fuel ratio output, the PWM2 output is filtered via a 10kW resistor and 100nF capacitor and amplified by op amp IC2a. This provides a wideband output at MV+, suitable for monitoring with a multimeter or a voltage and current (V/A) panel meter. The MV+ output is usually set to show 14.7V for petrol and 15.5V for LPG at lambda 1.0. Trimpots VR5 and VR6 set the gain of IC2a for the required air/fuel ratios. For the AF1 selection, the AND1/ RD1 output (pin 39) is set low (0V), allowing the gain to be set by VR5. The VR6 trimpot is connected to an analog input (AND0) at pin 38, which is effectively open-circuit. If the AF2 output is selected, the AND0 output is changed from an analog input to a low-level digital output. VR6 then sets the gain, with VR5 now connected to a high-impedance analog input (AND1). Jumper JP3 at the RC3 digital input (pin 37) selects between AF1 and AF2. When no shorting jumper is present, AF2 is selected. AF1 is selected when the jumper is shorted. Pin 37 has an internal pullup current configured to hold the input high when no jumper is connected. The AF1 and AF2 air/fuel ratios can also be displayed on a computer, tablet or smartphone via Bluetooth. VR7 at pin 43 (ANC5) sets the coefficient for AF1, while VR8 at pin 42 (ANC4) sets it for AF2. VR7 is adjusted so that the voltage at TP7 is one-tenth of the desired air/fuel ratio for lambda = 1.0 for AF1. So for a 14.7 stoichiometric air/fuel ratio, VR7 is adjusted for 1.47V. Similarly, VR8 is set for the AF2 air/fuel ratio value. For example, for a 15.5 air/fuel ratio for lambda = 1, VR8 is adjusted for 1.55V. Screen 1 shows the display on a computer via Bluetooth with a setting of 15.5:1 and a lambda of 1.0. Screen 2 shows the Android version but with at a lambda of 1.02 and 15.1:1 Air/ Fuel ratio. The VR7 and VR8 trimpots can be adjusted for different Air/ Fuel ratios. The software can also display lambda even if they are set for other values. It can even display AFR and lambda simultaneously. The lambda display has the decimal point moved left one digit compared to the air/fuel display version. These displays via Bluetooth work on recent Windows versions on a PC and run as a standalone executable file. Our prototype is run using Windows 11. As Processing is supported on macOS, the software should work on a Mac too, although we have not tested it. For Android, Processing does not Screen 1 (left): the Processing app can be made to run on Windows, Linux or Mac systems and shows the AFR and lambda values simultaneously. Screen 2 (right): the Android version, written in MIT App Inventor, is similar. You just have to choose the Bluetooth device and connect to it, after which you get live AFR and lambda displays. 76 Silicon Chip Australia's electronics magazine siliconchip.com.au have the required Bluetooth serial support, but MIT AppInventor does. So we have produced an app using AppInventor that mostly does the same job. We will make an APK file available, along with the source code. Dual panel meter display A multimeter output is also provided that shows the lambda value (as a voltage) and a current flow that can be displayed on a V/A panel meter. For this output, filtered PWM signal from the PWM2 output is buffered by op amp IC3b. The multimeter output is then taken via a voltage divider comprising trimpot VR9 and the 10kW resistor to ground. VR9 is adjusted for an output of 1V for a lambda of 1.0. For the current meter, IC3b sources current through a 330W resistor and trimpot VR10 (for calibration) to a shunt resistor. This 1W shunt resistor replaces the low-value shunt in the panel meter so that we don’t have to supply a huge current to get an appropriate reading. The meter can then show the lambda value, reading 1.00 when the lambda value is 1. This calibration is done with jumper shunt JP2 at the RC6 digital input of IC1 (pin 44). With JP2 shorted, the software within IC1 sets its outputs to show a lambda of 1 and a corresponding air/fuel ratio at a lambda of 1. The air/fuel ratio values produced at MV+ are also set with this calibration shunt. With JP2 in, the narrowband output produces 450mV (no adjustment is necessary). For the MV+ output, the voltage is adjusted to show the required air/fuel ratio using VR5 for the AF1 selection and VR6 for the AF2 selection (with JP3 in or out). So for a 14.7 air/fuel ratio at a lambda of 1, the voltage at MV+ is set to 14.7V, while MV+ is set at 15.5V for an air/ fuel ratio of 15.5 at lambda = 1. Sensor control Op amp IC4d is connected as a differential amplifier to monitor the voltage across the paralleled 62W and Rcal resistors. It operates with a gain of 25.45, as set by the 560kW and 22kW feedback resistors. The 3.3nF feedback capacitor rolls off high frequencies and prevents amplifier instability. IC4d’s output is referenced to the Vs/Ip voltage (at +3.3V) by the 560kW resistor between its pin 12 input and the Vs/Ip line, via op amp IC4c. As a result, when there is 0V across the siliconchip.com.au 12 multi-turn trimpots allow adjustments detailed in the text to be made with the case lid removed. 