I'm in need of a simple alarm to indicate a switch (120 VAC) having been left on too long, say more than five minutes. [I know, I could just buy a timer switch, but what's the fun in that?]
The State of California had a task force investigating water conservation and one of the force's recommendations was to make use of ground moisture sensors so that automatic lawn sprinklers would operate only when needed. The newspaper article about the force's report omitted any and all details about where one obtains such sensors. Inquiry of the local water district finds that there's serious questions about the sensors and making use of them. Anyone have any info about (reliable) soil moisture sensors? Any thoughts about how to incorporate the sensors into the typical lawn sprinkler controller?
Since you're posting in an embedded group, can we assume you want to use a microcontroller? This would be a simple project with a spare wall-wart adapter, 78x05 regulator and whatever smallest micro you want to use. You can turn a light on or drive a piezo audible indicator or whatever.
Have you checked your local DIY store (eg. Home Despot)? Sprinkler stuff is really cheap. Obvious approaches would be capacitance or AC conductivity. Some commercial gear uses radar, IIRC, but cost would be higher.
Best regards, Spehro Pefhany
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That's an interesting question to pose to a group of professionals conditioned to offer the cheapest, most reliable, most appropriate design.
In the absence of any other requirements, the answer is and always will be something like a timer switch depending on how one interprets what you wrote. So there must be some requirements that you're keeping to yourself.
If you Google SOIL MOISTURE SENSORS, you will find lots of hits. One of the common sensors is the gypsum block sensor. It requires that the excitation be reversing polarity so as not to build up contaminants on the sensor.
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