Houseplant Buzzer – When the Soil Gets Too Dry

Unless they are triffids (tall, prolific, carnivorous and venomous fictional plant species), outdoor plants are totally dependent for their welfare (water, sun and chemicals) on Mother Nature. Houseplants are not that lucky: they depend on man, who is often forgetful when it comes to watering or feeding them. To help such negligent owners/caretakers, at least as far as watering is concerned, a device is described that enables the moisture of the soil of houseplants to be monitored electronically. When the soil gets too dry, the monitor emits a series of squeaks to warn the owner or caretaker to add water. “Say it with flowers” is a well-known slogan and it is a fact that (most) people like flowers and plants in general. Of course, plants are of tremendous importance to animal life in providing food and oxygen. But, where many outdoor plants have great nutrimental or photosynthetic value, houseplants are normally kept for their decorative effect. This decorative effect soon becomes blemished, however, when the plant is not watered regularly. To help the many people who forget to look after their houseplants routinely, a simple monitoring device will sound a warning when the plant soil becomes too dry and will continue to do so until water has been added to the dry(ing) soil. Owing to the low current drain, the monitor can operate from a single dry battery for up to a year. The operation of the monitor depends on the property of water to conduct when it contains chemicals, such as alkalines or acids (found in abundance in good potplant soil and drinking water). This means that moist soil is a fairly good electric conductor. When the soil dries, it becomes less and less conductive. How It Works? The electrical resistance of the soil is measured by a pair of sharp pointed probes which are pushed into the soil. The resistance is continually monitored by a simple circuit. When the soil gets (too) dry, this circuit actuates a piezo-electric buzzer. Since the required moisture varies from plant to plant, a preset is usually used to set the minimum allowable moisture. The reliability of the device depends almost entirely on that of the probes. Were these to carry even only a tiny direct current, in the moist conditions in which they are used, they would soon be subject to rapid oxidation or electrochemical destruction. That is, depending on the direction of the direct current, one of the probes would soon be enveloped in a film of oxide, while the other would be (partly) dissolved. This electrolysis is negated by the use of an alternating current instead of a direct current.

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