62W resistor, IC4d’s output sits at 3.3V. Sensor cell voltage Op amp IC4a monitors the sensor cell voltage (Vs). When Vs is at 450mV, IC4a’s output is 2.5V. To achieve this, trimpot VR4 provides an offset voltage that’s buffered by op amp IC4b. The result is that IC4a’s output can swing symmetrically above and below 2.5V to drive IC1’s ANA7 input (pin 30). This voltage swing is an exaggerated (by 4.7 times) measurement of any variation above or below 450mV from the sensor cell. The reference current applied to the sensor cell is derived via a 62kW resistor between the +5V supply rail and the Vs terminal of the sensor cell. When the controller is running and measuring correctly, the Vs terminal is at the Vs/Ip voltage of 3.3V plus the 450mV of the sensor cell, ie, 3.750V. So there is 5V – 3.75V = 1.25V across the 62kW resistor and 20.2μA flows (1.25V ÷ 62kW). The actual current does not affect the accuracy of lambda measurement unless the current is reduced to near zero or is increased above 40μA. Engine start detection Trimpot VR13 sets the threshold voltage for detecting when the engine has started by monitoring the battery voltage. It is measured at the AND4 analog input of IC1 (pin 2) via a 20kW and 10kW voltage divider connected between the +12V input rail and 0V. This divider reduces the applied voltage by two-thirds and results in a Australia's electronics magazine maximum of +5V at the AND4 input for a battery voltage of 15V. Typically, a 12V lead acid battery is below 12.9V when the engine is off but rises above 12.9V when the engine starts and the alternator begins charging it. So the battery voltage is compared with the threshold voltage at TP17 (AND2 of IC1), as set by VR13. This threshold voltage can be set anywhere from 0-5V, corresponding to a battery voltage range of 0-15V. The TP17 voltage is set to 1/3rd the required engine-started battery voltage. For example, for a threshold of 13V, TP17 should be at 4.33V (13V ÷ 3). When the wideband controller is used as a portable air/fuel ratio measuring instrument, TP17 will need to be adjusted to slightly less than 4V so the controller will begin operation with a 12V DC supply. This ensures that the sensor is heated when power is first applied. However, it also means that the sensor must be protected from moisture ingress and physical shock when not in use. Driving the heater Mosfet Q1 drives the sensor’s heater with a voltage derived from a 122Hz PWM signal delivered from IC1’s PCB Dimensions Error The parts list last month stated the PCB measures 160.5 × 98.5mm and we priced it at $15 + postage on the shop page. The PCB is actually 103.5 × 63.5mm and as a result, we have changed the price to $10 + postage. May 2023  77 The HC-05 Bluetooth module shown enlarged for clarity. Normally the module is supplied with the heatshrink pre-attached. PWM6 output (pin 5). The heater current flows through the Mosfet and is monitored via the AND6 input at pin 4, ie, by monitoring the voltage across the 0.1W 3W resistor that’s low-pass filtered by the 22kW resistor and 10μF capacitor. The Mosfet current is measured during the sensor heating period, to detect if the sensor is connected and, specifically, if the heater is connected. It also checks for an over-current condition, such as a short circuit, although the fuse would probably blow in that case. The heater is switched off under fault conditions and the status LED (LED1) shows the fault. It’s driven from the RA3 digital output of IC1 (pin 22) via a 470W current-limiting resistor. It lights dimly when the sensor is heating and then flashes rapidly once the operating temperature is reached. It flashes more slowly if there is a sensor error. Pressure sensing The pressure sensing circuit comprises the pressure sensor plus trimpots VR11 and VR12. These trimpots connect to analog inputs AND5 (pin 3) and AND3 (pin 41), respectively. With a 5V supply and when there is equal pressure on each input port, the output from the sensor sits at 500mV. Its output rises when pressure is applied to the positive pressure port and varies by about 50mV/kPa. With the available 4.5V output range from 500mV to 5V, the maximum pressure measurement is 90kPa (900hPa). The Bosch pressure sensor. 