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WARNING This fuel mixture display should not be regarded as an accurate instrument since it will not necessarily be tailored to suit the voltage vs. lambda output curve for the particular oxygen sensor it is monitoring. To set the unit up as a calibrated instrument would require comparison with a known reference and subsequent adjustment of the internal software lookup table which converts the measured voltage into an air/fuel ratio. As published, the air/fuel mixture display is designed to follow the Bosch 0258104002 sensor output characteristics but even then, the calibration procedure will give approximate results only. Other typical narrow-band sensors can vary quite a lot in their output characteristics away from stoichiometric and, like the Bosch unit, also vary substantially in output voltage depending on temperature. For this reason, the Air/Fuel Ratio Meter is intended only for use as a dashboard unit to indicate air-fuel ratio trends during driving. It should not be relied on as an accurate instrument for tuning programmable engine management systems on a dynamometer or for making other engine adjustments. Part 2: final assembly, installation & calibration Mixture Display Last month, we described the circuit for the Fuel Mixture Display and showed you how to build the PC boards. This month, we complete the assembly and describe how the unit is installed and calibrated. By JOHN CLARKE Work can now begin on the case. First, remove the integral side pillars using a sharp chisel, then slide the microcontroller PC board in place and drill two mounting holes – one through the metal tab hole of the regulator and the other at bottom left, below the 0.1µF capacitor. These holes should be countersunk on the outside of the case (use an oversize drill to do this), if you intend using countersunk screws. Two holes are also required in the 66  Silicon Chip rear of the case (near the bottom) to accept the supply and sensor leads. The front panel label can now be affixed to the case lid and used as a template for making the display cutout and for drilling the hole for the LDR. The main display cutout is made by drilling a series of small holes right around the inside perimeter, then knocking out the centre piece and filing the job to a smooth finish. Make the cutout so that the red Perspex or acrylic window is a tight fit. Once it’s in place, the window can be further se­cured in place using several spots of super glue on the inside edges. Testing Before installing the micro­ con­ troller (IC1), it’s advisable to check that the regulator circuit is working correctly. This test is carried out on the micro­ controller board only; ie, the display board should not be plugged in. To check the regulator, connect automotive leads to the +12V and GND terminals, apply power and check that there is +5V on pins 4 & 14 of IC1’s socket. The metal tab of REG1 can be used as the ground connection during this procedure. If the +5V supply rail is correct, disconnect the power and install IC1 with pin 1 positioned as shown. The display board can then be plugged into the microcontroller board and Fig.4: this diagram shows how the two boards are stacked together and secured using screws, nuts and brass spacers. Notice that the righthand brass spacer is 9mm long, while the lefthand one is just 6mm long. Fig.5: the full-size front panel artwork is reproduced above, while at right are full-size etching patterns for the two PC boards. the assembly secured as shown in Fig.4. Check that there are no shorts between the two boards – some of the pigtails on the display board may have be trimmed to avoid this. Once the assembly is complete, reapply power with the EGO signal lead connected to ground. The display should light and show either an “L” (for Lean) or a high value (ie, a high air-fuel ratio). You can test the dimming feature by holding your finger over the LDR. Adjust VR1 until the display dims. This trimpot is best adjusted in the dark to obtain the desired bright­ness. You will need a 1V voltage source to set the span control, VR2. This can be derived from either a variable power supply or from a battery. Either way, you may need to divide the available voltage down so that your digital multimeter shows exactly 1V. Perhaps the easiest way to do this is to connect a trimpot (or a potentio­ meter) across the power