Yet another EGO sensor meter

Blurb

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.

Tuning

• Supply voltage can be just about anything, 5...25 V. A ~2.2 uF bypass capacitor may be needed with noisy power sources.
• Input signal is unamplified signal directly from the EGO sensor. You may have to use some signal conditioning and buffering depending on the environment. I don't have any noise in the signal, bringing the signal and ground wires directly from the sensor probably helps. Separate ground for the power isn't necessarily needed, but can be added too.
• R=10 Mohm, an impedance similar to a DMM.
• T1=4.7 kohm, used to scale the chip's 1.25 V reference voltage to a lower high-reference voltage (1.00 V). If you don't want to use a trimmer, try a 2.2 kohm resistor. Calibrate the display reading with an adjustable voltage source and a DMM, so you'll know which LED indicates what voltage level.
• T2=4.7 kohm, display LED current control. Don't fry the LM3914, maximum power dissipation is 1365 mW. Try a 2.2 kohm resistor for 5 mm LEDs, 1 kohm for 8 mm LEDs.
• Growing bar mode operation. For dot mode (only one LED lighted at the time) operation, cut the wire to the pin 9 and leave the pin floating. Dot mode operation can be useful when observing rapid signal fluctuations, I haven't tried it.
• LED number 1 lights first at 100 mV, then the others in 100 mV steps, last one at 1000 mV.
• the RKN:
  10. LED (quite enough, thanks)
  9. LED (power)
  8. LED (power)
  7. LED
  6. LED (a cataleptic what?)
  5. LED (a cataleptic what?)
  4. LED
  3. LED (cruise)
  2. LED (cruise)
  1. LED (cruise)
  

Updates

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

2-wire

wires: signal (black) and signal ground (gray)

3-wire

wires of the same color: sensor heater power and power ground (white)

the third wire: signal (black)

sensor body: signal ground

4-wire

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.