The Plant Minder manages the watering process for an indoor plant. It monitors the moisture content of the soil using several capacitive sensors, and controls the speed and running time of a low volume pump to drip-water the plant from a water reservoir. It automatically adapts the pump run time to the soil conditions, which can be greatly affected by the ambient temperature, humidity, and sun exposure. It is designed for larger plants, especially ones with large leaves, which need a significant volume of water every day to avoid damage. The system uses a pump rather than a gravity-fed system and includes moisture sensors located around the plant base, to reduce the possibility of water spilling on the floor.
The hardware is built around an Atmel ATmega48 AVR microcontroller, with a sprinkling of analog components for the moisture sensors and motor interface. The motor uses a simple PWM driver, along with a current sense amplifier to measure the drive current. The variation in current during each shaft rotation is used to compute the motor speed, and the average current indicates when the motor load drops if the reservoir is empty. An RS-232 level converter allows watering data to be downloaded to a PC. The system is powered by a 12V wall adapter, with a 3V lithium battery for clock and memory backup.
The system uses four sensors. Two are buried in the soil near the top and bottom of the container, one is outside the soil as a reference, and one is set on the floor to monitor for spills. The capacitance of each sensor is a function of how much water is in the surrounding area. This affects the time constant of an RC circuit, where C is the sensor capacitance and R is a fixed resistor. The sensors are driven by PWM outputs, and the ADC inputs measure the voltage at a fixed time after each PWM transition, which is affected by the time constant. This gives a simple yet reliable method for measuring a relative change in capacitance. Each sensor also has a shield driver to minimize the effect of the connecting cable’s capacitance. The indoor plant watering project is designed by Richard Wotiz from US.