The image shows a Dick Smith version of the original FMD designed for the Bosch LSM-11 sensor.
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.
http://www.techedge.com.au/vehicle/wbo2/fmd.htm