supply or battery and adjust the wiper until there is 1V between the negative terminal and the wiper. The pot should have a value of between 1kΩ and 100kΩ and, if you are using a variable power supply, this should be adjusted to Adjustment Switch off the power and adjust trimpots VR2 and VR3 to their centre positions. This done, solder a 1.8kΩ resistor (R2) to the copper side of the microcontroller PC board, between pin 9 of IC1 and the copper area beneath REG1 – see Fig.6. Next, connect the sensor input to ground and apply power. The display should show a reading which is close to 0.00 or it should show a “-” sign, indicating a negative value. Adjust VR3 until the display shows 0.00V. Note: adjust VR3 anticlockwise if the reading is above 0.00 and clockwise if it shows a “-”. 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. October 2000  67 This view shows one of the EGO sensors in a Holden VT Commodore. The VT’s engine has two sensors – one for each cylinder bank. You can use either. provide a low output voltage – eg, 5V or less – before the pot is connected. Alternatively, you can simply divide the 5V supply at the output of REG1 down to 1V using a pot, as described above. Now apply this 1V between the sensor input and ground and adjust VR2 until the display shows 10.0. This represents a vol­tage reading of 1.00V; ie, the decimal should be in the wrong place. This adjustment ensures that the unit operates over the standard 1V range provided by the EGO sensor. You may now wish to recheck the offset adjustment when the sensor The Suzuki Vitara’s EGO sensor is easy to find. This, by the way, is a 4-wire sensor – two for the heater, one for the signal and the other for the ground return. Fig.6: resistor R2 is installed to convert the unit to voltage mode so that the offset voltage can be correctly adjusted. Once the adjustment has been made, the resistor is removed. R3 is required if your engine runs on propane and is left out for unleaded petrol. input is connected to ground again – if it’s out, simply tweak VR3 for a 0.00 read­ing. When these adjustments have been completed, remove resistor R2 so that the display now shows the mixture ratio. Finally, if you are using propane (or LPG) instead of un­leaded petrol, you will have to install R3 as shown in Fig.6. Installation The Fuel Mixture Display can now be installed in the vehi­cle. Use automotive connectors and cable to make the +12V and GND (ground) What’s Inside An EGO Sensor? There are two types of oxygen sensor in general use, the first based on zirconium oxide (or zirconia, ZRO2) and the second based on titanium oxide. The zirconium oxide type is the most common as it generates a direct output voltage. These sensors are also often called Lambda (λ) sensors. Lambda is simply the current air-fuel ratio divided by the air-fuel ratio at stoichiometry. Its value is 1 at the stoichiometric point, is greater than 1 when the mixture is lean and less than 1 when the mixture is rich. Fig.7 shows the cross section of a typical zirconia EGO sensor. It uses a thimble-shaped section of zirconia with platinum electrodes attached on the inside and outside. One electrode is exposed to the at­ mosphere while the other electrode is exposed to the exhaust gas. 68  Silicon Chip Fig.7: cross-section of a typical zirconia EGO sensor. The EGO sensor generates a voltage due to the differing concentrations of oxygen ions at either electrode. Oxygen ions are negatively charged and the zirconia has the tendency to attract these oxygen ions and they accumulate on the surface of the electrodes. The electrode exposed to air has a greater con­centration of oxygen compared to the electrode exposed to the exhaust and so it becomes electrically negative. In practice, the negative electrode is connected to chassis and the exhaust electrode is positive. The magnitude of the voltage depends on the concentration of oxygen ions in the ex­haust gas and the temperature of the sensor. connections. In particular, note that the +12V supply must be derived via the ignition switch and a suitable connection point will usually be found inside the fusebox. Be sure to choose the fused side of the supply, so that the existing fuse is in series. The ground connection can be made to the chassis using an eyelet and self-tapping screw. Note that, for best results, this ground connection should be made at a point close to the EGO sensor