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We developed a compact sensor module for hydroponics that measures the nutrient concentration and water level using simple oscillator circuits. Oscillation frequencies are measured and are found to change in response to the resistance and capacitance between metal wires on a printed circuit board submerged in liquid fertilizer. A microprocessor is mounted on the sensor board for measurement and communication. Expressions relating the oscillating frequencies, liquid fertilizer concentration, water level and temperature were derived through experiments using the sensor module.
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Nishimura et al. (2017) studied this question.
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