D. Morrill O2 Led meter

John Vitamvas jpv at po.CWRU.Edu
Tue Feb 17 21:28:09 GMT 1998


Hello,

I came across your page not too long ago.  Quite impressive.  I decided
that I would build D. Morrill's 02 meter for my 87 RX-7 turbo, to use as a
monitoring device.  Rotaries are extremely sensitive to overly lean
mixtures, as any detonation can destroy them.  Hence, my interest...

It took about $25 including shipping from Jameco to build the meter, and
about a half hour of time.  I bench tested it on my Graymark 808 power
supply, using an HP multimeter.  I benched it at 5V supply instead of 14.4
automotive, but this is just the 3914 supply voltage and should be
irrelevant.  I found the following:

Given a 02 sensor range of 0-1.1V, where 1.1V = rich and 0V = lean...

# lit	Voltage
-----	-------
0	0.0 - .11V			
1	.12 - .25V
2	.26 - .37V
3	.38 - .50V
4	.51 - .62V
5	.63 - .74V
6	.75 - .87V
7	.88 - 1.00V
8	1.01 - 1.12V
9	1.13 - 1.23
10	1.24 ->

Hence, with stoich at .86 V (14.7:1), the meter centers on the 6th light.
The tenth light is technically out of range but not unreasonably so.  It is
linear all the way to the top, with approximately .114 V/LED average.  I
wired in switches for power and pin 9 (mode), mounted it in a nice case and
it will be mounted under the stereo in my car.

A few comments on the schematic:  It should be noted that what is referred
to as "LED 1" is actually the tenth led on the meter, representing the
highest voltage, or most rich.  This perhaps should be noted as I built it
the other way (intuitavely) and it was incorrect.  Anyone who intends to
use this in a car should install the 2.2uF cap.. otherwise it will be WAY
too noisy.  

Thanks for a great design!

John




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