Plants Humidity Sensor

IC1D is a CMOS Schmitt trigger oscillator at about 2KHz. It starts and continues to oscillate with a supply down to 1.24V (the lowest output voltage of my LM317 variable power supply) or less. IC1A is an inverter. IC1B is a Schmitt trigger NAND gate. Its output is low only when both inputs are at, or higher than the upper Schmitt trigger threshold voltage. With 47 ohms or less between the probes, an input is always low, so the output is always high. With a resistance of only R8 between the probes, the voltage across C3 is high most of the time, so the gate?s output is low for ½ the oscillator?s period. With a resistance that is halfway, then C3 is charged high by that resistance when the oscillator?s output is high, then is discharged when the oscillator?s output is low.
Plants Humidity Sensor - schematic

When C3 is being discharged, then pin 12 of the gate is high, and pin 13 is also high until the discharging voltage of C3 reaches the lower Schmitt threshold voltage. During this time, the gate's output is low. So the low time of the gate?s output depends on the value of the resistance between the probes. This is Pulse-Width-Modulation of the low output of the gate. 4) IC1C is another CMOS Schmitt trigger oscillator at about 2Hz. D1 and R4 discharge C4 quickly so that its output is low for only about 15ms with a 3V battery, and about 25ms with a 2V battery. 5) The series connection of Q1 and Q2 performs like a NOR gate, so that the LED lights only when both inputs to the transistors are low. 6) R7 is a current-limiting resistor for the 1.8V LED. With a 3V battery, the LED current is about 35mA. Circuit Operation: 1) When the soil is very dry, the LED flashes brightly, since the soil?s resistance is very high. 2) When the soil has been watered a few days before, but is drying, the LED flashes dimly, 3) When the soil is damp because it has been recently watered, the LED is off. Note that different soils have a different resistance. Also, sometimes, watered soil will continue to have a high resistance until the soil absorbs the water, a delay of about one hour. Although the LED?s current is 8mA with a 3V battery, it is lighted for only a maximum of only about 1/64th of the time, so its maximum average...

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