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supply rails. Even though the op amps are rail-to-rail types, they can’t supply much current at voltages right near their rails. Similarly, IC2 has a 33V positive supply so that the output from IC2a can deliver a voltage to indicate the air/fuel ratio at lean values, where the required voltage is well above 12V. Negative supply generation The -3V supply is derived using a voltage inverter that inverts the +5V supply, while the +33V supply is from a voltage tripler that increases the 12V supply by almost a factor of three. The -3V supply is generated by transistors Q2 & Q3, diodes D2-D4 and their associated capacitors. This circuit is driven by a pulse width modulated output of IC1 (PWM3) that delivers a 31.25kHz 5V peak-to-peak square wave signal. Q2 & Q3 buffer this signal and drive an inverting diode pump circuit consisting of D2 & D3 and two 10μF capacitors. The square wave at the emitters of Q2 and Q3 ranges from about 0.6V to 4.4V; it is not the full 0-5V swing due to the base-emitter voltage drop of each transistor. When Q2 is on, the 10μF capacitor connected to it charges via diode D2 to ground. The total voltage across the capacitor is 3.8V (4.4V – 0.6V). When the PWM3 output goes low (0V), transistor Q3 switches on, pulling the positive side of the capacitor to about 0.6V. The opposite side of the capacitor is pulled negative, causing diode D3 to conduct and charge the second 10μF capacitor to a negative voltage. This produces a negative supply rail of around -3V. We don’t get a full -5V SC6721 Kit ($120 + postage) Includes the PCB and all the parts that mount directly on it; the microcontroller comes pre-programmed (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. because of the transistor and diode voltage drops. Diode D4 clamps the negative rail, preventing it from going positive by +0.6V when the negative supply generator is not working, such as when the power is first applied and IC1 hasn’t started generating the square wave. Zener diode ZD3 limits the total voltage across IC3 to 15V. The 15V voltage limit is needed as the LM6482 has a total supply limit of 16V. So when the positive supply is 12V, as supplied by REG2, the negative supply is clamped at -3V. The alternative recommended IC for IC3 is the OPA2171, which can handle supply rails up to 36V in total. In that case, ZD3 could be left out. 33V supply generation The 33V supply for IC2 is from the voltage tripler driven from the PWM4 output of IC1 (pin 8). This produces a 31.25kHz square wave that drives a buffer comprising transistor Q4, Q5 and diode D5. When the PWM4 output is low, transistor Q4 is off, so its collector is pulled toward the 12V supply via the 1kW resistor. As this point also connects to the base of Q5, Q5 is on and its emitter is pulled up to around 11.4V. When the PWM4 output goes high, Q4 switches on and pulls its collector (and thus Q5’s base) down to around 0V. This means that Q5 is off, but D5 conducts, so its anode voltage drops to about 0.3V. Diodes D5-D9 are schottky types that have lower forward voltages than standard diodes. The resulting 11.3V to 0.3V swing at the emitter of Q4 and anode of D5 drives the voltage tripler circuitry via diodes D6, D7, D8 and D9 and the series of 1μF capacitors. ZD2 clamps the output voltage at 33V. Microcontroller details Pin 18 of IC1 is the MCLR reset input. It’s pulled high via a 10kW resistor and ensures that IC1 is reset on power up. The MCLR input, the clock (pin 16), the data line (pin 17) and the 5V and ground supply connect to an in-circuit serial programming header (ICSP) to allow IC1 to be programmed. The header isn’t required if the IC is already preprogrammed, such as the one included in our short-form kit. Link setting When installed, jumper JP1 ties IC1’s RC7 (pin 1) input low. This selects a test mode for checking that the sensor impedance is correct (300W). In this mode, the narrowband output produces a value corresponding to the sensor cell’s impedance. Since this impedance depends on the sensor temperature, it’s a good way to check whether that part of the control circuit is working and verify that the sensor is not being overheated by exhaust gas when installed in a vehicle. As mentioned earlier, when jumper JP2 is shorted, the WFMD produces fixed outputs at lambda = 1 for calibration. Next month, we will describe the construction procedure, how to set up and calibrate the WFMD and install the sensor in a vehicle’s exhaust system, as well as how to install and use the SC Bluetooth app. We replaced the narrowband sensor used in a 2000 VW Caravelle with the Bosch LSU4.9 wideband sensor and connected the narrowband ‘S’ curve output of the WFMD to the vehicle’s ECU to simulate a narrowband sensor signal. The yellow trace is the wideband output and cyan the narrowband output. It cycles between rich and lean about once every two seconds because the ECU is adjusting the fuel injector duty cycle based on the narrowband output. The wideband signal doesn’t visibly vary much because it’s only ranging over 0.98 to 1.02 lambda, as shown in the video at siliconchip.au/Videos/WFMD (taken from a computer using the Bluetooth interface). siliconchip.com.au Australia's electronics magazine May 2023  79