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The tester measures the dryness or wetness through soil resistance readings (0 to about 5 kilo-ohms) and indicates in dot mode by sequential lighting of LED1 through LED9. The first LED (LED1) lights up when the conductance is high (resistance is almost nil).

Normally, the soil resistivity varies from 0 to about 5 kilo-ohms. So for calibration, connect a 5-kilo-ohm potmeter between the two probes. Using trimpot VR1, set resistance to zero (minimum). LED1 should glow. Similarly, set resistance to 5 kilo-ohms (maximum). Now, LED9 should glow.

Circuit: https://www.dropbox.com/s/xywh8t1dvdb...

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