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DIY Air Fuel Ratio meter. 10 LED Rich/Lean Meter
My apologies to the owner of the website I got this from a long time ago. I know it was a Honda related page but I only saved
the JPG file and forgot the original site.  I made two already and they both work. The circuit depending on the resistors that you put in is simply a volt meter with variable peak voltage. This version has a 1 volt peak (+/-) suited for a 1 wire 02 sensor. I have no idea if it works with multi wire 02 sensor. I'll try to post a picture of the working prototype soon.
You will need the following parts

LM3914  National Semiconductor.
     10 LEDs 3 colors from lean to rich
     1 - 1k Ohm resistor.
     1 - 220 Ohm resistor.
          A prototyping board.

There are three inputs. 02 goes to the oxygen sensor wire, V is the + 12 volt source  , and the ground wire. Also check out National semiconductors' web site

some notes

the pins 1 and 18 are to the left of the letters LM exactly the was you see them at the dia gram
leds have polarity they won't light up if you put them in the wrong way. Check polarity (one leg long one short using 4.5 to 6 volt battery)
resistors have no polarity but it is critical that you put the right resistor in the right place do not interchange.

 go to National Semi Conductor web Site
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