Soil Moisture Sensors, How Do They Work?

In a way, the magic of a soil moisture sensor’s functionality boils down to a simple RC circuit. But of course, in practice there is a bit more to it than that. [rbaron] explains exactly how capacitive soil moisture sensors work simply, clearly, and concisely. He also shows, with a short video, exactly how their output changes in response to their environment, and explains how it informed his own sensor design.

At its heart, a moisture sensor measures how quickly (or slowly) a capacitor charges through a resistor, but in these sensors the capacitor is not a literal component, but is formed by two PCB traces that are near one another. Their capacitance — and therefore their charging rate — changes in response to how much water is around them. By measuring this effect on a probe sunk into dirt, the sensor can therefore indirectly measure the amount of water in the soil.

This ties into his own work on b-parasite: an open-source, all-in-one wireless soil moisture sensor (which was also a runner-up in our Earth Day contest) that broadcasts over BLE and even includes temperature readings. One thing to be mindful of if you are making your own PCBs or ordering them from a fab house is that passing current through metal in a moist environment is a recipe for oxidation, so it’s important not to expose bare traces to wet soil. A good coated PCB should avoid this problem, but one alternative we have seen proposed is to use graphite rods in place of metal.

Parts of the automated soil moisture monitoring station

Solar Stevenson Screen For Smart Sprinkler

It’s not infrequent that we see the combination of moisture sensors and water pumps to automate plant maintenance. Each one has a unique take on the idea, though, and solves problems in ways that could be useful for other applications as well. [Emiliano Valencia] approached the project with a few notable technologies worth gleaning, and did a nice writeup of his “Autonomous Solar Powered Irrigation Monitoring Station” (named Steve Waters as less of a mouthful).

Of particular interest was [Emiliano]’s solution for 3D printing a threaded rod; lay it flat and shave off the top and bottom. You didn’t need the whole thread anyway, did you? Despite the relatively limited number of GPIO pins on the ESP8266, the station has three analog sensors via an ADS1115 ADC to I2C, a BME280 for temperature, pressure, and humidity (also on the I2C bus), and two MOSFETs for controlling valves. For power, a solar cell on top of the enclosure charges an 18650 cell. Communication over wireless goes to Thingspeak, where a nice dashboard displays everything you could want. The whole idea of the Stevenson Screen is clever as well, and while this one is 3D printed, it seems any kind of stacking container could be modified to serve the same purpose and achieve any size by stacking more units. We’re skeptical about bugs getting in the electronics, though.

We recently saw an ESP32-based capacitive moisture sensor on a single PCB sending via MQTT, and we’ve seen [Emiliano] produce other high quality content etching PCBs with a vinyl cutter.

Give Your Smart Home A Green Thumb With MQTT

We have all been stuck inside for too long, and maybe that’s why we have recently seen a number of projects attempting to help humans take better care of their housemates from Kingdom Plantae. To survive, plants need nutrients, light, and water. That last one seems tricky to get right; not too dry and not drowning them either, so [rbaron’s] green solder-masked w-parasite wireless soil monitor turns this responsibility over to your existing home automation system.

w-parasite MQTT diagram

Like this low-power soil sensor project and the custom controller for six soil sensors, [rbaron’s] w-parasite uses a “parasitic capacitive” moisture sensor to determine if it’s time to water plants. This means that unlike resistive soil moisture sensors, here the copper traces are protected from corrosion by the solder mask. For those wondering how they work, [rbaron]’s Twitter thread has a great explanation.

