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Last Updated



DIY 10 Segment Air/Fuel Ratio Meter
I guess, the first question anyone would ask me is "Why?". Why build it yourself when you can get them so inexpensively? Let's face it, Summit Racing offers it for less than $30 complete! Cool factor and sense of accomplishment are the first things I respond, plus it gives you a range of mounting options you don't have in a pre-canned package, like how about a stealth mount in the dash or gauges. In addition, these have an adjustment feature that lets you "tune" the gauge for more detail that the big makers don't give you and an adjustable brightness control. In any case, these can be done for around $10-$15 easily and are every bit as accurate as many others that you pay bigger bucks for.
Parts List
Optional Parts:

The Run Down

      My first suggestion is to always use chip sockets when soldering to chips. This way the chip itself doesn't take any heat causing potential failure and undue headaches in trying to troubleshoot. If you solder a bunch of wires from the chip socket to the LED socket you can have some placement flexibility for the LED display, like in the dashboard. Be aware that LED's have a Positive side and Negative side. There's a flat spot in the corner of the BarGraph 'chip' to indicate Pin 1.

      There are many different ways to make this a clean, well executed project, but as always, the end result is from your own efforts. Such niceties as terminating the wires with connectors and what not. You can also use floppy drive ribbon cables to connect all the circuits together, which will make it easy and neat to mount the LED graph separately.    

      Most people want the wide band of the O2 measuring from 0-1v. But some might want a Finer Resolution. Where it'll measure from 0.4-1v. This would mean that the 1st LED will be ~0.4v and the 10th LED will be 1v. Engines equipped with a turbo, blowers, or Nitrous, might want a finer resolution as the Air / Fuel ratio is especially critical for those motors. In a Naturally Aspirated engine, the finer resolution might not be necessary as the proper mixture (14.7:1) is at about 0.5v from the O2 sensor.

      Resistor R1 is what determines the resolution for the LED display. Resistor R2 is what determines the relative brightness of the LED. You can stick potentiometers in place of these resistors in order to fine tune the bar graph resolution and display brightness and is highly encouraged for flexibility. If you decide to use the Potentiometer (POT) you need to get an Ohm meter and measure the range at which resistance ranges from 2.3k to 3.3k . As it is adjustable, you can adjust it on the fly.

      In order to run the display at the finest resolution possible, take out R1 and run it straight with no resistor in there. It should be that simple. During experimentation I had the resolution up to 0.7v as the 1st led, but I can't figure what was done to get this measurement. Feel free to play around with it and let us know if you find something that works better.

      Currently it is setup as BAR mode. If you prefer DOT mode, disconnect pins 3 and 9. DOT mode is when one LED will light up at a time, while BAR mode all lights will light up, building to the final value.

      Installation is as straight forward as it could be. Hook up the power to an to a switched power source, and connect up the ground to a solid ground. Connect up the Signal Wire to your O2 sensor. Remember that your ECU will need signal too for fuel adjustment. Note: the voltages from the O2 sensor are very low, so take extra care in taping the signal wire and make a high quality connection. It's highly suggested to solder the connection properly and use heat shrink tubing to seal it.

     The table below represents what the LEDs indicate on a non modified gauge in wide resolution mode. You can use a Volt/Ohm meter on the signal wire to take a reading directly from the O2 sensor to recalibrate your lights in the fine resolution mode. These should be pretty close, but don't hold me to these values.

LED 10 - 0.97V - 12.1:1 - Very Rich - Forced induction and nitrous. (All LEDs on)

LED 9 - 0.88V - 12.7:1

LED 8 - 0.78V - 13.2:1

LED 7 - 0.69V - 13.8:1

LED 6 - 0.59V - 14.4:1

LED 5 - 0.49V - 14.9:1 - Stoich- This is where NA motors should be at

LED 4 - 0.39V - 15.4:1

LED 3 - 0.30V - 16.0:1

LED 2 - 0.20V - 16.5:1

LED 1 - 0.10V - 17.1:1 - Lean (The only LED on)

Author: 85frankenstein

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