I don't know if anyone remembers, but awhile ago I built a PIC based
amphometer cct. The way that it worked was to detect the light level
change as a vehicle passed over two LDR's. This arrangement worked quite
well and gave consistent and accurate results as verified with high
The only problem was varying light levels which may be caused by
artificial lighting and/or clouds moving around temporarily blocking the
sun. I have played around quite a bit trying to erradicate this problem
because due to the nature of it's use, it needs another person to adjust
the threshold levels prior to using it. I have come up with a simple ALC
circuit, (Automatic Level Control).
The problem is that my op amp knowlege is not what it should be, so if
anyone has some time to have a quick peek at it I would appreciate
someone checking if it is ok.
I have posted the detector cct on my web site at
It works (I hope) by the fact that the (+) input is always 1 diode drop
below the (-) input until a rapid reduction in light level occurs, thus
briefly changing the comparators output state.
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