The “w” in the name is for WiFi as the built-in ESP-32 module then takes the moisture reading and sends an update wirelessly via MQTT. Depending on the IQ of your smart-home setup, you could log the data, route an alert to a cellphone, light up a smart-bulb, or even switch on an irrigation system.

w-parasite circuit board in a potted plant[rbaron] has shared a string of wireless hacks, controlling the A/C over Slack and a BLE Fitness Tracker that inspired more soldering than jogging. We like how streamlined this solution is, with the sensor, ESP-32 module, and battery all in a compact single board design. Are you asking yourself, “but how is a power-hungry ESP-32 going to last longer than it takes for my geraniums to dry out?” [rbaron] is using deep sleep that only consumes 15uA between very quick 500ms check-ins. The rechargeable LIR2450 Li-Ion coin cell shown here can transmit a reading every half hour for 90 days. If you need something that lasts longer than that, use [rbaron]’s handy spreadsheet to choose larger batteries that last a whole year. Though, let’s hope we don’t have to spend another whole year inside with our plant friends.

We may never know why the weeds in the cracks of city streets do better than our houseplants, but hopefully, we can keep our green roommates alive (slightly longer) with a little digital nudge.

 

ESP32 Soil Monitors Tap Into Ultra-Low Power Mode

Soil moisture sensors are cheap and easy to interface with, to the point that combining one with an Arduino and blinking an LED when your potted plant is feeling a bit parched is a common beginners project. But what about on the long term? Outside of a simple proof of concept, what would it take to actually read the data from these sensors over the course of weeks or months?

That’s precisely the question [derflob] recently had to answer. The goal was to build a device that could poll multiple soil sensors and push the data wirelessly into Home Assistant. But since it would be outside on the balcony, it needed to run exclusively on battery power. Luckily his chosen platform, the ESP32, has some phenomenal power saving features. You just need to know how to use them. Continue reading “ESP32 Soil Monitors Tap Into Ultra-Low Power Mode”

Vintage Ammeter Becomes Plant Moisture Gauge

It’s not uncommon to happen across vintage measurement equipment at the local flea market or garage sale. Often with an irresistible aesthetic, and built to last decades, these tools nonetheless tend to be sidelined when modern multimeters are available. [Build Comics] had just such a piece on hand, and decided to repurpose it with some modern hardware instead.

The build begins with a Hartmann & Braun 60 amp ammeter. Replete in a nice wooden box, it’s the perfect candidate for a modern refit. The device uses an indicator of the moving iron type. Intending to turn the device into a soil moisture monitor, [Build Comics] began by removing the original heavy-wound coil. In its place, a custom coil was installed instead, wound on a 3D printed bobbin using a modified sewing machine. This allows the meter to be easily driven by an Arduino with little more than a transistor on a GPIO pin. To detect moisture, a Iduino ME110 moisture probe was used. Complete with cloth-covered wire to maintain the vintage look. The original meter plate was also photographed, modified, and reprinted, to read moisture levels instead of current.

If you’re interested in these gauge restoration techniques but don’t have a green thumb, no worries. [Build Comics] used similar tricks to put together a gorgeous weather station that would look great on your desk.

Continue reading “Vintage Ammeter Becomes Plant Moisture Gauge”

A Capacitive Soil Sensor Hack For Lower Voltage Supplies

A frequent beginner project involves measuring soil moisture levels by measuring its resistance with a couple of electrodes. These electrodes are available ready-made as PCBs, but suffer badly from corrosion. Happily there is a solution in the form of capacitive sensor probes, and it is these that [Electrobob] is incorporating in to a home automation system. Unfortunately the commercial capacitive probes are designed to run from a 3.3 V supply and [Bob]’s project is using a pair of AA cells, so a quick hack was needed to enable them to be run from the lower voltage.

The explanation of the probe’s operation is an interesting part of the write-up, unexpectedly it uses a 555 configured as an astable oscillator. This feeds an RC low pass filter of which the capacitor is formed by the soil probe, which in turn feeds a rectifier to create a DC output. This can be measured to gain a reading of the soil moisture level.

The probe is fitted with a 3.3 V LDO regulator, which is simply bypassed. Measurements show its output to be linear, so if the supply voltage is also measured an accurate reading can be gleaned. These probes are still a slightly unknown quantity to many who might find a use for them, so it’s extremely useful to be given this insight into them.