78 Silicon Chip The pressure sensor we use is a particulate filter differential sensor designed to detect when the particulate filter for a diesel engine is clogged. It detects the pressure differential between the input and output of the filter; the higher the pressure difference, the more the filter is clogged. As we are using it to measure the exhaust pressure, only one input is needed; the other port is blocked off. VR11 is used to adjust the pressure sensor calibration to 25mV/kPa. For the sensor used, this means setting the trimpot to mid-way, reducing the 50mV/kPa output to 25mV/kPa. The no-pressure output of 500mV is also reduced to 250mV. VR11 is included so that another type of pressure sensor can be used, provided it has no less than a 25mV/ kPa output. For outputs over 25mV/ kPa, such as the one we use, VR11 reduces the output level applied to AND3 to set the correct calibration. VR12 is to set the voltage offset from the sensor, as measured at the AND5 input. That’s so that IC1 can calculate the pressure based on the fact that the voltage rises from the no-pressure voltage at 25mV/kPa. IC1 then makes the required compensation of Ip variation with pressure for up to 12% for lean values and 9% for rich values. These corrections are in accordance with the graph shown in Fig.8 from last month. In practice, VR12 is set so that the voltage at TP12 is the same as at TP11 with no pressure differential across the sensor inputs. The pressure sensor is set up by plugging (blocking) one of its differential air inlets to allow the sensor to work as an absolute pressure sensor rather than as a differential sensor. This is best done when the sensor is at sea level, at the standard air pressure of 1013hPa. If the input is plugged at higher altitudes, the sensor output will be referenced against the lower pressure in the plugged inlet, increasing the effective sensor offset. VR12 can also be used to counter this effect. Air pressure reduces by 11kPa per 1000m above sea level. Since the calibration is for 25mV/kPa, reduce the voltage by 27.5mV per 100m above sea level. This is suitable for altitudes up to about 900m, where the pressure versus altitude becomes non-linear. If the pressure sensor is not used, Australia's electronics magazine the AND5 input will be held low via VR11, indicating to IC1 that the sensor is not connected. No pressure corrections will then be made. The Bluetooth module The HC-05 Bluetooth module connects to the Tx (pin 10) and Rx (pin 11) of IC1 at the module’s serial Rx and Tx pins, respectively. The Rx input to the HC-05 module is supplied with a reduced voltage from the Tx output of IC1 via a resistive attenuator. This reduces the 5V output from the Tx pin to 3.3V. Some HC-05 modules are not 5V-tolerant and so require this attenuation. Data is sent to the Bluetooth module using 8-bit data, no parity and one stop bit at 9600 baud. The six data digits for the air/fuel ratio and lambda are sent in ASCII format with a line feed character at the end. Switch S1, connected to IC1’s RB1 digital input (pin 9), is included in case the HC-05 module requires manual pairing. When held closed during power-up, IC1’s RB4 digital output (pin 14) drives the EN (enable) input to the module low, allowing pairing with a Bluetooth receiver. The module we used did not require this procedure. Power supply Power for the circuit comes from the 12V vehicle battery. The +12V rail is fed via fuse F1 and applied directly to one side of the oxygen sensor heater (via H+ at Vbatt) and the input to REG2 (LM2940CT-12). REG2 can handle a reversed supply without damage; however, REG1 (the LM317T adjustable regulator) cannot, so power goes to the latter via reverse polarity protection diode D1. Fuse F1 will blow if the sensor is connected and the supply polarity is reversed. That’s because there would be a low-resistance current path through the heater element and the body diode in Q1. Trimpot VR1 allows REG1’s output to be set to precisely 5.00V, as this supply is used as an accurate reference voltage for the circuit. This rail also supplies microcontroller IC1 and dual op amp IC4. In contrast, dual op amp IC3 runs from +12V and -3V rails. That is mainly so that the pump current op amp (IC3a) can provide the required current right up to the 0V and 5V siliconchip.com.au