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Wideband Oxygen Sensor Controller Mk.2, Pt.2

Last month, we introduced our new Wideband Oxygen Sensor Controller Mk.2 and described the circuit. This month, we give the circuit for the display unit and the full construction details.

By John Clarke

While a voltmeter could be used to monitor the Wideband Controller’s 0-5V output, the measured voltage does not directly indicate the lambda value. Instead, you would need to use the equation lambda = [V x 0.228 + 0.70] to convert the controller’s wideband output voltage (V) to the corresponding lambda value.

That’s where the Wideband Oxygen Sensor Display comes in. It plugs directly into the controller unit and automatically calculates and displays the correct lambda value. What’s more, the Wideband Oxygen Sensor Display is set up to give the correct 0.70-1.84 lambda range by default, so you do not have to make any adjustments during construction.

Alternatively, you can alter the display to show the air-fuel ratio or you can program the unit to monitor any other signal source over a 0-5V range and display a corresponding readout (see panel).

As shown in the photos, the display unit is built into a small plastic case and this measures 83 x 54 x 31mm. Three 7-segment LED readouts are used to display the reading and these are visible through a red Perspex or acrylic window that takes the place of the original box lid. A single cable fitted with a 3.5mm stereo jack plug connects the unit to the wideband output on the controller and this carries both the signal and power (12V).

The unit itself consists of a PIC­16F88-I/P microcontroller, three 7-segment displays, a 3-terminal regulator and not much else. It features display dimming in low ambient light (so it’s not too bright at night), while four micro tactile switches allow the displayed values to be adjusted during set-up (if necessary).

Display Unit Features & Specifications


• 3-digit LED display

• Preset display range of 0.70 to 1.84 lambda

• 0-5V input range & linear display ranging

• Adjustable 0V and 5V endpoint values

• Decimal point positioning adjustable

• Automatic leading zero suppression

• Display dimming with minimum brightness adjustment

• Quieting period used for input measurement to ensure accuracy


• Power supply: 6-15V @ 240mA

• Input current loading: less than 1µA

• Digit update period: 250ms

• Wideband display reading range: 0-999

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