Silicon Chip Fuel Mixture Display

TE-WB (v1.5)   |   Oz DIY-WB (v1.0)   |   TE-5301   |   Wideband FMD   |   Silicon Chip FMD

The image shows a Dick Smith version of the original FMD designed for the Bosch LSM-11 sensor. Here's an image of the Jaycar version. A more accurate wideband fuel mixture kit using the NTK L1H1 UEGO sensor is available from Tech Edge. The Silicon Chip September and October 2000 issue features a fuel mixture (or air/fuel ratio) display. This display is offered as a kit or just a set of two PCBs by (prices correct @ 31 Oct 01): Altronics K4347 (AU$64.95) Jaycar KC5300 (AU$62.95) Dick Smith K4213 (AU$62.95) RCS 05109001 just PCBs (AU$16.50) It features Digital readout plus bargraph Display auto-dims at night Calibrated for Bosch LSM-11 11.8 to 20.6 AFR range for petrol

The Design

It uses a commonly available MicroChip PIC chip, the 16F84 (the lowest cost 4 Mhz version) which is a mature and readily available flash part that can be reprogrammed easily. The unit is built on two small single sided PCBs with just a few jumpers. The boards fit together with no messy inter-board wiring. One PCB holds the display components and the other the PIC circuitry. Just three wires connect to the vehicle - the sensor, Vbatt and GND.

The circuit below also has a larger view (1485 x 1086 = 218k bytes).

Software for the project is available as zipped source form or zipped hex from Silicon Chip's web site

The input from the Bosch LSM-11 sensor is via a 1M ohm resistor and 0.1 uF capacitor. This results in very low loading of the sensor but also damps any noise and slows the response to a readable rate - display refresh is around 2.5 times a second for the 3 digits, and twice that for the bar display.

The charge pump and regulator circuit Q6, D1, D2 and REF1 generate a small negative voltage that is used in conjunction with comparator IC2a and the PIC to create an eight bit analogue to digital (AtoD) converter using just two pins of the PIC. A square wave at 1953 Hz, and varying in duty cycle, is generated at RA3. The PIC's firmware varies this duty cycle to create a reference voltage at the comparator's +ve input pin 3 that is compared with the sensor's output voltage.

The PIC uses a lookup table to convert the voltage read from the sensors into an air-fuel ratio to multiplex onto the 3 seven segment LED displays and just 7 segments of the 10 segment dot/bar display. Transistor Q6, opamp 2b work with the light dependant resistor and VR1 to provide adjustable automatic dimming at night.

Transfer function The unit comes with the sensor voltage to A/F ratio transfer function shown in the graph (click on diagram for a larger view and details of how the dot/bar display is interpreted - 716 x 1144 = 152k bytes). As the source code for the PIC is provided, this transfer function can be modified to suit different sensors - including the DIY-WB unit's usable output swing of around 2.7 volts (1.4 - 3.1).

Modifications for DIY-WB operation

Modifications for operation with the Oz DIY-WB electronics are quite simple and can be done for around a dollar (although the PIC must be reprogrammed). Full details of all modifications, are available, but note that this design uses an NTK sensor that is quite different to the Bosch LSM-11. The NTK sensor requires the additional electronics provided by the DIY-WB unit.