Yet another EGO sensor meter


Here is my version of an LM3914 driven display for an EGO sensor (O2 sensor, lambda sensor). It uses the bar mode of a 3914 and is scaled from 0 V to 1 V, so all ten LEDs are in use. I used 8 mm jumbo-LEDs, which draw sufficient amount of current (just over specs that is ;) ). The ten LEDs are arranged using the RKN (Reverse Katajainen Notation): red means lean air-to-fuel mixture, green means rich mixture.

A parallel resistor of 10 Mohm is used (1 Mohm is ok too), thereby the voltage indicated by the meter matches my digital multi-meter. The input impedance of a bare 3914 is over 10 Mohm, which IMHO is more than enough for an EGO sensor.

schematic for the EGO sensor display



This device has proved to be a life-saver several times, frustration can be deadly :). Components for a minimal meter setup without a box cost under 50 marks here ($10). All corrections, comments and success stories to me.

Number 1 pin of an IC chip is sometimes marked with a small dot beside the pin, see the schematic. If not, orient the chip as in the schematic, the semicircle upwards. Pay attention to LED numbering, LED bar grows downwards in the schematic.

Note that this text describes a circuit to interface with a basic zirconium-based EGO (or HEGO) sensor only. Nowadays there are also other types of sensors, which require a different kind of interfacing circuit.

You are also supposed to know what can be done with the sensor, and what can not. A basic EGO sensor cannot measure true oxygen content in automotive environment, nor does it tell you the exact lambda value. If you're looking for an accurate exhaust gas analyzer, you won't find it here. There might be better ways to use a zirconium dioxide sensor to measure oxygen content, but we don't use them here.

The LED colors give only an approximate information of what can be happening with the A/F-mixture. You cannot connect any unambiguous mixture ratio values to different sensor voltage levels.

This is usually enough for the purposes I, and many others use these sensors. An EGO sensor can quickly detect grossly incorrect situations, like a way too lean or too rich mixture under hard acceleration, transient mixture spikes that can't be captured otherwise, and other weird situations affecting driveability.

A short introduction on EGO sensor connectors. Note that I have used only a few sensors, so this info is mainly gathered from others.

1-wire sensor
wire: signal (wire usually black in color)
sensor body: signal ground
wires: signal (black) and signal ground (gray)
wires of the same color: sensor heater power and power ground (white)
the third wire: signal (black)
sensor body: signal ground
two wires of the same color: sensor heater power and power ground (white)
two other wires: signal (black) and signal ground (gray)

See the National Semiconductor web site for more information and an LM3914 data sheet.

I have sketched a simple PCB layout for the EGO sensor display, but haven't actually tried the design in practice yet, so YMMV. An installation of six PCBs is also included in the archive file in PDF/EPS/Corel Draw formats.

A preview of the EGO sensor display PCB design