Air/Fuel ratio monitor

Written and conceived by Greg Gibson

Shown below is the circuit I'm currently using. The LM324 is used to amplify the high impedance signal coming out of the O2 sensor and filter out any noise present. The LM7805 5v regulator is used to keep the LM3914 chip from dissipating too much power when running in bar graph mode.

Set R15 such that LED10 just comes on when 0.85V is applied to the O2 sensor input. The O2 sensor input goes to the Green/Yellow wire at position D on the middle connector going into the ECU. The O2 ground input goes to one of the ground wires going into the ECU. The mode pin (pin 9) on the LM3914 can be tied to +12v for bar graph mode or left floating for dot mode.

The chart below shows the actual air to fuel ratio for the ten LEDs with the first red LED being LED 1 (pin 1 on LM3914) and the last green LED being LED 10 (pin 10 on LM3914). This chart is only correct if the circuit is calibrated as described in the paragraph avove.