HI Bill512,
If you are sure that in the presence of metals buried by the time the LED goes out, then you have to connect R5 to pin not inverting of LM358 and the cursor of P3 to R6 (inverting pin). While the phenomenon always causes an increase of the signal is possible that the phase change is acting contrary to your Llrl or in your country. In my LRL output of the CD4046 (pin 2) there is a negative change that is reversed by the LM358 because it is necessary that the signal increases to turn on the LED.
regards
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