> when I connect one pin of LDR to VDD (other pin goes to AN0
> and to ground through a resistor), there is obvious malfunction in
> the system

While the motor is running ? I've used LDRs and other opto sensors
and not found them noisy at all

> long time operation at 5.1V might cause some malfunction

Vdd limit is 5.5V for most, if not all, 5V PICs. I have many projects
with Vdd of 5.12V (for easier ADC maths)
--=20
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