Soil moisture-sensing by hacking a solar light
The pathway light chip (the XY8018 here), in its original circuit is meant to turn on an LED when the solar panel is not receiving light, acting like a short circuit (or “high”), and turn off the LED and have the solar panel charge the battery when the solar panel is receiving light (or “low”). Using this information about how the chip functions we can rewire our probes to the chip to be the dictating factor to whether the LED is on or not.
To set up a circuit that will turn the LED on when the soil is dry, we can wire one probe to pin 3 of the chip, where the solar panel was hooked up, and the other to ground, which will allow the LED to turn on when there is a high resistance between ground and pin 3 (when there is less water between the probes). The LED will turn off when there is more water between the probes and the resistance decreases between pin 3 and ground, pulling it low.