ground which is on the exhaust manifold. The sensor input wire connects to the EGO output wire. Sensor connection Oxygen sensors are commonly available in single or 3-wire configurations. If your car has a single-wire sensor, you simply connect the lead from the Fuel Mixture Display directly to the sensor output terminal, along with the existing lead to the engine management computer. To do this, first push a pin through the centre of the lead and bend the ends over. You can then solder the lead from the Fuel Mixture Display to this pin and wrap the joint in insulation tape. In the case of a 3-wire type of sensor, two of the leads are used to power an internal heater. These two leads are easy to identify, since one will be at +12V when the ignition is turned on and the other at 0V. The sensor lead will have a voltage somewhere between 0V and 1V, as measured on a high-impedance (ie, digital) multi­ meter, and the lead from the Fuel Mixture Display connects to this. Now start the engine and check that the display shows various readings from rich to lean. However, don’t expect the rich end of the display to light up until the EGO sensor has warmed up, even though the mixture is rich during the warm-up period. The EGO output is temperature dependent, which means that it must reach operating temperature before giving correct EGO indication. Once the engine is warm, the display should show the air-fuel ratio, while the bargraph should show the current trend (ie, the LEDs should move up and down the display as the mixture ratio varies). The display should show a rich (ie, low air-fuel ratio) reading or even an “r” (for rich) when using full throttle. Parts List 1 microcontroller PC board, code 05108001, 79 x 50mm 1 display PC board, code 05108002, 78 x 50mm 1 front panel label, 80 x 51mm 1 plastic utility case, 83 x 54 x 30mm 1 Perspex or acrylic transparent red sheet, 56 x 20 x 3mm 1 4MHz parallel resonant crystal (X1) 1 LDR (Jaycar RD-3480 or equivalent) 3 PC stakes 3 7-way pin header launchers 2 DIP-14 low-cost IC sockets with wiper contacts (cut for 3 x 7-way single in-line sockets) 1 9mm long x 3mm ID untapped brass spacer 1 6mm long x 3mm ID untapped brass spacer 2 6mm long M3 tapped spacers 2 M3 x 6mm countersunk screws or Nylon cheesehead 2 M3 washers 1mm thick or 1 M3 nut 2mm thick 2 M3 x 15mm brass screws 1 2m length of red automotive wire 1 2m length of yellow automotive wire 1 2m length of black or green automotive wire (ground wire) 1 500mm length of tinned copper wire for links Semiconductors 1 PIC16F84P microcontroller with AIRFUEL.HEX program (IC1) Conversely, if the throttle is abruptly lifted the reading should be high (lean) or should display an “L”. Calibration Unfortunately, it’s not possible to accurately calibrate the unit unless you have access to a known reference (see panel). However, the readings should be roughly “in the ballpark”, provided the unit is used with a 0-1V EGO sensor and VR2 & VR3 are adjusted as described previously. If necessary, you can adjust the calibration to suit the sensor on a trial and error basis. Adjust VR2 clockwise 1 LM358 dual op amp (IC2) 1 7805 5V 1A 3-terminal regulator (REG1) 4 BC327 PNP transistors (Q1-Q4) 2 BC337 NPN transistors (Q5,Q6) 3 HDSP5301, LTS542A common anode 7-segment LED displays (DISP1-DISP3) 1 10-LED red vertical bargraph (Jaycar Cat ZD-1704 or equiv.) 1 LM336-2.5 reference diode (REF1) 1 16V 1W zener diode (ZD1) 2 1N914 diodes (D1,D2) Capacitors 1 47µF 16VW PC electrolytic 3 10µF 35VW or 63VW PC electrolytic 4 0.1µF MKT polyester 2 15pF ceramic Resistors (0.25W 1%) 1 1MΩ 2 1.8kΩ 1 180kΩ 2 1kΩ 1 100kΩ 1 1kΩ 0.5W 1 12kΩ 4 680Ω 1 10kΩ 8 150Ω 2 3.3kΩ 1 10Ω 1W Potentiometers 1 500kΩ horizontal trimpot (VR1) 1 250kΩ horizontal trimpot (VR2) 1 20kΩ horizontal trimpot (VR3) Miscellaneous Automotive connectors, heatshrink tubing, cable ties, etc. if you want rich mixture indication to occur at a lower sensor voltage, for example. Similarly, adjust VR3 anticlockwise if you want lean readings at a higher sensor voltage. You may also wish to reduce the amount of display movement, particularly on the bargraph display. This can be done by in­creasing the value of the 0.1µF capacitor on pin 2 of IC2a. You can use an MKT style capacitor up to 1µF or a low leakage elec­trolytic from 1µF up to 10µF. The positive side of the capacitor should go towards the SC EGO input terminal. October 2000  69